Using defines for protocol characters

From 45733bb77f3ad2b7665e52c9e24b3aa11699f56e Mon Sep 17 00:00:00 2001
From: Dave Cramer <davecramer@gmail.com>
Date: Thu, 20 Apr 2023 15:40:03 -0400
Subject: [PATCH] Created protocol.h Protocol.h has defines for every protocol
 message both backend and frontend Instead of using hardcoded values for each
 protocol message use defines to make code easier to read

remove redundant comments

define CLOSE_PORTAL and CLOSE_PREPARED subcommands
fix some erroneous uses of DATA_ROW_RESPONSE which should have been DESCRIBE_REQUEST
use the names in the requests .

prepend PQMSG_REQ and PQMSG_RESP to commands and responses respectively
---
 src/backend/access/common/printsimple.c       |  5 +-
 src/backend/access/transam/parallel.c         | 17 ++--
 src/backend/backup/basebackup_copy.c          | 17 ++--
 src/backend/commands/async.c                  |  3 +-
 src/backend/commands/copyfromparse.c          | 23 +++---
 src/backend/commands/copyto.c                 |  5 +-
 src/backend/libpq/auth-sasl.c                 |  3 +-
 src/backend/libpq/auth.c                      |  9 ++-
 src/backend/postmaster/postmaster.c           |  3 +-
 src/backend/replication/walsender.c           | 19 ++---
 src/backend/tcop/dest.c                       |  5 +-
 src/backend/tcop/fastpath.c                   |  3 +-
 src/backend/tcop/postgres.c                   | 77 ++++++++++---------
 src/backend/utils/activity/backend_progress.c |  3 +-
 src/backend/utils/error/elog.c                |  3 +-
 src/backend/utils/misc/guc.c                  |  3 +-
 src/include/protocol.h                        | 61 +++++++++++++++
 src/interfaces/libpq/fe-auth.c                |  3 +-
 src/interfaces/libpq/fe-connect.c             | 15 ++--
 src/interfaces/libpq/fe-exec.c                | 53 ++++++-------
 src/interfaces/libpq/fe-protocol3.c           | 66 ++++++++--------
 src/interfaces/libpq/fe-trace.c               | 61 +++++++--------
 22 files changed, 270 insertions(+), 187 deletions(-)
 create mode 100644 src/include/protocol.h

diff --git a/src/backend/access/common/printsimple.c b/src/backend/access/common/printsimple.c
index ef818228ac..e7dc125f82 100644
--- a/src/backend/access/common/printsimple.c
+++ b/src/backend/access/common/printsimple.c
@@ -21,6 +21,7 @@
 #include "access/printsimple.h"
 #include "catalog/pg_type.h"
 #include "libpq/pqformat.h"
+#include "protocol.h"
 #include "utils/builtins.h"
 
 /*
@@ -32,7 +33,7 @@ printsimple_startup(DestReceiver *self, int operation, TupleDesc tupdesc)
 	StringInfoData buf;
 	int			i;
 
-	pq_beginmessage(&buf, 'T'); /* RowDescription */
+	pq_beginmessage(&buf, PQMSG_RESP_ROW_DESCRIPTION);
 	pq_sendint16(&buf, tupdesc->natts);
 
 	for (i = 0; i < tupdesc->natts; ++i)
@@ -65,7 +66,7 @@ printsimple(TupleTableSlot *slot, DestReceiver *self)
 	slot_getallattrs(slot);
 
 	/* Prepare and send message */
-	pq_beginmessage(&buf, 'D');
+	pq_beginmessage(&buf, PQMSG_RESP_DATA_ROW);
 	pq_sendint16(&buf, tupdesc->natts);
 
 	for (i = 0; i < tupdesc->natts; ++i)
diff --git a/src/backend/access/transam/parallel.c b/src/backend/access/transam/parallel.c
index 1738aecf1f..6deb1a697f 100644
--- a/src/backend/access/transam/parallel.c
+++ b/src/backend/access/transam/parallel.c
@@ -33,6 +33,7 @@
 #include "miscadmin.h"
 #include "optimizer/optimizer.h"
 #include "pgstat.h"
+#include "protocol.h"
 #include "storage/ipc.h"
 #include "storage/predicate.h"
 #include "storage/sinval.h"
@@ -1127,7 +1128,7 @@ HandleParallelMessage(ParallelContext *pcxt, int i, StringInfo msg)
 
 	switch (msgtype)
 	{
-		case 'K':				/* BackendKeyData */
+		case PQMSG_REQ_BACKEND_KEY_DATA:
 			{
 				int32		pid = pq_getmsgint(msg, 4);
 
@@ -1137,8 +1138,8 @@ HandleParallelMessage(ParallelContext *pcxt, int i, StringInfo msg)
 				break;
 			}
 
-		case 'E':				/* ErrorResponse */
-		case 'N':				/* NoticeResponse */
+		case PQMSG_RESP_ERROR:
+		case PQMSG_RESP_NOTICE:
 			{
 				ErrorData	edata;
 				ErrorContextCallback *save_error_context_stack;
@@ -1183,7 +1184,7 @@ HandleParallelMessage(ParallelContext *pcxt, int i, StringInfo msg)
 				break;
 			}
 
-		case 'A':				/* NotifyResponse */
+		case PQMSG_RESP_NOTIFY:
 			{
 				/* Propagate NotifyResponse. */
 				int32		pid;
@@ -1200,7 +1201,7 @@ HandleParallelMessage(ParallelContext *pcxt, int i, StringInfo msg)
 				break;
 			}
 
-		case 'P':				/* Parallel progress reporting */
+		case PQMSG_RESP_PARALLEL_PROGRESS:
 			{
 				/*
 				 * Only incremental progress reporting is currently supported.
@@ -1217,7 +1218,7 @@ HandleParallelMessage(ParallelContext *pcxt, int i, StringInfo msg)
 				break;
 			}
 
-		case 'X':				/* Terminate, indicating clean exit */
+		case PQMSG_REQ_TERMINATE:	/* Terminate, indicating clean exit */
 			{
 				shm_mq_detach(pcxt->worker[i].error_mqh);
 				pcxt->worker[i].error_mqh = NULL;
@@ -1372,7 +1373,7 @@ ParallelWorkerMain(Datum main_arg)
 	 * protocol message is defined, but it won't actually be used for anything
 	 * in this case.
 	 */
-	pq_beginmessage(&msgbuf, 'K');
+	pq_beginmessage(&msgbuf, PQMSG_REQ_BACKEND_KEY_DATA);
 	pq_sendint32(&msgbuf, (int32) MyProcPid);
 	pq_sendint32(&msgbuf, (int32) MyCancelKey);
 	pq_endmessage(&msgbuf);
@@ -1550,7 +1551,7 @@ ParallelWorkerMain(Datum main_arg)
 	DetachSession();
 
 	/* Report success. */
-	pq_putmessage('X', NULL, 0);
+	pq_putmessage(PQMSG_REQ_TERMINATE, NULL, 0);
 }
 
 /*
diff --git a/src/backend/backup/basebackup_copy.c b/src/backend/backup/basebackup_copy.c
index 1db80cde1b..44547bfa4d 100644
--- a/src/backend/backup/basebackup_copy.c
+++ b/src/backend/backup/basebackup_copy.c
@@ -32,6 +32,7 @@
 #include "executor/executor.h"
 #include "libpq/libpq.h"
 #include "libpq/pqformat.h"
+#include "protocol.h"
 #include "tcop/dest.h"
 #include "utils/builtins.h"
 #include "utils/timestamp.h"
@@ -169,7 +170,7 @@ bbsink_copystream_begin_archive(bbsink *sink, const char *archive_name)
 	StringInfoData buf;
 
 	ti = list_nth(state->tablespaces, state->tablespace_num);
-	pq_beginmessage(&buf, 'd'); /* CopyData */
+	pq_beginmessage(&buf, PQMSG_REQ_COPY_DATA);
 	pq_sendbyte(&buf, 'n');		/* New archive */
 	pq_sendstring(&buf, archive_name);
 	pq_sendstring(&buf, ti->path == NULL ? "" : ti->path);
@@ -220,8 +221,8 @@ bbsink_copystream_archive_contents(bbsink *sink, size_t len)
 		{
 			mysink->last_progress_report_time = now;
 
-			pq_beginmessage(&buf, 'd'); /* CopyData */
-			pq_sendbyte(&buf, 'p'); /* Progress report */
+			pq_beginmessage(&buf, PQMSG_REQ_COPY_DATA);
+			pq_sendbyte(&buf, PQMSG_REQ_COPY_PROGRESS);
 			pq_sendint64(&buf, state->bytes_done);
 			pq_endmessage(&buf);
 			pq_flush_if_writable();
@@ -246,8 +247,8 @@ bbsink_copystream_end_archive(bbsink *sink)
 
 	mysink->bytes_done_at_last_time_check = state->bytes_done;
 	mysink->last_progress_report_time = GetCurrentTimestamp();
-	pq_beginmessage(&buf, 'd'); /* CopyData */
-	pq_sendbyte(&buf, 'p');		/* Progress report */
+	pq_beginmessage(&buf, PQMSG_REQ_COPY_DATA);
+	pq_sendbyte(&buf, PQMSG_REQ_COPY_PROGRESS);
 	pq_sendint64(&buf, state->bytes_done);
 	pq_endmessage(&buf);
 	pq_flush_if_writable();
@@ -261,7 +262,7 @@ bbsink_copystream_begin_manifest(bbsink *sink)
 {
 	StringInfoData buf;
 
-	pq_beginmessage(&buf, 'd'); /* CopyData */
+	pq_beginmessage(&buf, PQMSG_REQ_COPY_DATA);
 	pq_sendbyte(&buf, 'm');		/* Manifest */
 	pq_endmessage(&buf);
 }
@@ -318,7 +319,7 @@ SendCopyOutResponse(void)
 {
 	StringInfoData buf;
 
-	pq_beginmessage(&buf, 'H');
+	pq_beginmessage(&buf, PQMSG_RESP_COPY_OUT);
 	pq_sendbyte(&buf, 0);		/* overall format */
 	pq_sendint16(&buf, 0);		/* natts */
 	pq_endmessage(&buf);
@@ -330,7 +331,7 @@ SendCopyOutResponse(void)
 static void
 SendCopyDone(void)
 {
-	pq_putemptymessage('c');
+	pq_putemptymessage(PQMSG_REQ_COPY_DONE);
 }
 
 /*
diff --git a/src/backend/commands/async.c b/src/backend/commands/async.c
index ef909cf4e0..e8de47bd41 100644
--- a/src/backend/commands/async.c
+++ b/src/backend/commands/async.c
@@ -141,6 +141,7 @@
 #include "libpq/libpq.h"
 #include "libpq/pqformat.h"
 #include "miscadmin.h"
+#include "protocol.h"
 #include "storage/ipc.h"
 #include "storage/lmgr.h"
 #include "storage/proc.h"
@@ -2281,7 +2282,7 @@ NotifyMyFrontEnd(const char *channel, const char *payload, int32 srcPid)
 	{
 		StringInfoData buf;
 
-		pq_beginmessage(&buf, 'A');
+		pq_beginmessage(&buf, PQMSG_RESP_NOTIFY);
 		pq_sendint32(&buf, srcPid);
 		pq_sendstring(&buf, channel);
 		pq_sendstring(&buf, payload);
diff --git a/src/backend/commands/copyfromparse.c b/src/backend/commands/copyfromparse.c
index 232768a6e1..7bae4a55c2 100644
--- a/src/backend/commands/copyfromparse.c
+++ b/src/backend/commands/copyfromparse.c
@@ -72,6 +72,7 @@
 #include "miscadmin.h"
 #include "pgstat.h"
 #include "port/pg_bswap.h"
+#include "protocol.h"
 #include "utils/builtins.h"
 #include "utils/memutils.h"
 #include "utils/rel.h"
@@ -174,7 +175,7 @@ ReceiveCopyBegin(CopyFromState cstate)
 	int16		format = (cstate->opts.binary ? 1 : 0);
 	int			i;
 
-	pq_beginmessage(&buf, 'G');
+	pq_beginmessage(&buf, PQMSG_RESP_COPY_IN);
 	pq_sendbyte(&buf, format);	/* overall format */
 	pq_sendint16(&buf, natts);
 	for (i = 0; i < natts; i++)
@@ -279,13 +280,13 @@ CopyGetData(CopyFromState cstate, void *databuf, int minread, int maxread)
 					/* Validate message type and set packet size limit */
 					switch (mtype)
 					{
-						case 'd':	/* CopyData */
+						case PQMSG_REQ_COPY_DATA:
 							maxmsglen = PQ_LARGE_MESSAGE_LIMIT;
 							break;
-						case 'c':	/* CopyDone */
-						case 'f':	/* CopyFail */
-						case 'H':	/* Flush */
-						case 'S':	/* Sync */
+						case PQMSG_REQ_COPY_DONE:
+						case PQMSG_REQ_COPY_FAIL:
+						case PQMSG_REQ_FLUSH_DATA:
+						case PQMSG_REQ_SYNC_DATA:
 							maxmsglen = PQ_SMALL_MESSAGE_LIMIT;
 							break;
 						default:
@@ -305,20 +306,20 @@ CopyGetData(CopyFromState cstate, void *databuf, int minread, int maxread)
 					/* ... and process it */
 					switch (mtype)
 					{
-						case 'd':	/* CopyData */
+						case PQMSG_REQ_COPY_DATA:
 							break;
-						case 'c':	/* CopyDone */
+						case PQMSG_REQ_COPY_DONE:
 							/* COPY IN correctly terminated by frontend */
 							cstate->raw_reached_eof = true;
 							return bytesread;
-						case 'f':	/* CopyFail */
+						case PQMSG_REQ_COPY_FAIL:
 							ereport(ERROR,
 									(errcode(ERRCODE_QUERY_CANCELED),
 									 errmsg("COPY from stdin failed: %s",
 											pq_getmsgstring(cstate->fe_msgbuf))));
 							break;
-						case 'H':	/* Flush */
-						case 'S':	/* Sync */
+						case PQMSG_REQ_FLUSH_DATA:
+						case PQMSG_REQ_SYNC_DATA:
 
 							/*
 							 * Ignore Flush/Sync for the convenience of client
diff --git a/src/backend/commands/copyto.c b/src/backend/commands/copyto.c
index 9e4b2437a5..f11b295787 100644
--- a/src/backend/commands/copyto.c
+++ b/src/backend/commands/copyto.c
@@ -34,6 +34,7 @@
 #include "miscadmin.h"
 #include "optimizer/optimizer.h"
 #include "pgstat.h"
+#include "protocol.h"
 #include "rewrite/rewriteHandler.h"
 #include "storage/fd.h"
 #include "tcop/tcopprot.h"
@@ -144,7 +145,7 @@ SendCopyBegin(CopyToState cstate)
 	int16		format = (cstate->opts.binary ? 1 : 0);
 	int			i;
 
-	pq_beginmessage(&buf, 'H');
+	pq_beginmessage(&buf, PQMSG_RESP_COPY_OUT);
 	pq_sendbyte(&buf, format);	/* overall format */
 	pq_sendint16(&buf, natts);
 	for (i = 0; i < natts; i++)
@@ -159,7 +160,7 @@ SendCopyEnd(CopyToState cstate)
 	/* Shouldn't have any unsent data */
 	Assert(cstate->fe_msgbuf->len == 0);
 	/* Send Copy Done message */
-	pq_putemptymessage('c');
+	pq_putemptymessage(PQMSG_REQ_COPY_DONE);
 }
 
 /*----------
diff --git a/src/backend/libpq/auth-sasl.c b/src/backend/libpq/auth-sasl.c
index 684680897b..6c2948a1dd 100644
--- a/src/backend/libpq/auth-sasl.c
+++ b/src/backend/libpq/auth-sasl.c
@@ -19,6 +19,7 @@
 #include "libpq/libpq.h"
 #include "libpq/pqformat.h"
 #include "libpq/sasl.h"
+#include "protocol.h"
 
 /*
  * Maximum accepted size of SASL messages.
@@ -87,7 +88,7 @@ CheckSASLAuth(const pg_be_sasl_mech *mech, Port *port, char *shadow_pass,
 	{
 		pq_startmsgread();
 		mtype = pq_getbyte();
-		if (mtype != 'p')
+		if (mtype != PQMSG_RESP_GSS)
 		{
 			/* Only log error if client didn't disconnect. */
 			if (mtype != EOF)
diff --git a/src/backend/libpq/auth.c b/src/backend/libpq/auth.c
index 315a24bb3f..61dfbb9a99 100644
--- a/src/backend/libpq/auth.c
+++ b/src/backend/libpq/auth.c
@@ -34,6 +34,7 @@
 #include "libpq/scram.h"
 #include "miscadmin.h"
 #include "port/pg_bswap.h"
+#include "protocol.h"
 #include "postmaster/postmaster.h"
 #include "replication/walsender.h"
 #include "storage/ipc.h"
@@ -665,7 +666,7 @@ sendAuthRequest(Port *port, AuthRequest areq, const char *extradata, int extrale
 
 	CHECK_FOR_INTERRUPTS();
 
-	pq_beginmessage(&buf, 'R');
+	pq_beginmessage(&buf, PQMSG_REQ_AUTHENTICATION);
 	pq_sendint32(&buf, (int32) areq);
 	if (extralen > 0)
 		pq_sendbytes(&buf, extradata, extralen);
@@ -698,7 +699,7 @@ recv_password_packet(Port *port)
 
 	/* Expect 'p' message type */
 	mtype = pq_getbyte();
-	if (mtype != 'p')
+	if (mtype != PQMSG_RESP_PASSWORD)
 	{
 		/*
 		 * If the client just disconnects without offering a password, don't
@@ -961,7 +962,7 @@ pg_GSS_recvauth(Port *port)
 		CHECK_FOR_INTERRUPTS();
 
 		mtype = pq_getbyte();
-		if (mtype != 'p')
+		if (mtype != PQMSG_RESP_GSS)
 		{
 			/* Only log error if client didn't disconnect. */
 			if (mtype != EOF)
@@ -1232,7 +1233,7 @@ pg_SSPI_recvauth(Port *port)
 	{
 		pq_startmsgread();
 		mtype = pq_getbyte();
-		if (mtype != 'p')
+		if (mtype != PQMSG_RESP_GSS)
 		{
 			if (sspictx != NULL)
 			{
diff --git a/src/backend/postmaster/postmaster.c b/src/backend/postmaster/postmaster.c
index 9c8ec779f9..52c15bebb7 100644
--- a/src/backend/postmaster/postmaster.c
+++ b/src/backend/postmaster/postmaster.c
@@ -114,6 +114,7 @@
 #include "postmaster/pgarch.h"
 #include "postmaster/postmaster.h"
 #include "postmaster/syslogger.h"
+#include "protocol.h"
 #include "replication/logicallauncher.h"
 #include "replication/walsender.h"
 #include "storage/fd.h"
@@ -2357,7 +2358,7 @@ SendNegotiateProtocolVersion(List *unrecognized_protocol_options)
 	StringInfoData buf;
 	ListCell   *lc;
 
-	pq_beginmessage(&buf, 'v'); /* NegotiateProtocolVersion */
+	pq_beginmessage(&buf, PQMSG_RESP_NEGOTIATE_PROTOCOL);
 	pq_sendint32(&buf, PG_PROTOCOL_LATEST);
 	pq_sendint32(&buf, list_length(unrecognized_protocol_options));
 	foreach(lc, unrecognized_protocol_options)
diff --git a/src/backend/replication/walsender.c b/src/backend/replication/walsender.c
index d27ef2985d..4bc2a1018c 100644
--- a/src/backend/replication/walsender.c
+++ b/src/backend/replication/walsender.c
@@ -69,6 +69,7 @@
 #include "nodes/replnodes.h"
 #include "pgstat.h"
 #include "postmaster/interrupt.h"
+#include "protocol.h"
 #include "replication/decode.h"
 #include "replication/logical.h"
 #include "replication/slot.h"
@@ -603,7 +604,7 @@ SendTimeLineHistory(TimeLineHistoryCmd *cmd)
 	dest->rStartup(dest, CMD_SELECT, tupdesc);
 
 	/* Send a DataRow message */
-	pq_beginmessage(&buf, 'D');
+	pq_beginmessage(&buf, PQMSG_RESP_DATA_ROW);
 	pq_sendint16(&buf, 2);		/* # of columns */
 	len = strlen(histfname);
 	pq_sendint32(&buf, len);	/* col1 len */
@@ -801,7 +802,7 @@ StartReplication(StartReplicationCmd *cmd)
 		WalSndSetState(WALSNDSTATE_CATCHUP);
 
 		/* Send a CopyBothResponse message, and start streaming */
-		pq_beginmessage(&buf, 'W');
+		pq_beginmessage(&buf, PQMSG_RESP_COPY_BOTH);
 		pq_sendbyte(&buf, 0);
 		pq_sendint16(&buf, 0);
 		pq_endmessage(&buf);
@@ -1294,7 +1295,7 @@ StartLogicalReplication(StartReplicationCmd *cmd)
 	WalSndSetState(WALSNDSTATE_CATCHUP);
 
 	/* Send a CopyBothResponse message, and start streaming */
-	pq_beginmessage(&buf, 'W');
+	pq_beginmessage(&buf, PQMSG_RESP_COPY_BOTH);
 	pq_sendbyte(&buf, 0);
 	pq_sendint16(&buf, 0);
 	pq_endmessage(&buf);
@@ -1923,11 +1924,11 @@ ProcessRepliesIfAny(void)
 		/* Validate message type and set packet size limit */
 		switch (firstchar)
 		{
-			case 'd':
+			case PQMSG_REQ_COPY_DATA:
 				maxmsglen = PQ_LARGE_MESSAGE_LIMIT;
 				break;
-			case 'c':
-			case 'X':
+			case PQMSG_REQ_COPY_DONE:
+			case PQMSG_REQ_TERMINATE:
 				maxmsglen = PQ_SMALL_MESSAGE_LIMIT;
 				break;
 			default:
@@ -1955,7 +1956,7 @@ ProcessRepliesIfAny(void)
 				/*
 				 * 'd' means a standby reply wrapped in a CopyData packet.
 				 */
-			case 'd':
+			case PQMSG_REQ_COPY_DATA:
 				ProcessStandbyMessage();
 				received = true;
 				break;
@@ -1964,7 +1965,7 @@ ProcessRepliesIfAny(void)
 				 * CopyDone means the standby requested to finish streaming.
 				 * Reply with CopyDone, if we had not sent that already.
 				 */
-			case 'c':
+			case PQMSG_REQ_COPY_DONE:
 				if (!streamingDoneSending)
 				{
 					pq_putmessage_noblock('c', NULL, 0);
@@ -1978,7 +1979,7 @@ ProcessRepliesIfAny(void)
 				/*
 				 * 'X' means that the standby is closing down the socket.
 				 */
-			case 'X':
+			case PQMSG_REQ_TERMINATE:
 				proc_exit(0);
 
 			default:
diff --git a/src/backend/tcop/dest.c b/src/backend/tcop/dest.c
index c0406e2ee5..a9bb80ac91 100644
--- a/src/backend/tcop/dest.c
+++ b/src/backend/tcop/dest.c
@@ -39,6 +39,7 @@
 #include "executor/tstoreReceiver.h"
 #include "libpq/libpq.h"
 #include "libpq/pqformat.h"
+#include "protocol.h"
 #include "utils/portal.h"
 
 
@@ -220,7 +221,7 @@ NullCommand(CommandDest dest)
 		case DestRemoteSimple:
 
 			/* Tell the FE that we saw an empty query string */
-			pq_putemptymessage('I');
+			pq_putemptymessage(PQMSG_RESP_EMPTY_QUERY);
 			break;
 
 		case DestNone:
@@ -258,7 +259,7 @@ ReadyForQuery(CommandDest dest)
 			{
 				StringInfoData buf;
 
-				pq_beginmessage(&buf, 'Z');
+				pq_beginmessage(&buf, PQMSG_RESP_READY_FOR_QUERY);
 				pq_sendbyte(&buf, TransactionBlockStatusCode());
 				pq_endmessage(&buf);
 			}
diff --git a/src/backend/tcop/fastpath.c b/src/backend/tcop/fastpath.c
index 2f70ebd5fa..384e53a651 100644
--- a/src/backend/tcop/fastpath.c
+++ b/src/backend/tcop/fastpath.c
@@ -27,6 +27,7 @@
 #include "mb/pg_wchar.h"
 #include "miscadmin.h"
 #include "port/pg_bswap.h"
+#include "protocol.h"
 #include "tcop/fastpath.h"
 #include "tcop/tcopprot.h"
 #include "utils/acl.h"
@@ -69,7 +70,7 @@ SendFunctionResult(Datum retval, bool isnull, Oid rettype, int16 format)
 {
 	StringInfoData buf;
 
-	pq_beginmessage(&buf, 'V');
+	pq_beginmessage(&buf, PQMSG_RESP_FUNCTION_CALL);
 
 	if (isnull)
 	{
diff --git a/src/backend/tcop/postgres.c b/src/backend/tcop/postgres.c
index 36cc99ec9c..dd8cce7a9a 100644
--- a/src/backend/tcop/postgres.c
+++ b/src/backend/tcop/postgres.c
@@ -55,6 +55,7 @@
 #include "postmaster/autovacuum.h"
 #include "postmaster/interrupt.h"
 #include "postmaster/postmaster.h"
+#include "protocol.h"
 #include "replication/logicallauncher.h"
 #include "replication/logicalworker.h"
 #include "replication/slot.h"
@@ -402,37 +403,37 @@ SocketBackend(StringInfo inBuf)
 	 */
 	switch (qtype)
 	{
-		case 'Q':				/* simple query */
+		case PQMSG_REQ_SIMPLE_QUERY:
 			maxmsglen = PQ_LARGE_MESSAGE_LIMIT;
 			doing_extended_query_message = false;
 			break;
 
-		case 'F':				/* fastpath function call */
+		case PQMSG_REQ_FUNCTION_CALL:
 			maxmsglen = PQ_LARGE_MESSAGE_LIMIT;
 			doing_extended_query_message = false;
 			break;
 
-		case 'X':				/* terminate */
+		case PQMSG_REQ_TERMINATE:
 			maxmsglen = PQ_SMALL_MESSAGE_LIMIT;
 			doing_extended_query_message = false;
 			ignore_till_sync = false;
 			break;
 
-		case 'B':				/* bind */
-		case 'P':				/* parse */
+		case PQMSG_REQ_BIND :
+		case PQMSG_REQ_PARSE:
 			maxmsglen = PQ_LARGE_MESSAGE_LIMIT;
 			doing_extended_query_message = true;
 			break;
 
-		case 'C':				/* close */
-		case 'D':				/* describe */
-		case 'E':				/* execute */
-		case 'H':				/* flush */
+		case PQMSG_REQ_CLOSE:
+		case PQMSG_REQ_DESCRIBE:
+		case PQMSG_REQ_EXECUTE:
+		case PQMSG_REQ_FLUSH_DATA:
 			maxmsglen = PQ_SMALL_MESSAGE_LIMIT;
 			doing_extended_query_message = true;
 			break;
 
-		case 'S':				/* sync */
+		case PQMSG_REQ_SYNC_DATA:
 			maxmsglen = PQ_SMALL_MESSAGE_LIMIT;
 			/* stop any active skip-till-Sync */
 			ignore_till_sync = false;
@@ -440,13 +441,13 @@ SocketBackend(StringInfo inBuf)
 			doing_extended_query_message = false;
 			break;
 
-		case 'd':				/* copy data */
+		case PQMSG_REQ_COPY_DATA:
 			maxmsglen = PQ_LARGE_MESSAGE_LIMIT;
 			doing_extended_query_message = false;
 			break;
 
-		case 'c':				/* copy done */
-		case 'f':				/* copy fail */
+		case PQMSG_REQ_COPY_DONE:
+		case PQMSG_REQ_COPY_FAIL:
 			maxmsglen = PQ_SMALL_MESSAGE_LIMIT;
 			doing_extended_query_message = false;
 			break;
@@ -1589,7 +1590,7 @@ exec_parse_message(const char *query_string,	/* string to execute */
 	 * Send ParseComplete.
 	 */
 	if (whereToSendOutput == DestRemote)
-		pq_putemptymessage('1');
+		pq_putemptymessage(PQMSG_RESP_PARSE_COMPLETE);
 
 	/*
 	 * Emit duration logging if appropriate.
@@ -2047,7 +2048,7 @@ exec_bind_message(StringInfo input_message)
 	 * Send BindComplete.
 	 */
 	if (whereToSendOutput == DestRemote)
-		pq_putemptymessage('2');
+		pq_putemptymessage(PQMSG_RESP_BIND_COMPLETE);
 
 	/*
 	 * Emit duration logging if appropriate.
@@ -2290,7 +2291,7 @@ exec_execute_message(const char *portal_name, long max_rows)
 	{
 		/* Portal run not complete, so send PortalSuspended */
 		if (whereToSendOutput == DestRemote)
-			pq_putemptymessage('s');
+			pq_putemptymessage(PQMSG_RESP_PORTAL_SUSPENDED);
 
 		/*
 		 * Set XACT_FLAGS_PIPELINING whenever we suspend an Execute message,
@@ -2683,7 +2684,7 @@ exec_describe_statement_message(const char *stmt_name)
 								  NULL);
 	}
 	else
-		pq_putemptymessage('n');	/* NoData */
+		pq_putemptymessage(PQMSG_RESP_NO_DATA);
 }
 
 /*
@@ -2736,7 +2737,7 @@ exec_describe_portal_message(const char *portal_name)
 								  FetchPortalTargetList(portal),
 								  portal->formats);
 	else
-		pq_putemptymessage('n');	/* NoData */
+		pq_putemptymessage(PQMSG_RESP_NO_DATA);
 }
 
 
@@ -4239,7 +4240,7 @@ PostgresMain(const char *dbname, const char *username)
 	{
 		StringInfoData buf;
 
-		pq_beginmessage(&buf, 'K');
+		pq_beginmessage(&buf, PQMSG_REQ_BACKEND_KEY_DATA);
 		pq_sendint32(&buf, (int32) MyProcPid);
 		pq_sendint32(&buf, (int32) MyCancelKey);
 		pq_endmessage(&buf);
@@ -4618,7 +4619,7 @@ PostgresMain(const char *dbname, const char *username)
 
 		switch (firstchar)
 		{
-			case 'Q':			/* simple query */
+			case PQMSG_REQ_SIMPLE_QUERY:
 				{
 					const char *query_string;
 
@@ -4642,7 +4643,7 @@ PostgresMain(const char *dbname, const char *username)
 				}
 				break;
 
-			case 'P':			/* parse */
+			case PQMSG_REQ_PARSE:
 				{
 					const char *stmt_name;
 					const char *query_string;
@@ -4672,7 +4673,7 @@ PostgresMain(const char *dbname, const char *username)
 				}
 				break;
 
-			case 'B':			/* bind */
+			case PQMSG_REQ_BIND:
 				forbidden_in_wal_sender(firstchar);
 
 				/* Set statement_timestamp() */
@@ -4687,7 +4688,7 @@ PostgresMain(const char *dbname, const char *username)
 				/* exec_bind_message does valgrind_report_error_query */
 				break;
 
-			case 'E':			/* execute */
+			case PQMSG_REQ_EXECUTE:
 				{
 					const char *portal_name;
 					int			max_rows;
@@ -4707,7 +4708,7 @@ PostgresMain(const char *dbname, const char *username)
 				}
 				break;
 
-			case 'F':			/* fastpath function call */
+			case PQMSG_REQ_FUNCTION_CALL:
 				forbidden_in_wal_sender(firstchar);
 
 				/* Set statement_timestamp() */
@@ -4742,7 +4743,7 @@ PostgresMain(const char *dbname, const char *username)
 				send_ready_for_query = true;
 				break;
 
-			case 'C':			/* close */
+			case PQMSG_REQ_CLOSE:
 				{
 					int			close_type;
 					const char *close_target;
@@ -4755,7 +4756,7 @@ PostgresMain(const char *dbname, const char *username)
 
 					switch (close_type)
 					{
-						case 'S':
+						case PREPARED_SUB_COMMAND:
 							if (close_target[0] != '\0')
 								DropPreparedStatement(close_target, false);
 							else
@@ -4764,7 +4765,7 @@ PostgresMain(const char *dbname, const char *username)
 								drop_unnamed_stmt();
 							}
 							break;
-						case 'P':
+						case PORTAL_SUB_COMMAND:
 							{
 								Portal		portal;
 
@@ -4782,13 +4783,13 @@ PostgresMain(const char *dbname, const char *username)
 					}
 
 					if (whereToSendOutput == DestRemote)
-						pq_putemptymessage('3');	/* CloseComplete */
+						pq_putemptymessage(PQMSG_RESP_CLOSE_COMPLETE);
 
 					valgrind_report_error_query("CLOSE message");
 				}
 				break;
 
-			case 'D':			/* describe */
+			case PQMSG_REQ_DESCRIBE:
 				{
 					int			describe_type;
 					const char *describe_target;
@@ -4804,10 +4805,10 @@ PostgresMain(const char *dbname, const char *username)
 
 					switch (describe_type)
 					{
-						case 'S':
+						case PREPARED_SUB_COMMAND:
 							exec_describe_statement_message(describe_target);
 							break;
-						case 'P':
+						case PORTAL_SUB_COMMAND:
 							exec_describe_portal_message(describe_target);
 							break;
 						default:
@@ -4822,13 +4823,13 @@ PostgresMain(const char *dbname, const char *username)
 				}
 				break;
 
-			case 'H':			/* flush */
+			case PQMSG_REQ_FLUSH_DATA:
 				pq_getmsgend(&input_message);
 				if (whereToSendOutput == DestRemote)
 					pq_flush();
 				break;
 
-			case 'S':			/* sync */
+			case PQMSG_REQ_SYNC_DATA:
 				pq_getmsgend(&input_message);
 				finish_xact_command();
 				valgrind_report_error_query("SYNC message");
@@ -4847,7 +4848,7 @@ PostgresMain(const char *dbname, const char *username)
 
 				/* FALLTHROUGH */
 
-			case 'X':
+			case PQMSG_REQ_TERMINATE:
 
 				/*
 				 * Reset whereToSendOutput to prevent ereport from attempting
@@ -4865,9 +4866,9 @@ PostgresMain(const char *dbname, const char *username)
 				 */
 				proc_exit(0);
 
-			case 'd':			/* copy data */
-			case 'c':			/* copy done */
-			case 'f':			/* copy fail */
+			case PQMSG_REQ_COPY_DATA:
+			case PQMSG_REQ_COPY_DONE:
+			case PQMSG_REQ_COPY_FAIL:
 
 				/*
 				 * Accept but ignore these messages, per protocol spec; we
@@ -4897,7 +4898,7 @@ forbidden_in_wal_sender(char firstchar)
 {
 	if (am_walsender)
 	{
-		if (firstchar == 'F')
+		if (firstchar == PQMSG_REQ_FUNCTION_CALL)
 			ereport(ERROR,
 					(errcode(ERRCODE_PROTOCOL_VIOLATION),
 					 errmsg("fastpath function calls not supported in a replication connection")));
diff --git a/src/backend/utils/activity/backend_progress.c b/src/backend/utils/activity/backend_progress.c
index 67447ef03a..6200433594 100644
--- a/src/backend/utils/activity/backend_progress.c
+++ b/src/backend/utils/activity/backend_progress.c
@@ -10,6 +10,7 @@
  */
 #include "postgres.h"
 
+#include "protocol.h"
 #include "access/parallel.h"
 #include "libpq/pqformat.h"
 #include "port/atomics.h"		/* for memory barriers */
@@ -102,7 +103,7 @@ pgstat_progress_parallel_incr_param(int index, int64 incr)
 
 		initStringInfo(&progress_message);
 
-		pq_beginmessage(&progress_message, 'P');
+		pq_beginmessage(&progress_message, PQMSG_RESP_PARALLEL_PROGRESS);
 		pq_sendint32(&progress_message, index);
 		pq_sendint64(&progress_message, incr);
 		pq_endmessage(&progress_message);
diff --git a/src/backend/utils/error/elog.c b/src/backend/utils/error/elog.c
index 5898100acb..2e94ef096a 100644
--- a/src/backend/utils/error/elog.c
+++ b/src/backend/utils/error/elog.c
@@ -77,6 +77,7 @@
 #include "postmaster/bgworker.h"
 #include "postmaster/postmaster.h"
 #include "postmaster/syslogger.h"
+#include "protocol.h"
 #include "storage/ipc.h"
 #include "storage/proc.h"
 #include "tcop/tcopprot.h"
@@ -3465,7 +3466,7 @@ send_message_to_frontend(ErrorData *edata)
 		char		tbuf[12];
 
 		/* 'N' (Notice) is for nonfatal conditions, 'E' is for errors */
-		pq_beginmessage(&msgbuf, (edata->elevel < ERROR) ? 'N' : 'E');
+		pq_beginmessage(&msgbuf, (edata->elevel < ERROR) ? PQMSG_RESP_NOTICE : PQMSG_RESP_ERROR);
 
 		sev = error_severity(edata->elevel);
 		pq_sendbyte(&msgbuf, PG_DIAG_SEVERITY);
diff --git a/src/backend/utils/misc/guc.c b/src/backend/utils/misc/guc.c
index 5308896c87..1b9632942e 100644
--- a/src/backend/utils/misc/guc.c
+++ b/src/backend/utils/misc/guc.c
@@ -37,6 +37,7 @@
 #include "libpq/pqformat.h"
 #include "parser/scansup.h"
 #include "port/pg_bitutils.h"
+#include "protocol.h"
 #include "storage/fd.h"
 #include "storage/lwlock.h"
 #include "storage/shmem.h"
@@ -2593,7 +2594,7 @@ ReportGUCOption(struct config_generic *record)
 	{
 		StringInfoData msgbuf;
 
-		pq_beginmessage(&msgbuf, 'S');
+		pq_beginmessage(&msgbuf, PQMSG_RESP_PARAMETER_STATUS);
 		pq_sendstring(&msgbuf, record->name);
 		pq_sendstring(&msgbuf, val);
 		pq_endmessage(&msgbuf);
diff --git a/src/include/protocol.h b/src/include/protocol.h
new file mode 100644
index 0000000000..0540b36264
--- /dev/null
+++ b/src/include/protocol.h
@@ -0,0 +1,61 @@
+/*-------------------------------------------------------------------------
+ *
+ * protocol.h
+ *	  Exports from postmaster/postmaster.c.
+ *
+ * Portions Copyright (c) 1996-2023, PostgreSQL Global Development Group
+ *
+ * src/include/protocol.h
+ *
+ *-------------------------------------------------------------------------
+ */
+#ifndef _PROTOCOL_H
+#define _PROTOCOL_H
+
+#define PQMSG_REQ_BIND              'B'
+#define PQMSG_REQ_CLOSE             'C'
+#define PQMSG_REQ_DESCRIBE          'D'
+#define PQMSG_REQ_EXECUTE           'E'
+#define PQMSG_REQ_FUNCTION_CALL     'F'
+#define PQMSG_REQ_FLUSH_DATA        'H'
+#define PQMSG_REQ_BACKEND_KEY_DATA  'K'
+#define PQMSG_REQ_PARSE             'P'
+#define PQMSG_REQ_AUTHENTICATION    'R'
+#define PQMSG_REQ_SYNC_DATA         'S'
+#define PQMSG_REQ_SIMPLE_QUERY      'Q'
+#define PQMSG_REQ_TERMINATE         'X'
+#define PQMSG_REQ_COPY_FAIL         'f'
+#define PQMSG_REQ_COPY_DONE         'c'
+#define PQMSG_REQ_COPY_DATA         'd'
+#define PQMSG_REQ_COPY_PROGRESS     'p'
+#define PREPARED_SUB_COMMAND        'S'
+#define PORTAL_SUB_COMMAND          'P'
+
+
+/*
+Responses
+*/
+#define PQMSG_RESP_PARSE_COMPLETE   '1'
+#define PQMSG_RESP_BIND_COMPLETE    '2'
+#define PQMSG_RESP_CLOSE_COMPLETE   '3'
+#define PQMSG_RESP_NOTIFY           'A'
+#define PQMSG_RESP_COMMAND_COMPLETE 'C'
+#define PQMSG_RESP_DATA_ROW         'D'
+#define PQMSG_RESP_ERROR            'E'
+#define PQMSG_RESP_COPY_IN          'G'
+#define PQMSG_RESP_COPY_OUT         'H'
+#define PQMSG_RESP_EMPTY_QUERY      'I'
+#define PQMSG_RESP_NOTICE           'N'
+#define PQMSG_RESP_PARALLEL_PROGRESS 'P'
+#define PQMSG_RESP_FUNCTION_CALL    'V'
+#define PQMSG_RESP_PARAMETER_STATUS 'S'
+#define PQMSG_RESP_ROW_DESCRIPTION  'T'
+#define PQMSG_RESP_COPY_BOTH        'W'
+#define PQMSG_RESP_READY_FOR_QUERY  'Z'
+#define PQMSG_RESP_NO_DATA          'n'
+#define PQMSG_RESP_PASSWORD         'p'
+#define PQMSG_RESP_GSS              'p'
+#define PQMSG_RESP_PORTAL_SUSPENDED 's'
+#define PQMSG_RESP_PARAMETER_DESCRIPTION 't'
+#define PQMSG_RESP_NEGOTIATE_PROTOCOL    'v'
+#endif
\ No newline at end of file
diff --git a/src/interfaces/libpq/fe-auth.c b/src/interfaces/libpq/fe-auth.c
index 887ca5e9e1..1980e745dd 100644
--- a/src/interfaces/libpq/fe-auth.c
+++ b/src/interfaces/libpq/fe-auth.c
@@ -43,6 +43,7 @@
 #include "fe-auth.h"
 #include "fe-auth-sasl.h"
 #include "libpq-fe.h"
+#include "protocol.h"
 
 #ifdef ENABLE_GSS
 /*
@@ -586,7 +587,7 @@ pg_SASL_init(PGconn *conn, int payloadlen)
 	/*
 	 * Build a SASLInitialResponse message, and send it.
 	 */
-	if (pqPutMsgStart('p', conn))
+	if (pqPutMsgStart(PQMSG_RESP_GSS, conn))
 		goto error;
 	if (pqPuts(selected_mechanism, conn))
 		goto error;
diff --git a/src/interfaces/libpq/fe-connect.c b/src/interfaces/libpq/fe-connect.c
index 837c5321aa..18f27b3cd0 100644
--- a/src/interfaces/libpq/fe-connect.c
+++ b/src/interfaces/libpq/fe-connect.c
@@ -32,6 +32,7 @@
 #include "mb/pg_wchar.h"
 #include "pg_config_paths.h"
 #include "port/pg_bswap.h"
+#include "protocol.h"
 
 #ifdef WIN32
 #include "win32.h"
@@ -3591,7 +3592,7 @@ keep_going:						/* We will come back to here until there is
 				 * Anything else probably means it's not Postgres on the other
 				 * end at all.
 				 */
-				if (!(beresp == 'R' || beresp == 'v' || beresp == 'E'))
+				if (!(beresp == PQMSG_REQ_AUTHENTICATION || beresp == PQMSG_RESP_NEGOTIATE_PROTOCOL || beresp == PQMSG_RESP_ERROR))
 				{
 					libpq_append_conn_error(conn, "expected authentication request from server, but received %c",
 											beresp);
@@ -3618,19 +3619,19 @@ keep_going:						/* We will come back to here until there is
 				 * version 14, the server also used the old protocol for
 				 * errors that happened before processing the startup packet.)
 				 */
-				if (beresp == 'R' && (msgLength < 8 || msgLength > 2000))
+				if (beresp == PQMSG_REQ_AUTHENTICATION && (msgLength < 8 || msgLength > 2000))
 				{
 					libpq_append_conn_error(conn, "received invalid authentication request");
 					goto error_return;
 				}
-				if (beresp == 'v' && (msgLength < 8 || msgLength > 2000))
+				if (beresp == PQMSG_RESP_NEGOTIATE_PROTOCOL && (msgLength < 8 || msgLength > 2000))
 				{
 					libpq_append_conn_error(conn, "received invalid protocol negotiation message");
 					goto error_return;
 				}
 
 #define MAX_ERRLEN 30000
-				if (beresp == 'E' && (msgLength < 8 || msgLength > MAX_ERRLEN))
+				if (beresp == PQMSG_RESP_ERROR && (msgLength < 8 || msgLength > MAX_ERRLEN))
 				{
 					/* Handle error from a pre-3.0 server */
 					conn->inCursor = conn->inStart + 1; /* reread data */
@@ -3693,7 +3694,7 @@ keep_going:						/* We will come back to here until there is
 				}
 
 				/* Handle errors. */
-				if (beresp == 'E')
+				if (beresp == PQMSG_RESP_ERROR)
 				{
 					if (pqGetErrorNotice3(conn, true))
 					{
@@ -3770,7 +3771,7 @@ keep_going:						/* We will come back to here until there is
 
 					goto error_return;
 				}
-				else if (beresp == 'v')
+				else if (beresp == PQMSG_RESP_NEGOTIATE_PROTOCOL)
 				{
 					if (pqGetNegotiateProtocolVersion3(conn))
 					{
@@ -4540,7 +4541,7 @@ sendTerminateConn(PGconn *conn)
 		 * Try to send "close connection" message to backend. Ignore any
 		 * error.
 		 */
-		pqPutMsgStart('X', conn);
+		pqPutMsgStart(PQMSG_REQ_TERMINATE, conn);
 		pqPutMsgEnd(conn);
 		(void) pqFlush(conn);
 	}
diff --git a/src/interfaces/libpq/fe-exec.c b/src/interfaces/libpq/fe-exec.c
index a868284ff8..e105132173 100644
--- a/src/interfaces/libpq/fe-exec.c
+++ b/src/interfaces/libpq/fe-exec.c
@@ -27,6 +27,7 @@
 #include "libpq-fe.h"
 #include "libpq-int.h"
 #include "mb/pg_wchar.h"
+#include "protocol.h"
 
 /* keep this in same order as ExecStatusType in libpq-fe.h */
 char	   *const pgresStatus[] = {
@@ -1458,7 +1459,7 @@ PQsendQueryInternal(PGconn *conn, const char *query, bool newQuery)
 
 	/* Send the query message(s) */
 	/* construct the outgoing Query message */
-	if (pqPutMsgStart('Q', conn) < 0 ||
+	if (pqPutMsgStart(PQMSG_REQ_SIMPLE_QUERY, conn) < 0 ||
 		pqPuts(query, conn) < 0 ||
 		pqPutMsgEnd(conn) < 0)
 	{
@@ -1571,7 +1572,7 @@ PQsendPrepare(PGconn *conn,
 		return 0;				/* error msg already set */
 
 	/* construct the Parse message */
-	if (pqPutMsgStart('P', conn) < 0 ||
+	if (pqPutMsgStart(PQMSG_REQ_PARSE, conn) < 0 ||
 		pqPuts(stmtName, conn) < 0 ||
 		pqPuts(query, conn) < 0)
 		goto sendFailed;
@@ -1599,7 +1600,7 @@ PQsendPrepare(PGconn *conn,
 	/* Add a Sync, unless in pipeline mode. */
 	if (conn->pipelineStatus == PQ_PIPELINE_OFF)
 	{
-		if (pqPutMsgStart('S', conn) < 0 ||
+		if (pqPutMsgStart(PQMSG_REQ_SYNC_DATA, conn) < 0 ||
 			pqPutMsgEnd(conn) < 0)
 			goto sendFailed;
 	}
@@ -1784,7 +1785,7 @@ PQsendQueryGuts(PGconn *conn,
 	if (command)
 	{
 		/* construct the Parse message */
-		if (pqPutMsgStart('P', conn) < 0 ||
+		if (pqPutMsgStart(PQMSG_REQ_PARSE, conn) < 0 ||
 			pqPuts(stmtName, conn) < 0 ||
 			pqPuts(command, conn) < 0)
 			goto sendFailed;
@@ -1808,7 +1809,7 @@ PQsendQueryGuts(PGconn *conn,
 	}
 
 	/* Construct the Bind message */
-	if (pqPutMsgStart('B', conn) < 0 ||
+	if (pqPutMsgStart(PQMSG_REQ_BIND, conn) < 0 ||
 		pqPuts("", conn) < 0 ||
 		pqPuts(stmtName, conn) < 0)
 		goto sendFailed;
@@ -1874,14 +1875,14 @@ PQsendQueryGuts(PGconn *conn,
 		goto sendFailed;
 
 	/* construct the Describe Portal message */
-	if (pqPutMsgStart('D', conn) < 0 ||
-		pqPutc('P', conn) < 0 ||
+	if (pqPutMsgStart(PQMSG_REQ_DESCRIBE, conn) < 0 ||
+		pqPutc(PORTAL_SUB_COMMAND, conn) < 0 ||
 		pqPuts("", conn) < 0 ||
 		pqPutMsgEnd(conn) < 0)
 		goto sendFailed;
 
 	/* construct the Execute message */
-	if (pqPutMsgStart('E', conn) < 0 ||
+	if (pqPutMsgStart(PQMSG_REQ_EXECUTE, conn) < 0 ||
 		pqPuts("", conn) < 0 ||
 		pqPutInt(0, 4, conn) < 0 ||
 		pqPutMsgEnd(conn) < 0)
@@ -1890,7 +1891,7 @@ PQsendQueryGuts(PGconn *conn,
 	/* construct the Sync message if not in pipeline mode */
 	if (conn->pipelineStatus == PQ_PIPELINE_OFF)
 	{
-		if (pqPutMsgStart('S', conn) < 0 ||
+		if (pqPutMsgStart(PQMSG_REQ_SYNC_DATA, conn) < 0 ||
 			pqPutMsgEnd(conn) < 0)
 			goto sendFailed;
 	}
@@ -2422,7 +2423,7 @@ PQdescribePrepared(PGconn *conn, const char *stmt)
 {
 	if (!PQexecStart(conn))
 		return NULL;
-	if (!PQsendTypedCommand(conn, 'D', 'S', stmt))
+	if (!PQsendTypedCommand(conn, PQMSG_REQ_DESCRIBE, PREPARED_SUB_COMMAND, stmt))
 		return NULL;
 	return PQexecFinish(conn);
 }
@@ -2441,7 +2442,7 @@ PQdescribePortal(PGconn *conn, const char *portal)
 {
 	if (!PQexecStart(conn))
 		return NULL;
-	if (!PQsendTypedCommand(conn, 'D', 'P', portal))
+	if (!PQsendTypedCommand(conn, PQMSG_REQ_DESCRIBE, PORTAL_SUB_COMMAND, portal))
 		return NULL;
 	return PQexecFinish(conn);
 }
@@ -2456,7 +2457,7 @@ PQdescribePortal(PGconn *conn, const char *portal)
 int
 PQsendDescribePrepared(PGconn *conn, const char *stmt)
 {
-	return PQsendTypedCommand(conn, 'D', 'S', stmt);
+	return PQsendTypedCommand(conn, PQMSG_REQ_DESCRIBE, PREPARED_SUB_COMMAND, stmt);
 }
 
 /*
@@ -2469,7 +2470,7 @@ PQsendDescribePrepared(PGconn *conn, const char *stmt)
 int
 PQsendDescribePortal(PGconn *conn, const char *portal)
 {
-	return PQsendTypedCommand(conn, 'D', 'P', portal);
+	return PQsendTypedCommand(conn, PQMSG_REQ_DESCRIBE, PORTAL_SUB_COMMAND, portal);
 }
 
 /*
@@ -2488,7 +2489,7 @@ PQclosePrepared(PGconn *conn, const char *stmt)
 {
 	if (!PQexecStart(conn))
 		return NULL;
-	if (!PQsendTypedCommand(conn, 'C', 'S', stmt))
+	if (!PQsendTypedCommand(conn, PQMSG_REQ_CLOSE, PREPARED_SUB_COMMAND, stmt))
 		return NULL;
 	return PQexecFinish(conn);
 }
@@ -2506,7 +2507,7 @@ PQclosePortal(PGconn *conn, const char *portal)
 {
 	if (!PQexecStart(conn))
 		return NULL;
-	if (!PQsendTypedCommand(conn, 'C', 'P', portal))
+	if (!PQsendTypedCommand(conn, PQMSG_REQ_CLOSE, PORTAL_SUB_COMMAND, portal))
 		return NULL;
 	return PQexecFinish(conn);
 }
@@ -2521,7 +2522,7 @@ PQclosePortal(PGconn *conn, const char *portal)
 int
 PQsendClosePrepared(PGconn *conn, const char *stmt)
 {
-	return PQsendTypedCommand(conn, 'C', 'S', stmt);
+	return PQsendTypedCommand(conn, PQMSG_REQ_CLOSE, PREPARED_SUB_COMMAND, stmt);
 }
 
 /*
@@ -2534,7 +2535,7 @@ PQsendClosePrepared(PGconn *conn, const char *stmt)
 int
 PQsendClosePortal(PGconn *conn, const char *portal)
 {
-	return PQsendTypedCommand(conn, 'C', 'P', portal);
+	return PQsendTypedCommand(conn, PQMSG_REQ_CLOSE, PORTAL_SUB_COMMAND, portal);
 }
 
 /*
@@ -2577,17 +2578,17 @@ PQsendTypedCommand(PGconn *conn, char command, char type, const char *target)
 	/* construct the Sync message */
 	if (conn->pipelineStatus == PQ_PIPELINE_OFF)
 	{
-		if (pqPutMsgStart('S', conn) < 0 ||
+		if (pqPutMsgStart(PQMSG_REQ_SYNC_DATA, conn) < 0 ||
 			pqPutMsgEnd(conn) < 0)
 			goto sendFailed;
 	}
 
 	/* remember if we are doing a Close or a Describe */
-	if (command == 'C')
+	if (command == PQMSG_REQ_CLOSE)
 	{
 		entry->queryclass = PGQUERY_CLOSE;
 	}
-	else if (command == 'D')
+	else if (command == PQMSG_REQ_DESCRIBE)
 	{
 		entry->queryclass = PGQUERY_DESCRIBE;
 	}
@@ -2696,7 +2697,7 @@ PQputCopyData(PGconn *conn, const char *buffer, int nbytes)
 				return pqIsnonblocking(conn) ? 0 : -1;
 		}
 		/* Send the data (too simple to delegate to fe-protocol files) */
-		if (pqPutMsgStart('d', conn) < 0 ||
+		if (pqPutMsgStart(PQMSG_REQ_COPY_DATA, conn) < 0 ||
 			pqPutnchar(buffer, nbytes, conn) < 0 ||
 			pqPutMsgEnd(conn) < 0)
 			return -1;
@@ -2731,7 +2732,7 @@ PQputCopyEnd(PGconn *conn, const char *errormsg)
 	if (errormsg)
 	{
 		/* Send COPY FAIL */
-		if (pqPutMsgStart('f', conn) < 0 ||
+		if (pqPutMsgStart(PQMSG_REQ_COPY_FAIL, conn) < 0 ||
 			pqPuts(errormsg, conn) < 0 ||
 			pqPutMsgEnd(conn) < 0)
 			return -1;
@@ -2739,7 +2740,7 @@ PQputCopyEnd(PGconn *conn, const char *errormsg)
 	else
 	{
 		/* Send COPY DONE */
-		if (pqPutMsgStart('c', conn) < 0 ||
+		if (pqPutMsgStart(PQMSG_REQ_COPY_DONE, conn) < 0 ||
 			pqPutMsgEnd(conn) < 0)
 			return -1;
 	}
@@ -2751,7 +2752,7 @@ PQputCopyEnd(PGconn *conn, const char *errormsg)
 	if (conn->cmd_queue_head &&
 		conn->cmd_queue_head->queryclass != PGQUERY_SIMPLE)
 	{
-		if (pqPutMsgStart('S', conn) < 0 ||
+		if (pqPutMsgStart(PQMSG_REQ_SYNC_DATA, conn) < 0 ||
 			pqPutMsgEnd(conn) < 0)
 			return -1;
 	}
@@ -3263,7 +3264,7 @@ PQpipelineSync(PGconn *conn)
 	entry->query = NULL;
 
 	/* construct the Sync message */
-	if (pqPutMsgStart('S', conn) < 0 ||
+	if (pqPutMsgStart(PQMSG_REQ_SYNC_DATA, conn) < 0 ||
 		pqPutMsgEnd(conn) < 0)
 		goto sendFailed;
 
@@ -3311,7 +3312,7 @@ PQsendFlushRequest(PGconn *conn)
 		return 0;
 	}
 
-	if (pqPutMsgStart('H', conn) < 0 ||
+	if (pqPutMsgStart(PQMSG_REQ_FLUSH_DATA, conn) < 0 ||
 		pqPutMsgEnd(conn) < 0)
 	{
 		return 0;
diff --git a/src/interfaces/libpq/fe-protocol3.c b/src/interfaces/libpq/fe-protocol3.c
index 7bc6355d17..8de12f4aca 100644
--- a/src/interfaces/libpq/fe-protocol3.c
+++ b/src/interfaces/libpq/fe-protocol3.c
@@ -28,14 +28,16 @@
 #include "libpq-int.h"
 #include "mb/pg_wchar.h"
 #include "port/pg_bswap.h"
+#include "protocol.h"
 
 /*
  * This macro lists the backend message types that could be "long" (more
  * than a couple of kilobytes).
  */
 #define VALID_LONG_MESSAGE_TYPE(id) \
-	((id) == 'T' || (id) == 'D' || (id) == 'd' || (id) == 'V' || \
-	 (id) == 'E' || (id) == 'N' || (id) == 'A')
+	((id) == PQMSG_RESP_ROW_DESCRIPTION || (id) == PQMSG_RESP_DATA_ROW || (id) == PQMSG_REQ_COPY_DATA || \
+	 (id) == PQMSG_RESP_FUNCTION_CALL ||  (id) == PQMSG_RESP_ERROR || (id) == PQMSG_RESP_NOTICE || \
+	 (id) == PQMSG_RESP_NOTIFY)
 
 
 static void handleSyncLoss(PGconn *conn, char id, int msgLength);
@@ -140,12 +142,12 @@ pqParseInput3(PGconn *conn)
 		 * from config file due to SIGHUP), but otherwise we hold off until
 		 * BUSY state.
 		 */
-		if (id == 'A')
+		if (id == PQMSG_RESP_NOTIFY)
 		{
 			if (getNotify(conn))
 				return;
 		}
-		else if (id == 'N')
+		else if (id == PQMSG_RESP_NOTICE)
 		{
 			if (pqGetErrorNotice3(conn, false))
 				return;
@@ -165,12 +167,12 @@ pqParseInput3(PGconn *conn)
 			 * it is about to close the connection, so we don't want to just
 			 * discard it...)
 			 */
-			if (id == 'E')
+			if (id == PQMSG_RESP_ERROR)
 			{
 				if (pqGetErrorNotice3(conn, false /* treat as notice */ ))
 					return;
 			}
-			else if (id == 'S')
+			else if (id == PQMSG_RESP_PARAMETER_STATUS)
 			{
 				if (getParameterStatus(conn))
 					return;
@@ -192,7 +194,7 @@ pqParseInput3(PGconn *conn)
 			 */
 			switch (id)
 			{
-				case 'C':		/* command complete */
+				case PQMSG_RESP_COMMAND_COMPLETE:
 					if (pqGets(&conn->workBuffer, conn))
 						return;
 					if (!pgHavePendingResult(conn))
@@ -210,12 +212,12 @@ pqParseInput3(PGconn *conn)
 								CMDSTATUS_LEN);
 					conn->asyncStatus = PGASYNC_READY;
 					break;
-				case 'E':		/* error return */
+				case PQMSG_RESP_ERROR:
 					if (pqGetErrorNotice3(conn, true))
 						return;
 					conn->asyncStatus = PGASYNC_READY;
 					break;
-				case 'Z':		/* sync response, backend is ready for new
+				case PQMSG_RESP_READY_FOR_QUERY:		/* sync response, backend is ready for new
 								 * query */
 					if (getReadyForQuery(conn))
 						return;
@@ -246,7 +248,7 @@ pqParseInput3(PGconn *conn)
 						conn->asyncStatus = PGASYNC_IDLE;
 					}
 					break;
-				case 'I':		/* empty query */
+				case PQMSG_RESP_EMPTY_QUERY:
 					if (!pgHavePendingResult(conn))
 					{
 						conn->result = PQmakeEmptyPGresult(conn,
@@ -259,7 +261,7 @@ pqParseInput3(PGconn *conn)
 					}
 					conn->asyncStatus = PGASYNC_READY;
 					break;
-				case '1':		/* Parse Complete */
+				case PQMSG_RESP_PARSE_COMPLETE:
 					/* If we're doing PQprepare, we're done; else ignore */
 					if (conn->cmd_queue_head &&
 						conn->cmd_queue_head->queryclass == PGQUERY_PREPARE)
@@ -277,10 +279,10 @@ pqParseInput3(PGconn *conn)
 						conn->asyncStatus = PGASYNC_READY;
 					}
 					break;
-				case '2':		/* Bind Complete */
+				case PQMSG_RESP_BIND_COMPLETE:
 					/* Nothing to do for this message type */
 					break;
-				case '3':		/* Close Complete */
+				case PQMSG_RESP_CLOSE_COMPLETE:
 					/* If we're doing PQsendClose, we're done; else ignore */
 					if (conn->cmd_queue_head &&
 						conn->cmd_queue_head->queryclass == PGQUERY_CLOSE)
@@ -298,11 +300,11 @@ pqParseInput3(PGconn *conn)
 						conn->asyncStatus = PGASYNC_READY;
 					}
 					break;
-				case 'S':		/* parameter status */
+				case PQMSG_RESP_PARAMETER_STATUS:
 					if (getParameterStatus(conn))
 						return;
 					break;
-				case 'K':		/* secret key data from the backend */
+				case PQMSG_REQ_BACKEND_KEY_DATA:		/* secret key data from the backend */
 
 					/*
 					 * This is expected only during backend startup, but it's
@@ -314,7 +316,7 @@ pqParseInput3(PGconn *conn)
 					if (pqGetInt(&(conn->be_key), 4, conn))
 						return;
 					break;
-				case 'T':		/* Row Description */
+				case PQMSG_RESP_ROW_DESCRIPTION:
 					if (conn->error_result ||
 						(conn->result != NULL &&
 						 conn->result->resultStatus == PGRES_FATAL_ERROR))
@@ -346,7 +348,7 @@ pqParseInput3(PGconn *conn)
 						return;
 					}
 					break;
-				case 'n':		/* No Data */
+				case PQMSG_RESP_NO_DATA:
 
 					/*
 					 * NoData indicates that we will not be seeing a
@@ -374,11 +376,11 @@ pqParseInput3(PGconn *conn)
 						conn->asyncStatus = PGASYNC_READY;
 					}
 					break;
-				case 't':		/* Parameter Description */
+				case PQMSG_RESP_PARAMETER_DESCRIPTION:
 					if (getParamDescriptions(conn, msgLength))
 						return;
 					break;
-				case 'D':		/* Data Row */
+				case PQMSG_RESP_DATA_ROW:
 					if (conn->result != NULL &&
 						conn->result->resultStatus == PGRES_TUPLES_OK)
 					{
@@ -405,24 +407,24 @@ pqParseInput3(PGconn *conn)
 						conn->inCursor += msgLength;
 					}
 					break;
-				case 'G':		/* Start Copy In */
+				case PQMSG_RESP_COPY_IN:
 					if (getCopyStart(conn, PGRES_COPY_IN))
 						return;
 					conn->asyncStatus = PGASYNC_COPY_IN;
 					break;
-				case 'H':		/* Start Copy Out */
+				case PQMSG_RESP_COPY_OUT:
 					if (getCopyStart(conn, PGRES_COPY_OUT))
 						return;
 					conn->asyncStatus = PGASYNC_COPY_OUT;
 					conn->copy_already_done = 0;
 					break;
-				case 'W':		/* Start Copy Both */
+				case PQMSG_RESP_COPY_BOTH:
 					if (getCopyStart(conn, PGRES_COPY_BOTH))
 						return;
 					conn->asyncStatus = PGASYNC_COPY_BOTH;
 					conn->copy_already_done = 0;
 					break;
-				case 'd':		/* Copy Data */
+				case PQMSG_REQ_COPY_DATA:
 
 					/*
 					 * If we see Copy Data, just silently drop it.  This would
@@ -431,7 +433,7 @@ pqParseInput3(PGconn *conn)
 					 */
 					conn->inCursor += msgLength;
 					break;
-				case 'c':		/* Copy Done */
+				case PQMSG_REQ_COPY_DONE:
 
 					/*
 					 * If we see Copy Done, just silently drop it.  This is
@@ -1692,21 +1694,21 @@ getCopyDataMessage(PGconn *conn)
 		 */
 		switch (id)
 		{
-			case 'A':			/* NOTIFY */
+			case PQMSG_RESP_NOTIFY:
 				if (getNotify(conn))
 					return 0;
 				break;
-			case 'N':			/* NOTICE */
+			case PQMSG_RESP_NOTICE:
 				if (pqGetErrorNotice3(conn, false))
 					return 0;
 				break;
-			case 'S':			/* ParameterStatus */
+			case PQMSG_RESP_PARAMETER_STATUS:
 				if (getParameterStatus(conn))
 					return 0;
 				break;
-			case 'd':			/* Copy Data, pass it back to caller */
+			case PQMSG_REQ_COPY_DATA:			/* Copy Data, pass it back to caller */
 				return msgLength;
-			case 'c':
+			case PQMSG_REQ_COPY_DONE:
 
 				/*
 				 * If this is a CopyDone message, exit COPY_OUT mode and let
@@ -1929,7 +1931,7 @@ pqEndcopy3(PGconn *conn)
 	if (conn->asyncStatus == PGASYNC_COPY_IN ||
 		conn->asyncStatus == PGASYNC_COPY_BOTH)
 	{
-		if (pqPutMsgStart('c', conn) < 0 ||
+		if (pqPutMsgStart(PQMSG_REQ_COPY_DONE, conn) < 0 ||
 			pqPutMsgEnd(conn) < 0)
 			return 1;
 
@@ -1940,7 +1942,7 @@ pqEndcopy3(PGconn *conn)
 		if (conn->cmd_queue_head &&
 			conn->cmd_queue_head->queryclass != PGQUERY_SIMPLE)
 		{
-			if (pqPutMsgStart('S', conn) < 0 ||
+			if (pqPutMsgStart(PQMSG_REQ_SYNC_DATA, conn) < 0 ||
 				pqPutMsgEnd(conn) < 0)
 				return 1;
 		}
@@ -2023,7 +2025,7 @@ pqFunctionCall3(PGconn *conn, Oid fnid,
 
 	/* PQfn already validated connection state */
 
-	if (pqPutMsgStart('F', conn) < 0 || /* function call msg */
+	if (pqPutMsgStart(PQMSG_REQ_FUNCTION_CALL, conn) < 0 || /* function call msg */
 		pqPutInt(fnid, 4, conn) < 0 ||	/* function id */
 		pqPutInt(1, 2, conn) < 0 || /* # of format codes */
 		pqPutInt(1, 2, conn) < 0 || /* format code: BINARY */
diff --git a/src/interfaces/libpq/fe-trace.c b/src/interfaces/libpq/fe-trace.c
index 402784f40e..83c3328444 100644
--- a/src/interfaces/libpq/fe-trace.c
+++ b/src/interfaces/libpq/fe-trace.c
@@ -28,6 +28,7 @@
 #include "libpq-fe.h"
 #include "libpq-int.h"
 #include "port/pg_bswap.h"
+#include "protocol.h"
 
 
 /* Enable tracing */
@@ -562,110 +563,110 @@ pqTraceOutputMessage(PGconn *conn, const char *message, bool toServer)
 
 	switch (id)
 	{
-		case '1':
+		case PQMSG_RESP_PARSE_COMPLETE:
 			fprintf(conn->Pfdebug, "ParseComplete");
 			/* No message content */
 			break;
-		case '2':
+		case PQMSG_RESP_BIND_COMPLETE:
 			fprintf(conn->Pfdebug, "BindComplete");
 			/* No message content */
 			break;
-		case '3':
+		case PQMSG_RESP_CLOSE_COMPLETE:
 			fprintf(conn->Pfdebug, "CloseComplete");
 			/* No message content */
 			break;
-		case 'A':				/* Notification Response */
+		case PQMSG_RESP_NOTIFY:
 			pqTraceOutputA(conn->Pfdebug, message, &logCursor, regress);
 			break;
-		case 'B':				/* Bind */
+		case PQMSG_REQ_BIND:
 			pqTraceOutputB(conn->Pfdebug, message, &logCursor);
 			break;
-		case 'c':
+		case PQMSG_REQ_COPY_DONE:
 			fprintf(conn->Pfdebug, "CopyDone");
 			/* No message content */
 			break;
-		case 'C':				/* Close(F) or Command Complete(B) */
+		case PQMSG_RESP_COMMAND_COMPLETE:				/* Close(F) or Command Complete(B) */
 			pqTraceOutputC(conn->Pfdebug, toServer, message, &logCursor);
 			break;
-		case 'd':				/* Copy Data */
+		case PQMSG_REQ_COPY_DATA:
 			/* Drop COPY data to reduce the overhead of logging. */
 			break;
-		case 'D':				/* Describe(F) or Data Row(B) */
+		case PQMSG_REQ_DESCRIBE:		/* Describe(F) or Data Row(B) */
 			pqTraceOutputD(conn->Pfdebug, toServer, message, &logCursor);
 			break;
-		case 'E':				/* Execute(F) or Error Response(B) */
+		case PQMSG_REQ_EXECUTE:		/* Execute(F) or Error Response(B) */
 			pqTraceOutputE(conn->Pfdebug, toServer, message, &logCursor,
 						   regress);
 			break;
-		case 'f':				/* Copy Fail */
+		case PQMSG_REQ_COPY_FAIL:
 			pqTraceOutputf(conn->Pfdebug, message, &logCursor);
 			break;
-		case 'F':				/* Function Call */
+		case PQMSG_REQ_FUNCTION_CALL:
 			pqTraceOutputF(conn->Pfdebug, message, &logCursor, regress);
 			break;
-		case 'G':				/* Start Copy In */
+		case PQMSG_RESP_COPY_IN:
 			pqTraceOutputG(conn->Pfdebug, message, &logCursor);
 			break;
-		case 'H':				/* Flush(F) or Start Copy Out(B) */
+		case PQMSG_REQ_FLUSH_DATA:	/* Flush(F) or Start Copy Out(B) */
 			if (!toServer)
 				pqTraceOutputH(conn->Pfdebug, message, &logCursor);
 			else
 				fprintf(conn->Pfdebug, "Flush");	/* no message content */
 			break;
-		case 'I':
+		case PQMSG_RESP_EMPTY_QUERY:
 			fprintf(conn->Pfdebug, "EmptyQueryResponse");
 			/* No message content */
 			break;
-		case 'K':				/* secret key data from the backend */
+		case PQMSG_REQ_BACKEND_KEY_DATA:		/* secret key data from the backend */
 			pqTraceOutputK(conn->Pfdebug, message, &logCursor, regress);
 			break;
-		case 'n':
+		case PQMSG_RESP_NO_DATA:
 			fprintf(conn->Pfdebug, "NoData");
 			/* No message content */
 			break;
-		case 'N':
+		case PQMSG_RESP_NOTICE:
 			pqTraceOutputNR(conn->Pfdebug, "NoticeResponse", message,
 							&logCursor, regress);
 			break;
-		case 'P':				/* Parse */
+		case PQMSG_REQ_PARSE:
 			pqTraceOutputP(conn->Pfdebug, message, &logCursor, regress);
 			break;
-		case 'Q':				/* Query */
+		case PQMSG_REQ_SIMPLE_QUERY:
 			pqTraceOutputQ(conn->Pfdebug, message, &logCursor);
 			break;
-		case 'R':				/* Authentication */
+		case PQMSG_REQ_AUTHENTICATION:
 			pqTraceOutputR(conn->Pfdebug, message, &logCursor);
 			break;
-		case 's':
+		case PQMSG_RESP_PORTAL_SUSPENDED:
 			fprintf(conn->Pfdebug, "PortalSuspended");
 			/* No message content */
 			break;
-		case 'S':				/* Parameter Status(B) or Sync(F) */
+		case PQMSG_REQ_SYNC_DATA:	/* Parameter Status(B) or Sync(F) */
 			if (!toServer)
 				pqTraceOutputS(conn->Pfdebug, message, &logCursor);
 			else
 				fprintf(conn->Pfdebug, "Sync"); /* no message content */
 			break;
-		case 't':				/* Parameter Description */
+		case PQMSG_RESP_PARAMETER_DESCRIPTION:
 			pqTraceOutputt(conn->Pfdebug, message, &logCursor, regress);
 			break;
-		case 'T':				/* Row Description */
+		case PQMSG_RESP_ROW_DESCRIPTION:
 			pqTraceOutputT(conn->Pfdebug, message, &logCursor, regress);
 			break;
-		case 'v':				/* Negotiate Protocol Version */
+		case PQMSG_RESP_NEGOTIATE_PROTOCOL:
 			pqTraceOutputv(conn->Pfdebug, message, &logCursor);
 			break;
-		case 'V':				/* Function Call response */
+		case PQMSG_RESP_FUNCTION_CALL:
 			pqTraceOutputV(conn->Pfdebug, message, &logCursor);
 			break;
-		case 'W':				/* Start Copy Both */
+		case PQMSG_RESP_COPY_BOTH:
 			pqTraceOutputW(conn->Pfdebug, message, &logCursor, length);
 			break;
-		case 'X':
+		case PQMSG_REQ_TERMINATE:
 			fprintf(conn->Pfdebug, "Terminate");
 			/* No message content */
 			break;
-		case 'Z':				/* Ready For Query */
+		case PQMSG_RESP_READY_FOR_QUERY:
 			pqTraceOutputZ(conn->Pfdebug, message, &logCursor);
 			break;
 		default:
-- 
2.37.1 (Apple Git-137.1)

From b3d54e460c6bf0736c1bb55b3c509beb34d2f259 Mon Sep 17 00:00:00 2001
From: Dave Cramer <davecramer@gmail.com>
Date: Thu, 20 Apr 2023 15:40:03 -0400
Subject: [PATCH] Created protocol.h Protocol.h has defines for every protocol
 message both backend and frontend Instead of using hardcoded values for each
 protocol message use defines to make code easier to read

remove redundant comments

define CLOSE_PORTAL and CLOSE_PREPARED subcommands
fix some erroneous uses of DATA_ROW_RESPONSE which should have been DESCRIBE_REQUEST
use the names in the requests .

prepend PQMSG_REQ and PQMSG_RESP to commands and responses respectively

Use commands for documenting fe-trace.c
More commands instead of characters
---
 src/backend/access/common/printsimple.c       |   5 +-
 src/backend/access/transam/parallel.c         |  17 +--
 src/backend/backup/basebackup_copy.c          |  21 ++--
 src/backend/commands/async.c                  |   3 +-
 src/backend/commands/copyfromparse.c          |  23 ++--
 src/backend/commands/copyto.c                 |   7 +-
 src/backend/libpq/auth-sasl.c                 |   3 +-
 src/backend/libpq/auth.c                      |   9 +-
 src/backend/postmaster/postmaster.c           |   3 +-
 src/backend/replication/walsender.c           |  19 ++--
 src/backend/tcop/dest.c                       |   9 +-
 src/backend/tcop/fastpath.c                   |   3 +-
 src/backend/tcop/postgres.c                   |  77 ++++++-------
 src/backend/utils/activity/backend_progress.c |   3 +-
 src/backend/utils/error/elog.c                |   3 +-
 src/backend/utils/misc/guc.c                  |   3 +-
 src/include/protocol.h                        |  60 +++++++++++
 src/interfaces/libpq/fe-auth.c                |   3 +-
 src/interfaces/libpq/fe-connect.c             |  15 +--
 src/interfaces/libpq/fe-exec.c                |  53 ++++-----
 src/interfaces/libpq/fe-protocol3.c           |  66 ++++++------
 src/interfaces/libpq/fe-trace.c               | 101 +++++++++---------
 22 files changed, 294 insertions(+), 212 deletions(-)
 create mode 100644 src/include/protocol.h

diff --git a/src/backend/access/common/printsimple.c b/src/backend/access/common/printsimple.c
index ef818228ac..e7dc125f82 100644
--- a/src/backend/access/common/printsimple.c
+++ b/src/backend/access/common/printsimple.c
@@ -21,6 +21,7 @@
 #include "access/printsimple.h"
 #include "catalog/pg_type.h"
 #include "libpq/pqformat.h"
+#include "protocol.h"
 #include "utils/builtins.h"
 
 /*
@@ -32,7 +33,7 @@ printsimple_startup(DestReceiver *self, int operation, TupleDesc tupdesc)
 	StringInfoData buf;
 	int			i;
 
-	pq_beginmessage(&buf, 'T'); /* RowDescription */
+	pq_beginmessage(&buf, PQMSG_RESP_ROW_DESCRIPTION);
 	pq_sendint16(&buf, tupdesc->natts);
 
 	for (i = 0; i < tupdesc->natts; ++i)
@@ -65,7 +66,7 @@ printsimple(TupleTableSlot *slot, DestReceiver *self)
 	slot_getallattrs(slot);
 
 	/* Prepare and send message */
-	pq_beginmessage(&buf, 'D');
+	pq_beginmessage(&buf, PQMSG_RESP_DATA_ROW);
 	pq_sendint16(&buf, tupdesc->natts);
 
 	for (i = 0; i < tupdesc->natts; ++i)
diff --git a/src/backend/access/transam/parallel.c b/src/backend/access/transam/parallel.c
index 1738aecf1f..6deb1a697f 100644
--- a/src/backend/access/transam/parallel.c
+++ b/src/backend/access/transam/parallel.c
@@ -33,6 +33,7 @@
 #include "miscadmin.h"
 #include "optimizer/optimizer.h"
 #include "pgstat.h"
+#include "protocol.h"
 #include "storage/ipc.h"
 #include "storage/predicate.h"
 #include "storage/sinval.h"
@@ -1127,7 +1128,7 @@ HandleParallelMessage(ParallelContext *pcxt, int i, StringInfo msg)
 
 	switch (msgtype)
 	{
-		case 'K':				/* BackendKeyData */
+		case PQMSG_REQ_BACKEND_KEY_DATA:
 			{
 				int32		pid = pq_getmsgint(msg, 4);
 
@@ -1137,8 +1138,8 @@ HandleParallelMessage(ParallelContext *pcxt, int i, StringInfo msg)
 				break;
 			}
 
-		case 'E':				/* ErrorResponse */
-		case 'N':				/* NoticeResponse */
+		case PQMSG_RESP_ERROR:
+		case PQMSG_RESP_NOTICE:
 			{
 				ErrorData	edata;
 				ErrorContextCallback *save_error_context_stack;
@@ -1183,7 +1184,7 @@ HandleParallelMessage(ParallelContext *pcxt, int i, StringInfo msg)
 				break;
 			}
 
-		case 'A':				/* NotifyResponse */
+		case PQMSG_RESP_NOTIFY:
 			{
 				/* Propagate NotifyResponse. */
 				int32		pid;
@@ -1200,7 +1201,7 @@ HandleParallelMessage(ParallelContext *pcxt, int i, StringInfo msg)
 				break;
 			}
 
-		case 'P':				/* Parallel progress reporting */
+		case PQMSG_RESP_PARALLEL_PROGRESS:
 			{
 				/*
 				 * Only incremental progress reporting is currently supported.
@@ -1217,7 +1218,7 @@ HandleParallelMessage(ParallelContext *pcxt, int i, StringInfo msg)
 				break;
 			}
 
-		case 'X':				/* Terminate, indicating clean exit */
+		case PQMSG_REQ_TERMINATE:	/* Terminate, indicating clean exit */
 			{
 				shm_mq_detach(pcxt->worker[i].error_mqh);
 				pcxt->worker[i].error_mqh = NULL;
@@ -1372,7 +1373,7 @@ ParallelWorkerMain(Datum main_arg)
 	 * protocol message is defined, but it won't actually be used for anything
 	 * in this case.
 	 */
-	pq_beginmessage(&msgbuf, 'K');
+	pq_beginmessage(&msgbuf, PQMSG_REQ_BACKEND_KEY_DATA);
 	pq_sendint32(&msgbuf, (int32) MyProcPid);
 	pq_sendint32(&msgbuf, (int32) MyCancelKey);
 	pq_endmessage(&msgbuf);
@@ -1550,7 +1551,7 @@ ParallelWorkerMain(Datum main_arg)
 	DetachSession();
 
 	/* Report success. */
-	pq_putmessage('X', NULL, 0);
+	pq_putmessage(PQMSG_REQ_TERMINATE, NULL, 0);
 }
 
 /*
diff --git a/src/backend/backup/basebackup_copy.c b/src/backend/backup/basebackup_copy.c
index 1db80cde1b..c2dfce2235 100644
--- a/src/backend/backup/basebackup_copy.c
+++ b/src/backend/backup/basebackup_copy.c
@@ -32,6 +32,7 @@
 #include "executor/executor.h"
 #include "libpq/libpq.h"
 #include "libpq/pqformat.h"
+#include "protocol.h"
 #include "tcop/dest.h"
 #include "utils/builtins.h"
 #include "utils/timestamp.h"
@@ -152,7 +153,7 @@ bbsink_copystream_begin_backup(bbsink *sink)
 	SendTablespaceList(state->tablespaces);
 
 	/* Send a CommandComplete message */
-	pq_puttextmessage('C', "SELECT");
+	pq_puttextmessage(PQMSG_RESP_COMMAND_COMPLETE, "SELECT");
 
 	/* Begin COPY stream. This will be used for all archives + manifest. */
 	SendCopyOutResponse();
@@ -169,7 +170,7 @@ bbsink_copystream_begin_archive(bbsink *sink, const char *archive_name)
 	StringInfoData buf;
 
 	ti = list_nth(state->tablespaces, state->tablespace_num);
-	pq_beginmessage(&buf, 'd'); /* CopyData */
+	pq_beginmessage(&buf, PQMSG_REQ_COPY_DATA);
 	pq_sendbyte(&buf, 'n');		/* New archive */
 	pq_sendstring(&buf, archive_name);
 	pq_sendstring(&buf, ti->path == NULL ? "" : ti->path);
@@ -220,8 +221,8 @@ bbsink_copystream_archive_contents(bbsink *sink, size_t len)
 		{
 			mysink->last_progress_report_time = now;
 
-			pq_beginmessage(&buf, 'd'); /* CopyData */
-			pq_sendbyte(&buf, 'p'); /* Progress report */
+			pq_beginmessage(&buf, PQMSG_REQ_COPY_DATA);
+			pq_sendbyte(&buf, PQMSG_REQ_COPY_PROGRESS);
 			pq_sendint64(&buf, state->bytes_done);
 			pq_endmessage(&buf);
 			pq_flush_if_writable();
@@ -246,8 +247,8 @@ bbsink_copystream_end_archive(bbsink *sink)
 
 	mysink->bytes_done_at_last_time_check = state->bytes_done;
 	mysink->last_progress_report_time = GetCurrentTimestamp();
-	pq_beginmessage(&buf, 'd'); /* CopyData */
-	pq_sendbyte(&buf, 'p');		/* Progress report */
+	pq_beginmessage(&buf, PQMSG_REQ_COPY_DATA);
+	pq_sendbyte(&buf, PQMSG_REQ_COPY_PROGRESS);
 	pq_sendint64(&buf, state->bytes_done);
 	pq_endmessage(&buf);
 	pq_flush_if_writable();
@@ -261,7 +262,7 @@ bbsink_copystream_begin_manifest(bbsink *sink)
 {
 	StringInfoData buf;
 
-	pq_beginmessage(&buf, 'd'); /* CopyData */
+	pq_beginmessage(&buf, PQMSG_REQ_COPY_DATA);
 	pq_sendbyte(&buf, 'm');		/* Manifest */
 	pq_endmessage(&buf);
 }
@@ -318,7 +319,7 @@ SendCopyOutResponse(void)
 {
 	StringInfoData buf;
 
-	pq_beginmessage(&buf, 'H');
+	pq_beginmessage(&buf, PQMSG_RESP_COPY_OUT);
 	pq_sendbyte(&buf, 0);		/* overall format */
 	pq_sendint16(&buf, 0);		/* natts */
 	pq_endmessage(&buf);
@@ -330,7 +331,7 @@ SendCopyOutResponse(void)
 static void
 SendCopyDone(void)
 {
-	pq_putemptymessage('c');
+	pq_putemptymessage(PQMSG_REQ_COPY_DONE);
 }
 
 /*
@@ -368,7 +369,7 @@ SendXlogRecPtrResult(XLogRecPtr ptr, TimeLineID tli)
 	end_tup_output(tstate);
 
 	/* Send a CommandComplete message */
-	pq_puttextmessage('C', "SELECT");
+	pq_puttextmessage(PQMSG_RESP_COMMAND_COMPLETE, "SELECT");
 }
 
 /*
diff --git a/src/backend/commands/async.c b/src/backend/commands/async.c
index ef909cf4e0..e8de47bd41 100644
--- a/src/backend/commands/async.c
+++ b/src/backend/commands/async.c
@@ -141,6 +141,7 @@
 #include "libpq/libpq.h"
 #include "libpq/pqformat.h"
 #include "miscadmin.h"
+#include "protocol.h"
 #include "storage/ipc.h"
 #include "storage/lmgr.h"
 #include "storage/proc.h"
@@ -2281,7 +2282,7 @@ NotifyMyFrontEnd(const char *channel, const char *payload, int32 srcPid)
 	{
 		StringInfoData buf;
 
-		pq_beginmessage(&buf, 'A');
+		pq_beginmessage(&buf, PQMSG_RESP_NOTIFY);
 		pq_sendint32(&buf, srcPid);
 		pq_sendstring(&buf, channel);
 		pq_sendstring(&buf, payload);
diff --git a/src/backend/commands/copyfromparse.c b/src/backend/commands/copyfromparse.c
index 232768a6e1..7bae4a55c2 100644
--- a/src/backend/commands/copyfromparse.c
+++ b/src/backend/commands/copyfromparse.c
@@ -72,6 +72,7 @@
 #include "miscadmin.h"
 #include "pgstat.h"
 #include "port/pg_bswap.h"
+#include "protocol.h"
 #include "utils/builtins.h"
 #include "utils/memutils.h"
 #include "utils/rel.h"
@@ -174,7 +175,7 @@ ReceiveCopyBegin(CopyFromState cstate)
 	int16		format = (cstate->opts.binary ? 1 : 0);
 	int			i;
 
-	pq_beginmessage(&buf, 'G');
+	pq_beginmessage(&buf, PQMSG_RESP_COPY_IN);
 	pq_sendbyte(&buf, format);	/* overall format */
 	pq_sendint16(&buf, natts);
 	for (i = 0; i < natts; i++)
@@ -279,13 +280,13 @@ CopyGetData(CopyFromState cstate, void *databuf, int minread, int maxread)
 					/* Validate message type and set packet size limit */
 					switch (mtype)
 					{
-						case 'd':	/* CopyData */
+						case PQMSG_REQ_COPY_DATA:
 							maxmsglen = PQ_LARGE_MESSAGE_LIMIT;
 							break;
-						case 'c':	/* CopyDone */
-						case 'f':	/* CopyFail */
-						case 'H':	/* Flush */
-						case 'S':	/* Sync */
+						case PQMSG_REQ_COPY_DONE:
+						case PQMSG_REQ_COPY_FAIL:
+						case PQMSG_REQ_FLUSH_DATA:
+						case PQMSG_REQ_SYNC_DATA:
 							maxmsglen = PQ_SMALL_MESSAGE_LIMIT;
 							break;
 						default:
@@ -305,20 +306,20 @@ CopyGetData(CopyFromState cstate, void *databuf, int minread, int maxread)
 					/* ... and process it */
 					switch (mtype)
 					{
-						case 'd':	/* CopyData */
+						case PQMSG_REQ_COPY_DATA:
 							break;
-						case 'c':	/* CopyDone */
+						case PQMSG_REQ_COPY_DONE:
 							/* COPY IN correctly terminated by frontend */
 							cstate->raw_reached_eof = true;
 							return bytesread;
-						case 'f':	/* CopyFail */
+						case PQMSG_REQ_COPY_FAIL:
 							ereport(ERROR,
 									(errcode(ERRCODE_QUERY_CANCELED),
 									 errmsg("COPY from stdin failed: %s",
 											pq_getmsgstring(cstate->fe_msgbuf))));
 							break;
-						case 'H':	/* Flush */
-						case 'S':	/* Sync */
+						case PQMSG_REQ_FLUSH_DATA:
+						case PQMSG_REQ_SYNC_DATA:
 
 							/*
 							 * Ignore Flush/Sync for the convenience of client
diff --git a/src/backend/commands/copyto.c b/src/backend/commands/copyto.c
index 9e4b2437a5..df36dd8ccd 100644
--- a/src/backend/commands/copyto.c
+++ b/src/backend/commands/copyto.c
@@ -34,6 +34,7 @@
 #include "miscadmin.h"
 #include "optimizer/optimizer.h"
 #include "pgstat.h"
+#include "protocol.h"
 #include "rewrite/rewriteHandler.h"
 #include "storage/fd.h"
 #include "tcop/tcopprot.h"
@@ -144,7 +145,7 @@ SendCopyBegin(CopyToState cstate)
 	int16		format = (cstate->opts.binary ? 1 : 0);
 	int			i;
 
-	pq_beginmessage(&buf, 'H');
+	pq_beginmessage(&buf, PQMSG_RESP_COPY_OUT);
 	pq_sendbyte(&buf, format);	/* overall format */
 	pq_sendint16(&buf, natts);
 	for (i = 0; i < natts; i++)
@@ -159,7 +160,7 @@ SendCopyEnd(CopyToState cstate)
 	/* Shouldn't have any unsent data */
 	Assert(cstate->fe_msgbuf->len == 0);
 	/* Send Copy Done message */
-	pq_putemptymessage('c');
+	pq_putemptymessage(PQMSG_REQ_COPY_DONE);
 }
 
 /*----------
@@ -247,7 +248,7 @@ CopySendEndOfRow(CopyToState cstate)
 				CopySendChar(cstate, '\n');
 
 			/* Dump the accumulated row as one CopyData message */
-			(void) pq_putmessage('d', fe_msgbuf->data, fe_msgbuf->len);
+			(void) pq_putmessage(PQMSG_REQ_COPY_DATA, fe_msgbuf->data, fe_msgbuf->len);
 			break;
 		case COPY_CALLBACK:
 			cstate->data_dest_cb(fe_msgbuf->data, fe_msgbuf->len);
diff --git a/src/backend/libpq/auth-sasl.c b/src/backend/libpq/auth-sasl.c
index 684680897b..cb4a4f9227 100644
--- a/src/backend/libpq/auth-sasl.c
+++ b/src/backend/libpq/auth-sasl.c
@@ -19,6 +19,7 @@
 #include "libpq/libpq.h"
 #include "libpq/pqformat.h"
 #include "libpq/sasl.h"
+#include "protocol.h"
 
 /*
  * Maximum accepted size of SASL messages.
@@ -87,7 +88,7 @@ CheckSASLAuth(const pg_be_sasl_mech *mech, Port *port, char *shadow_pass,
 	{
 		pq_startmsgread();
 		mtype = pq_getbyte();
-		if (mtype != 'p')
+		if (mtype != PQMSG_RESP_PASSWORD)
 		{
 			/* Only log error if client didn't disconnect. */
 			if (mtype != EOF)
diff --git a/src/backend/libpq/auth.c b/src/backend/libpq/auth.c
index 315a24bb3f..bc1979423a 100644
--- a/src/backend/libpq/auth.c
+++ b/src/backend/libpq/auth.c
@@ -34,6 +34,7 @@
 #include "libpq/scram.h"
 #include "miscadmin.h"
 #include "port/pg_bswap.h"
+#include "protocol.h"
 #include "postmaster/postmaster.h"
 #include "replication/walsender.h"
 #include "storage/ipc.h"
@@ -665,7 +666,7 @@ sendAuthRequest(Port *port, AuthRequest areq, const char *extradata, int extrale
 
 	CHECK_FOR_INTERRUPTS();
 
-	pq_beginmessage(&buf, 'R');
+	pq_beginmessage(&buf, PQMSG_REQ_AUTHENTICATION);
 	pq_sendint32(&buf, (int32) areq);
 	if (extralen > 0)
 		pq_sendbytes(&buf, extradata, extralen);
@@ -698,7 +699,7 @@ recv_password_packet(Port *port)
 
 	/* Expect 'p' message type */
 	mtype = pq_getbyte();
-	if (mtype != 'p')
+	if (mtype != PQMSG_RESP_PASSWORD)
 	{
 		/*
 		 * If the client just disconnects without offering a password, don't
@@ -961,7 +962,7 @@ pg_GSS_recvauth(Port *port)
 		CHECK_FOR_INTERRUPTS();
 
 		mtype = pq_getbyte();
-		if (mtype != 'p')
+		if (mtype != PQMSG_RESP_PASSWORD)
 		{
 			/* Only log error if client didn't disconnect. */
 			if (mtype != EOF)
@@ -1232,7 +1233,7 @@ pg_SSPI_recvauth(Port *port)
 	{
 		pq_startmsgread();
 		mtype = pq_getbyte();
-		if (mtype != 'p')
+		if (mtype != PQMSG_RESP_PASSWORD)
 		{
 			if (sspictx != NULL)
 			{
diff --git a/src/backend/postmaster/postmaster.c b/src/backend/postmaster/postmaster.c
index 9c8ec779f9..52c15bebb7 100644
--- a/src/backend/postmaster/postmaster.c
+++ b/src/backend/postmaster/postmaster.c
@@ -114,6 +114,7 @@
 #include "postmaster/pgarch.h"
 #include "postmaster/postmaster.h"
 #include "postmaster/syslogger.h"
+#include "protocol.h"
 #include "replication/logicallauncher.h"
 #include "replication/walsender.h"
 #include "storage/fd.h"
@@ -2357,7 +2358,7 @@ SendNegotiateProtocolVersion(List *unrecognized_protocol_options)
 	StringInfoData buf;
 	ListCell   *lc;
 
-	pq_beginmessage(&buf, 'v'); /* NegotiateProtocolVersion */
+	pq_beginmessage(&buf, PQMSG_RESP_NEGOTIATE_PROTOCOL);
 	pq_sendint32(&buf, PG_PROTOCOL_LATEST);
 	pq_sendint32(&buf, list_length(unrecognized_protocol_options));
 	foreach(lc, unrecognized_protocol_options)
diff --git a/src/backend/replication/walsender.c b/src/backend/replication/walsender.c
index d27ef2985d..4bc2a1018c 100644
--- a/src/backend/replication/walsender.c
+++ b/src/backend/replication/walsender.c
@@ -69,6 +69,7 @@
 #include "nodes/replnodes.h"
 #include "pgstat.h"
 #include "postmaster/interrupt.h"
+#include "protocol.h"
 #include "replication/decode.h"
 #include "replication/logical.h"
 #include "replication/slot.h"
@@ -603,7 +604,7 @@ SendTimeLineHistory(TimeLineHistoryCmd *cmd)
 	dest->rStartup(dest, CMD_SELECT, tupdesc);
 
 	/* Send a DataRow message */
-	pq_beginmessage(&buf, 'D');
+	pq_beginmessage(&buf, PQMSG_RESP_DATA_ROW);
 	pq_sendint16(&buf, 2);		/* # of columns */
 	len = strlen(histfname);
 	pq_sendint32(&buf, len);	/* col1 len */
@@ -801,7 +802,7 @@ StartReplication(StartReplicationCmd *cmd)
 		WalSndSetState(WALSNDSTATE_CATCHUP);
 
 		/* Send a CopyBothResponse message, and start streaming */
-		pq_beginmessage(&buf, 'W');
+		pq_beginmessage(&buf, PQMSG_RESP_COPY_BOTH);
 		pq_sendbyte(&buf, 0);
 		pq_sendint16(&buf, 0);
 		pq_endmessage(&buf);
@@ -1294,7 +1295,7 @@ StartLogicalReplication(StartReplicationCmd *cmd)
 	WalSndSetState(WALSNDSTATE_CATCHUP);
 
 	/* Send a CopyBothResponse message, and start streaming */
-	pq_beginmessage(&buf, 'W');
+	pq_beginmessage(&buf, PQMSG_RESP_COPY_BOTH);
 	pq_sendbyte(&buf, 0);
 	pq_sendint16(&buf, 0);
 	pq_endmessage(&buf);
@@ -1923,11 +1924,11 @@ ProcessRepliesIfAny(void)
 		/* Validate message type and set packet size limit */
 		switch (firstchar)
 		{
-			case 'd':
+			case PQMSG_REQ_COPY_DATA:
 				maxmsglen = PQ_LARGE_MESSAGE_LIMIT;
 				break;
-			case 'c':
-			case 'X':
+			case PQMSG_REQ_COPY_DONE:
+			case PQMSG_REQ_TERMINATE:
 				maxmsglen = PQ_SMALL_MESSAGE_LIMIT;
 				break;
 			default:
@@ -1955,7 +1956,7 @@ ProcessRepliesIfAny(void)
 				/*
 				 * 'd' means a standby reply wrapped in a CopyData packet.
 				 */
-			case 'd':
+			case PQMSG_REQ_COPY_DATA:
 				ProcessStandbyMessage();
 				received = true;
 				break;
@@ -1964,7 +1965,7 @@ ProcessRepliesIfAny(void)
 				 * CopyDone means the standby requested to finish streaming.
 				 * Reply with CopyDone, if we had not sent that already.
 				 */
-			case 'c':
+			case PQMSG_REQ_COPY_DONE:
 				if (!streamingDoneSending)
 				{
 					pq_putmessage_noblock('c', NULL, 0);
@@ -1978,7 +1979,7 @@ ProcessRepliesIfAny(void)
 				/*
 				 * 'X' means that the standby is closing down the socket.
 				 */
-			case 'X':
+			case PQMSG_REQ_TERMINATE:
 				proc_exit(0);
 
 			default:
diff --git a/src/backend/tcop/dest.c b/src/backend/tcop/dest.c
index c0406e2ee5..2fb80d3889 100644
--- a/src/backend/tcop/dest.c
+++ b/src/backend/tcop/dest.c
@@ -39,6 +39,7 @@
 #include "executor/tstoreReceiver.h"
 #include "libpq/libpq.h"
 #include "libpq/pqformat.h"
+#include "protocol.h"
 #include "utils/portal.h"
 
 
@@ -176,7 +177,7 @@ EndCommand(const QueryCompletion *qc, CommandDest dest, bool force_undecorated_o
 
 			len = BuildQueryCompletionString(completionTag, qc,
 											 force_undecorated_output);
-			pq_putmessage('C', completionTag, len + 1);
+			pq_putmessage(PQMSG_REQ_CLOSE, completionTag, len + 1);
 
 		case DestNone:
 		case DestDebug:
@@ -200,7 +201,7 @@ EndCommand(const QueryCompletion *qc, CommandDest dest, bool force_undecorated_o
 void
 EndReplicationCommand(const char *commandTag)
 {
-	pq_putmessage('C', commandTag, strlen(commandTag) + 1);
+	pq_putmessage(PQMSG_REQ_CLOSE, commandTag, strlen(commandTag) + 1);
 }
 
 /* ----------------
@@ -220,7 +221,7 @@ NullCommand(CommandDest dest)
 		case DestRemoteSimple:
 
 			/* Tell the FE that we saw an empty query string */
-			pq_putemptymessage('I');
+			pq_putemptymessage(PQMSG_RESP_EMPTY_QUERY);
 			break;
 
 		case DestNone:
@@ -258,7 +259,7 @@ ReadyForQuery(CommandDest dest)
 			{
 				StringInfoData buf;
 
-				pq_beginmessage(&buf, 'Z');
+				pq_beginmessage(&buf, PQMSG_RESP_READY_FOR_QUERY);
 				pq_sendbyte(&buf, TransactionBlockStatusCode());
 				pq_endmessage(&buf);
 			}
diff --git a/src/backend/tcop/fastpath.c b/src/backend/tcop/fastpath.c
index 2f70ebd5fa..384e53a651 100644
--- a/src/backend/tcop/fastpath.c
+++ b/src/backend/tcop/fastpath.c
@@ -27,6 +27,7 @@
 #include "mb/pg_wchar.h"
 #include "miscadmin.h"
 #include "port/pg_bswap.h"
+#include "protocol.h"
 #include "tcop/fastpath.h"
 #include "tcop/tcopprot.h"
 #include "utils/acl.h"
@@ -69,7 +70,7 @@ SendFunctionResult(Datum retval, bool isnull, Oid rettype, int16 format)
 {
 	StringInfoData buf;
 
-	pq_beginmessage(&buf, 'V');
+	pq_beginmessage(&buf, PQMSG_RESP_FUNCTION_CALL);
 
 	if (isnull)
 	{
diff --git a/src/backend/tcop/postgres.c b/src/backend/tcop/postgres.c
index 36cc99ec9c..1e81c58880 100644
--- a/src/backend/tcop/postgres.c
+++ b/src/backend/tcop/postgres.c
@@ -55,6 +55,7 @@
 #include "postmaster/autovacuum.h"
 #include "postmaster/interrupt.h"
 #include "postmaster/postmaster.h"
+#include "protocol.h"
 #include "replication/logicallauncher.h"
 #include "replication/logicalworker.h"
 #include "replication/slot.h"
@@ -402,37 +403,37 @@ SocketBackend(StringInfo inBuf)
 	 */
 	switch (qtype)
 	{
-		case 'Q':				/* simple query */
+		case PQMSG_REQ_SIMPLE_QUERY:
 			maxmsglen = PQ_LARGE_MESSAGE_LIMIT;
 			doing_extended_query_message = false;
 			break;
 
-		case 'F':				/* fastpath function call */
+		case PQMSG_REQ_FUNCTION_CALL:
 			maxmsglen = PQ_LARGE_MESSAGE_LIMIT;
 			doing_extended_query_message = false;
 			break;
 
-		case 'X':				/* terminate */
+		case PQMSG_REQ_TERMINATE:
 			maxmsglen = PQ_SMALL_MESSAGE_LIMIT;
 			doing_extended_query_message = false;
 			ignore_till_sync = false;
 			break;
 
-		case 'B':				/* bind */
-		case 'P':				/* parse */
+		case PQMSG_REQ_BIND :
+		case PQMSG_REQ_PARSE:
 			maxmsglen = PQ_LARGE_MESSAGE_LIMIT;
 			doing_extended_query_message = true;
 			break;
 
-		case 'C':				/* close */
-		case 'D':				/* describe */
-		case 'E':				/* execute */
-		case 'H':				/* flush */
+		case PQMSG_REQ_CLOSE:
+		case PQMSG_REQ_DESCRIBE:
+		case PQMSG_REQ_EXECUTE:
+		case PQMSG_REQ_FLUSH_DATA:
 			maxmsglen = PQ_SMALL_MESSAGE_LIMIT;
 			doing_extended_query_message = true;
 			break;
 
-		case 'S':				/* sync */
+		case PQMSG_REQ_SYNC_DATA:
 			maxmsglen = PQ_SMALL_MESSAGE_LIMIT;
 			/* stop any active skip-till-Sync */
 			ignore_till_sync = false;
@@ -440,13 +441,13 @@ SocketBackend(StringInfo inBuf)
 			doing_extended_query_message = false;
 			break;
 
-		case 'd':				/* copy data */
+		case PQMSG_REQ_COPY_DATA:
 			maxmsglen = PQ_LARGE_MESSAGE_LIMIT;
 			doing_extended_query_message = false;
 			break;
 
-		case 'c':				/* copy done */
-		case 'f':				/* copy fail */
+		case PQMSG_REQ_COPY_DONE:
+		case PQMSG_REQ_COPY_FAIL:
 			maxmsglen = PQ_SMALL_MESSAGE_LIMIT;
 			doing_extended_query_message = false;
 			break;
@@ -1589,7 +1590,7 @@ exec_parse_message(const char *query_string,	/* string to execute */
 	 * Send ParseComplete.
 	 */
 	if (whereToSendOutput == DestRemote)
-		pq_putemptymessage('1');
+		pq_putemptymessage(PQMSG_RESP_PARSE_COMPLETE);
 
 	/*
 	 * Emit duration logging if appropriate.
@@ -2047,7 +2048,7 @@ exec_bind_message(StringInfo input_message)
 	 * Send BindComplete.
 	 */
 	if (whereToSendOutput == DestRemote)
-		pq_putemptymessage('2');
+		pq_putemptymessage(PQMSG_RESP_BIND_COMPLETE);
 
 	/*
 	 * Emit duration logging if appropriate.
@@ -2290,7 +2291,7 @@ exec_execute_message(const char *portal_name, long max_rows)
 	{
 		/* Portal run not complete, so send PortalSuspended */
 		if (whereToSendOutput == DestRemote)
-			pq_putemptymessage('s');
+			pq_putemptymessage(PQMSG_RESP_PORTAL_SUSPENDED);
 
 		/*
 		 * Set XACT_FLAGS_PIPELINING whenever we suspend an Execute message,
@@ -2683,7 +2684,7 @@ exec_describe_statement_message(const char *stmt_name)
 								  NULL);
 	}
 	else
-		pq_putemptymessage('n');	/* NoData */
+		pq_putemptymessage(PQMSG_RESP_NO_DATA);
 }
 
 /*
@@ -2736,7 +2737,7 @@ exec_describe_portal_message(const char *portal_name)
 								  FetchPortalTargetList(portal),
 								  portal->formats);
 	else
-		pq_putemptymessage('n');	/* NoData */
+		pq_putemptymessage(PQMSG_RESP_NO_DATA);
 }
 
 
@@ -4239,7 +4240,7 @@ PostgresMain(const char *dbname, const char *username)
 	{
 		StringInfoData buf;
 
-		pq_beginmessage(&buf, 'K');
+		pq_beginmessage(&buf, PQMSG_REQ_BACKEND_KEY_DATA);
 		pq_sendint32(&buf, (int32) MyProcPid);
 		pq_sendint32(&buf, (int32) MyCancelKey);
 		pq_endmessage(&buf);
@@ -4618,7 +4619,7 @@ PostgresMain(const char *dbname, const char *username)
 
 		switch (firstchar)
 		{
-			case 'Q':			/* simple query */
+			case PQMSG_REQ_SIMPLE_QUERY:
 				{
 					const char *query_string;
 
@@ -4642,7 +4643,7 @@ PostgresMain(const char *dbname, const char *username)
 				}
 				break;
 
-			case 'P':			/* parse */
+			case PQMSG_REQ_PARSE:
 				{
 					const char *stmt_name;
 					const char *query_string;
@@ -4672,7 +4673,7 @@ PostgresMain(const char *dbname, const char *username)
 				}
 				break;
 
-			case 'B':			/* bind */
+			case PQMSG_REQ_BIND:
 				forbidden_in_wal_sender(firstchar);
 
 				/* Set statement_timestamp() */
@@ -4687,7 +4688,7 @@ PostgresMain(const char *dbname, const char *username)
 				/* exec_bind_message does valgrind_report_error_query */
 				break;
 
-			case 'E':			/* execute */
+			case PQMSG_REQ_EXECUTE:
 				{
 					const char *portal_name;
 					int			max_rows;
@@ -4707,7 +4708,7 @@ PostgresMain(const char *dbname, const char *username)
 				}
 				break;
 
-			case 'F':			/* fastpath function call */
+			case PQMSG_REQ_FUNCTION_CALL:
 				forbidden_in_wal_sender(firstchar);
 
 				/* Set statement_timestamp() */
@@ -4742,7 +4743,7 @@ PostgresMain(const char *dbname, const char *username)
 				send_ready_for_query = true;
 				break;
 
-			case 'C':			/* close */
+			case PQMSG_REQ_CLOSE:
 				{
 					int			close_type;
 					const char *close_target;
@@ -4755,7 +4756,7 @@ PostgresMain(const char *dbname, const char *username)
 
 					switch (close_type)
 					{
-						case 'S':
+						case PQMSG_REQ_PREPARED:
 							if (close_target[0] != '\0')
 								DropPreparedStatement(close_target, false);
 							else
@@ -4764,7 +4765,7 @@ PostgresMain(const char *dbname, const char *username)
 								drop_unnamed_stmt();
 							}
 							break;
-						case 'P':
+						case PQMSG_REQ_PORTAL:
 							{
 								Portal		portal;
 
@@ -4782,13 +4783,13 @@ PostgresMain(const char *dbname, const char *username)
 					}
 
 					if (whereToSendOutput == DestRemote)
-						pq_putemptymessage('3');	/* CloseComplete */
+						pq_putemptymessage(PQMSG_RESP_CLOSE_COMPLETE);
 
 					valgrind_report_error_query("CLOSE message");
 				}
 				break;
 
-			case 'D':			/* describe */
+			case PQMSG_REQ_DESCRIBE:
 				{
 					int			describe_type;
 					const char *describe_target;
@@ -4804,10 +4805,10 @@ PostgresMain(const char *dbname, const char *username)
 
 					switch (describe_type)
 					{
-						case 'S':
+						case PQMSG_REQ_PREPARED:
 							exec_describe_statement_message(describe_target);
 							break;
-						case 'P':
+						case PQMSG_REQ_PORTAL:
 							exec_describe_portal_message(describe_target);
 							break;
 						default:
@@ -4822,13 +4823,13 @@ PostgresMain(const char *dbname, const char *username)
 				}
 				break;
 
-			case 'H':			/* flush */
+			case PQMSG_REQ_FLUSH_DATA:
 				pq_getmsgend(&input_message);
 				if (whereToSendOutput == DestRemote)
 					pq_flush();
 				break;
 
-			case 'S':			/* sync */
+			case PQMSG_REQ_SYNC_DATA:
 				pq_getmsgend(&input_message);
 				finish_xact_command();
 				valgrind_report_error_query("SYNC message");
@@ -4847,7 +4848,7 @@ PostgresMain(const char *dbname, const char *username)
 
 				/* FALLTHROUGH */
 
-			case 'X':
+			case PQMSG_REQ_TERMINATE:
 
 				/*
 				 * Reset whereToSendOutput to prevent ereport from attempting
@@ -4865,9 +4866,9 @@ PostgresMain(const char *dbname, const char *username)
 				 */
 				proc_exit(0);
 
-			case 'd':			/* copy data */
-			case 'c':			/* copy done */
-			case 'f':			/* copy fail */
+			case PQMSG_REQ_COPY_DATA:
+			case PQMSG_REQ_COPY_DONE:
+			case PQMSG_REQ_COPY_FAIL:
 
 				/*
 				 * Accept but ignore these messages, per protocol spec; we
@@ -4897,7 +4898,7 @@ forbidden_in_wal_sender(char firstchar)
 {
 	if (am_walsender)
 	{
-		if (firstchar == 'F')
+		if (firstchar == PQMSG_REQ_FUNCTION_CALL)
 			ereport(ERROR,
 					(errcode(ERRCODE_PROTOCOL_VIOLATION),
 					 errmsg("fastpath function calls not supported in a replication connection")));
diff --git a/src/backend/utils/activity/backend_progress.c b/src/backend/utils/activity/backend_progress.c
index 67447ef03a..6200433594 100644
--- a/src/backend/utils/activity/backend_progress.c
+++ b/src/backend/utils/activity/backend_progress.c
@@ -10,6 +10,7 @@
  */
 #include "postgres.h"
 
+#include "protocol.h"
 #include "access/parallel.h"
 #include "libpq/pqformat.h"
 #include "port/atomics.h"		/* for memory barriers */
@@ -102,7 +103,7 @@ pgstat_progress_parallel_incr_param(int index, int64 incr)
 
 		initStringInfo(&progress_message);
 
-		pq_beginmessage(&progress_message, 'P');
+		pq_beginmessage(&progress_message, PQMSG_RESP_PARALLEL_PROGRESS);
 		pq_sendint32(&progress_message, index);
 		pq_sendint64(&progress_message, incr);
 		pq_endmessage(&progress_message);
diff --git a/src/backend/utils/error/elog.c b/src/backend/utils/error/elog.c
index 5898100acb..2e94ef096a 100644
--- a/src/backend/utils/error/elog.c
+++ b/src/backend/utils/error/elog.c
@@ -77,6 +77,7 @@
 #include "postmaster/bgworker.h"
 #include "postmaster/postmaster.h"
 #include "postmaster/syslogger.h"
+#include "protocol.h"
 #include "storage/ipc.h"
 #include "storage/proc.h"
 #include "tcop/tcopprot.h"
@@ -3465,7 +3466,7 @@ send_message_to_frontend(ErrorData *edata)
 		char		tbuf[12];
 
 		/* 'N' (Notice) is for nonfatal conditions, 'E' is for errors */
-		pq_beginmessage(&msgbuf, (edata->elevel < ERROR) ? 'N' : 'E');
+		pq_beginmessage(&msgbuf, (edata->elevel < ERROR) ? PQMSG_RESP_NOTICE : PQMSG_RESP_ERROR);
 
 		sev = error_severity(edata->elevel);
 		pq_sendbyte(&msgbuf, PG_DIAG_SEVERITY);
diff --git a/src/backend/utils/misc/guc.c b/src/backend/utils/misc/guc.c
index 5308896c87..1b9632942e 100644
--- a/src/backend/utils/misc/guc.c
+++ b/src/backend/utils/misc/guc.c
@@ -37,6 +37,7 @@
 #include "libpq/pqformat.h"
 #include "parser/scansup.h"
 #include "port/pg_bitutils.h"
+#include "protocol.h"
 #include "storage/fd.h"
 #include "storage/lwlock.h"
 #include "storage/shmem.h"
@@ -2593,7 +2594,7 @@ ReportGUCOption(struct config_generic *record)
 	{
 		StringInfoData msgbuf;
 
-		pq_beginmessage(&msgbuf, 'S');
+		pq_beginmessage(&msgbuf, PQMSG_RESP_PARAMETER_STATUS);
 		pq_sendstring(&msgbuf, record->name);
 		pq_sendstring(&msgbuf, val);
 		pq_endmessage(&msgbuf);
diff --git a/src/include/protocol.h b/src/include/protocol.h
new file mode 100644
index 0000000000..1dd7fe3051
--- /dev/null
+++ b/src/include/protocol.h
@@ -0,0 +1,60 @@
+/*-------------------------------------------------------------------------
+ *
+ * protocol.h
+ *	  Exports from postmaster/postmaster.c.
+ *
+ * Portions Copyright (c) 1996-2023, PostgreSQL Global Development Group
+ *
+ * src/include/protocol.h
+ *
+ *-------------------------------------------------------------------------
+ */
+#ifndef _PROTOCOL_H
+#define _PROTOCOL_H
+
+#define PQMSG_REQ_BIND              'B'
+#define PQMSG_REQ_CLOSE             'C'
+#define PQMSG_REQ_DESCRIBE          'D'
+#define PQMSG_REQ_EXECUTE           'E'
+#define PQMSG_REQ_FUNCTION_CALL     'F'
+#define PQMSG_REQ_FLUSH_DATA        'H'
+#define PQMSG_REQ_BACKEND_KEY_DATA  'K'
+#define PQMSG_REQ_PARSE             'P'
+#define PQMSG_REQ_AUTHENTICATION    'R'
+#define PQMSG_REQ_SYNC_DATA         'S'
+#define PQMSG_REQ_SIMPLE_QUERY      'Q'
+#define PQMSG_REQ_TERMINATE         'X'
+#define PQMSG_REQ_COPY_FAIL         'f'
+#define PQMSG_REQ_COPY_DONE         'c'
+#define PQMSG_REQ_COPY_DATA         'd'
+#define PQMSG_REQ_COPY_PROGRESS     'p'
+#define PQMSG_REQ_PREPARED          'S'
+#define PQMSG_REQ_PORTAL            'P'
+
+
+/*
+Responses
+*/
+#define PQMSG_RESP_PARSE_COMPLETE   '1'
+#define PQMSG_RESP_BIND_COMPLETE    '2'
+#define PQMSG_RESP_CLOSE_COMPLETE   '3'
+#define PQMSG_RESP_NOTIFY           'A'
+#define PQMSG_RESP_COMMAND_COMPLETE 'C'
+#define PQMSG_RESP_DATA_ROW         'D'
+#define PQMSG_RESP_ERROR            'E'
+#define PQMSG_RESP_COPY_IN          'G'
+#define PQMSG_RESP_COPY_OUT         'H'
+#define PQMSG_RESP_EMPTY_QUERY      'I'
+#define PQMSG_RESP_NOTICE           'N'
+#define PQMSG_RESP_PARALLEL_PROGRESS 'P'
+#define PQMSG_RESP_FUNCTION_CALL    'V'
+#define PQMSG_RESP_PARAMETER_STATUS 'S'
+#define PQMSG_RESP_ROW_DESCRIPTION  'T'
+#define PQMSG_RESP_COPY_BOTH        'W'
+#define PQMSG_RESP_READY_FOR_QUERY  'Z'
+#define PQMSG_RESP_NO_DATA          'n'
+#define PQMSG_RESP_PASSWORD         'p'
+#define PQMSG_RESP_PORTAL_SUSPENDED 's'
+#define PQMSG_RESP_PARAMETER_DESCRIPTION 't'
+#define PQMSG_RESP_NEGOTIATE_PROTOCOL    'v'
+#endif
\ No newline at end of file
diff --git a/src/interfaces/libpq/fe-auth.c b/src/interfaces/libpq/fe-auth.c
index 887ca5e9e1..36ef32bd68 100644
--- a/src/interfaces/libpq/fe-auth.c
+++ b/src/interfaces/libpq/fe-auth.c
@@ -43,6 +43,7 @@
 #include "fe-auth.h"
 #include "fe-auth-sasl.h"
 #include "libpq-fe.h"
+#include "protocol.h"
 
 #ifdef ENABLE_GSS
 /*
@@ -586,7 +587,7 @@ pg_SASL_init(PGconn *conn, int payloadlen)
 	/*
 	 * Build a SASLInitialResponse message, and send it.
 	 */
-	if (pqPutMsgStart('p', conn))
+	if (pqPutMsgStart(PQMSG_RESP_PASSWORD, conn))
 		goto error;
 	if (pqPuts(selected_mechanism, conn))
 		goto error;
diff --git a/src/interfaces/libpq/fe-connect.c b/src/interfaces/libpq/fe-connect.c
index 837c5321aa..18f27b3cd0 100644
--- a/src/interfaces/libpq/fe-connect.c
+++ b/src/interfaces/libpq/fe-connect.c
@@ -32,6 +32,7 @@
 #include "mb/pg_wchar.h"
 #include "pg_config_paths.h"
 #include "port/pg_bswap.h"
+#include "protocol.h"
 
 #ifdef WIN32
 #include "win32.h"
@@ -3591,7 +3592,7 @@ keep_going:						/* We will come back to here until there is
 				 * Anything else probably means it's not Postgres on the other
 				 * end at all.
 				 */
-				if (!(beresp == 'R' || beresp == 'v' || beresp == 'E'))
+				if (!(beresp == PQMSG_REQ_AUTHENTICATION || beresp == PQMSG_RESP_NEGOTIATE_PROTOCOL || beresp == PQMSG_RESP_ERROR))
 				{
 					libpq_append_conn_error(conn, "expected authentication request from server, but received %c",
 											beresp);
@@ -3618,19 +3619,19 @@ keep_going:						/* We will come back to here until there is
 				 * version 14, the server also used the old protocol for
 				 * errors that happened before processing the startup packet.)
 				 */
-				if (beresp == 'R' && (msgLength < 8 || msgLength > 2000))
+				if (beresp == PQMSG_REQ_AUTHENTICATION && (msgLength < 8 || msgLength > 2000))
 				{
 					libpq_append_conn_error(conn, "received invalid authentication request");
 					goto error_return;
 				}
-				if (beresp == 'v' && (msgLength < 8 || msgLength > 2000))
+				if (beresp == PQMSG_RESP_NEGOTIATE_PROTOCOL && (msgLength < 8 || msgLength > 2000))
 				{
 					libpq_append_conn_error(conn, "received invalid protocol negotiation message");
 					goto error_return;
 				}
 
 #define MAX_ERRLEN 30000
-				if (beresp == 'E' && (msgLength < 8 || msgLength > MAX_ERRLEN))
+				if (beresp == PQMSG_RESP_ERROR && (msgLength < 8 || msgLength > MAX_ERRLEN))
 				{
 					/* Handle error from a pre-3.0 server */
 					conn->inCursor = conn->inStart + 1; /* reread data */
@@ -3693,7 +3694,7 @@ keep_going:						/* We will come back to here until there is
 				}
 
 				/* Handle errors. */
-				if (beresp == 'E')
+				if (beresp == PQMSG_RESP_ERROR)
 				{
 					if (pqGetErrorNotice3(conn, true))
 					{
@@ -3770,7 +3771,7 @@ keep_going:						/* We will come back to here until there is
 
 					goto error_return;
 				}
-				else if (beresp == 'v')
+				else if (beresp == PQMSG_RESP_NEGOTIATE_PROTOCOL)
 				{
 					if (pqGetNegotiateProtocolVersion3(conn))
 					{
@@ -4540,7 +4541,7 @@ sendTerminateConn(PGconn *conn)
 		 * Try to send "close connection" message to backend. Ignore any
 		 * error.
 		 */
-		pqPutMsgStart('X', conn);
+		pqPutMsgStart(PQMSG_REQ_TERMINATE, conn);
 		pqPutMsgEnd(conn);
 		(void) pqFlush(conn);
 	}
diff --git a/src/interfaces/libpq/fe-exec.c b/src/interfaces/libpq/fe-exec.c
index a868284ff8..dc2dcab9b7 100644
--- a/src/interfaces/libpq/fe-exec.c
+++ b/src/interfaces/libpq/fe-exec.c
@@ -27,6 +27,7 @@
 #include "libpq-fe.h"
 #include "libpq-int.h"
 #include "mb/pg_wchar.h"
+#include "protocol.h"
 
 /* keep this in same order as ExecStatusType in libpq-fe.h */
 char	   *const pgresStatus[] = {
@@ -1458,7 +1459,7 @@ PQsendQueryInternal(PGconn *conn, const char *query, bool newQuery)
 
 	/* Send the query message(s) */
 	/* construct the outgoing Query message */
-	if (pqPutMsgStart('Q', conn) < 0 ||
+	if (pqPutMsgStart(PQMSG_REQ_SIMPLE_QUERY, conn) < 0 ||
 		pqPuts(query, conn) < 0 ||
 		pqPutMsgEnd(conn) < 0)
 	{
@@ -1571,7 +1572,7 @@ PQsendPrepare(PGconn *conn,
 		return 0;				/* error msg already set */
 
 	/* construct the Parse message */
-	if (pqPutMsgStart('P', conn) < 0 ||
+	if (pqPutMsgStart(PQMSG_REQ_PARSE, conn) < 0 ||
 		pqPuts(stmtName, conn) < 0 ||
 		pqPuts(query, conn) < 0)
 		goto sendFailed;
@@ -1599,7 +1600,7 @@ PQsendPrepare(PGconn *conn,
 	/* Add a Sync, unless in pipeline mode. */
 	if (conn->pipelineStatus == PQ_PIPELINE_OFF)
 	{
-		if (pqPutMsgStart('S', conn) < 0 ||
+		if (pqPutMsgStart(PQMSG_REQ_SYNC_DATA, conn) < 0 ||
 			pqPutMsgEnd(conn) < 0)
 			goto sendFailed;
 	}
@@ -1784,7 +1785,7 @@ PQsendQueryGuts(PGconn *conn,
 	if (command)
 	{
 		/* construct the Parse message */
-		if (pqPutMsgStart('P', conn) < 0 ||
+		if (pqPutMsgStart(PQMSG_REQ_PARSE, conn) < 0 ||
 			pqPuts(stmtName, conn) < 0 ||
 			pqPuts(command, conn) < 0)
 			goto sendFailed;
@@ -1808,7 +1809,7 @@ PQsendQueryGuts(PGconn *conn,
 	}
 
 	/* Construct the Bind message */
-	if (pqPutMsgStart('B', conn) < 0 ||
+	if (pqPutMsgStart(PQMSG_REQ_BIND, conn) < 0 ||
 		pqPuts("", conn) < 0 ||
 		pqPuts(stmtName, conn) < 0)
 		goto sendFailed;
@@ -1874,14 +1875,14 @@ PQsendQueryGuts(PGconn *conn,
 		goto sendFailed;
 
 	/* construct the Describe Portal message */
-	if (pqPutMsgStart('D', conn) < 0 ||
-		pqPutc('P', conn) < 0 ||
+	if (pqPutMsgStart(PQMSG_REQ_DESCRIBE, conn) < 0 ||
+		pqPutc(PQMSG_REQ_PORTAL, conn) < 0 ||
 		pqPuts("", conn) < 0 ||
 		pqPutMsgEnd(conn) < 0)
 		goto sendFailed;
 
 	/* construct the Execute message */
-	if (pqPutMsgStart('E', conn) < 0 ||
+	if (pqPutMsgStart(PQMSG_REQ_EXECUTE, conn) < 0 ||
 		pqPuts("", conn) < 0 ||
 		pqPutInt(0, 4, conn) < 0 ||
 		pqPutMsgEnd(conn) < 0)
@@ -1890,7 +1891,7 @@ PQsendQueryGuts(PGconn *conn,
 	/* construct the Sync message if not in pipeline mode */
 	if (conn->pipelineStatus == PQ_PIPELINE_OFF)
 	{
-		if (pqPutMsgStart('S', conn) < 0 ||
+		if (pqPutMsgStart(PQMSG_REQ_SYNC_DATA, conn) < 0 ||
 			pqPutMsgEnd(conn) < 0)
 			goto sendFailed;
 	}
@@ -2422,7 +2423,7 @@ PQdescribePrepared(PGconn *conn, const char *stmt)
 {
 	if (!PQexecStart(conn))
 		return NULL;
-	if (!PQsendTypedCommand(conn, 'D', 'S', stmt))
+	if (!PQsendTypedCommand(conn, PQMSG_REQ_DESCRIBE, PQMSG_REQ_PREPARED, stmt))
 		return NULL;
 	return PQexecFinish(conn);
 }
@@ -2441,7 +2442,7 @@ PQdescribePortal(PGconn *conn, const char *portal)
 {
 	if (!PQexecStart(conn))
 		return NULL;
-	if (!PQsendTypedCommand(conn, 'D', 'P', portal))
+	if (!PQsendTypedCommand(conn, PQMSG_REQ_DESCRIBE, PQMSG_REQ_PORTAL, portal))
 		return NULL;
 	return PQexecFinish(conn);
 }
@@ -2456,7 +2457,7 @@ PQdescribePortal(PGconn *conn, const char *portal)
 int
 PQsendDescribePrepared(PGconn *conn, const char *stmt)
 {
-	return PQsendTypedCommand(conn, 'D', 'S', stmt);
+	return PQsendTypedCommand(conn, PQMSG_REQ_DESCRIBE, PQMSG_REQ_PREPARED, stmt);
 }
 
 /*
@@ -2469,7 +2470,7 @@ PQsendDescribePrepared(PGconn *conn, const char *stmt)
 int
 PQsendDescribePortal(PGconn *conn, const char *portal)
 {
-	return PQsendTypedCommand(conn, 'D', 'P', portal);
+	return PQsendTypedCommand(conn, PQMSG_REQ_DESCRIBE, PQMSG_REQ_PORTAL, portal);
 }
 
 /*
@@ -2488,7 +2489,7 @@ PQclosePrepared(PGconn *conn, const char *stmt)
 {
 	if (!PQexecStart(conn))
 		return NULL;
-	if (!PQsendTypedCommand(conn, 'C', 'S', stmt))
+	if (!PQsendTypedCommand(conn, PQMSG_REQ_CLOSE, PQMSG_REQ_PREPARED, stmt))
 		return NULL;
 	return PQexecFinish(conn);
 }
@@ -2506,7 +2507,7 @@ PQclosePortal(PGconn *conn, const char *portal)
 {
 	if (!PQexecStart(conn))
 		return NULL;
-	if (!PQsendTypedCommand(conn, 'C', 'P', portal))
+	if (!PQsendTypedCommand(conn, PQMSG_REQ_CLOSE, PQMSG_REQ_PORTAL, portal))
 		return NULL;
 	return PQexecFinish(conn);
 }
@@ -2521,7 +2522,7 @@ PQclosePortal(PGconn *conn, const char *portal)
 int
 PQsendClosePrepared(PGconn *conn, const char *stmt)
 {
-	return PQsendTypedCommand(conn, 'C', 'S', stmt);
+	return PQsendTypedCommand(conn, PQMSG_REQ_CLOSE, PQMSG_REQ_PREPARED, stmt);
 }
 
 /*
@@ -2534,7 +2535,7 @@ PQsendClosePrepared(PGconn *conn, const char *stmt)
 int
 PQsendClosePortal(PGconn *conn, const char *portal)
 {
-	return PQsendTypedCommand(conn, 'C', 'P', portal);
+	return PQsendTypedCommand(conn, PQMSG_REQ_CLOSE, PQMSG_REQ_PORTAL, portal);
 }
 
 /*
@@ -2577,17 +2578,17 @@ PQsendTypedCommand(PGconn *conn, char command, char type, const char *target)
 	/* construct the Sync message */
 	if (conn->pipelineStatus == PQ_PIPELINE_OFF)
 	{
-		if (pqPutMsgStart('S', conn) < 0 ||
+		if (pqPutMsgStart(PQMSG_REQ_SYNC_DATA, conn) < 0 ||
 			pqPutMsgEnd(conn) < 0)
 			goto sendFailed;
 	}
 
 	/* remember if we are doing a Close or a Describe */
-	if (command == 'C')
+	if (command == PQMSG_REQ_CLOSE)
 	{
 		entry->queryclass = PGQUERY_CLOSE;
 	}
-	else if (command == 'D')
+	else if (command == PQMSG_REQ_DESCRIBE)
 	{
 		entry->queryclass = PGQUERY_DESCRIBE;
 	}
@@ -2696,7 +2697,7 @@ PQputCopyData(PGconn *conn, const char *buffer, int nbytes)
 				return pqIsnonblocking(conn) ? 0 : -1;
 		}
 		/* Send the data (too simple to delegate to fe-protocol files) */
-		if (pqPutMsgStart('d', conn) < 0 ||
+		if (pqPutMsgStart(PQMSG_REQ_COPY_DATA, conn) < 0 ||
 			pqPutnchar(buffer, nbytes, conn) < 0 ||
 			pqPutMsgEnd(conn) < 0)
 			return -1;
@@ -2731,7 +2732,7 @@ PQputCopyEnd(PGconn *conn, const char *errormsg)
 	if (errormsg)
 	{
 		/* Send COPY FAIL */
-		if (pqPutMsgStart('f', conn) < 0 ||
+		if (pqPutMsgStart(PQMSG_REQ_COPY_FAIL, conn) < 0 ||
 			pqPuts(errormsg, conn) < 0 ||
 			pqPutMsgEnd(conn) < 0)
 			return -1;
@@ -2739,7 +2740,7 @@ PQputCopyEnd(PGconn *conn, const char *errormsg)
 	else
 	{
 		/* Send COPY DONE */
-		if (pqPutMsgStart('c', conn) < 0 ||
+		if (pqPutMsgStart(PQMSG_REQ_COPY_DONE, conn) < 0 ||
 			pqPutMsgEnd(conn) < 0)
 			return -1;
 	}
@@ -2751,7 +2752,7 @@ PQputCopyEnd(PGconn *conn, const char *errormsg)
 	if (conn->cmd_queue_head &&
 		conn->cmd_queue_head->queryclass != PGQUERY_SIMPLE)
 	{
-		if (pqPutMsgStart('S', conn) < 0 ||
+		if (pqPutMsgStart(PQMSG_REQ_SYNC_DATA, conn) < 0 ||
 			pqPutMsgEnd(conn) < 0)
 			return -1;
 	}
@@ -3263,7 +3264,7 @@ PQpipelineSync(PGconn *conn)
 	entry->query = NULL;
 
 	/* construct the Sync message */
-	if (pqPutMsgStart('S', conn) < 0 ||
+	if (pqPutMsgStart(PQMSG_REQ_SYNC_DATA, conn) < 0 ||
 		pqPutMsgEnd(conn) < 0)
 		goto sendFailed;
 
@@ -3311,7 +3312,7 @@ PQsendFlushRequest(PGconn *conn)
 		return 0;
 	}
 
-	if (pqPutMsgStart('H', conn) < 0 ||
+	if (pqPutMsgStart(PQMSG_REQ_FLUSH_DATA, conn) < 0 ||
 		pqPutMsgEnd(conn) < 0)
 	{
 		return 0;
diff --git a/src/interfaces/libpq/fe-protocol3.c b/src/interfaces/libpq/fe-protocol3.c
index 7bc6355d17..8de12f4aca 100644
--- a/src/interfaces/libpq/fe-protocol3.c
+++ b/src/interfaces/libpq/fe-protocol3.c
@@ -28,14 +28,16 @@
 #include "libpq-int.h"
 #include "mb/pg_wchar.h"
 #include "port/pg_bswap.h"
+#include "protocol.h"
 
 /*
  * This macro lists the backend message types that could be "long" (more
  * than a couple of kilobytes).
  */
 #define VALID_LONG_MESSAGE_TYPE(id) \
-	((id) == 'T' || (id) == 'D' || (id) == 'd' || (id) == 'V' || \
-	 (id) == 'E' || (id) == 'N' || (id) == 'A')
+	((id) == PQMSG_RESP_ROW_DESCRIPTION || (id) == PQMSG_RESP_DATA_ROW || (id) == PQMSG_REQ_COPY_DATA || \
+	 (id) == PQMSG_RESP_FUNCTION_CALL ||  (id) == PQMSG_RESP_ERROR || (id) == PQMSG_RESP_NOTICE || \
+	 (id) == PQMSG_RESP_NOTIFY)
 
 
 static void handleSyncLoss(PGconn *conn, char id, int msgLength);
@@ -140,12 +142,12 @@ pqParseInput3(PGconn *conn)
 		 * from config file due to SIGHUP), but otherwise we hold off until
 		 * BUSY state.
 		 */
-		if (id == 'A')
+		if (id == PQMSG_RESP_NOTIFY)
 		{
 			if (getNotify(conn))
 				return;
 		}
-		else if (id == 'N')
+		else if (id == PQMSG_RESP_NOTICE)
 		{
 			if (pqGetErrorNotice3(conn, false))
 				return;
@@ -165,12 +167,12 @@ pqParseInput3(PGconn *conn)
 			 * it is about to close the connection, so we don't want to just
 			 * discard it...)
 			 */
-			if (id == 'E')
+			if (id == PQMSG_RESP_ERROR)
 			{
 				if (pqGetErrorNotice3(conn, false /* treat as notice */ ))
 					return;
 			}
-			else if (id == 'S')
+			else if (id == PQMSG_RESP_PARAMETER_STATUS)
 			{
 				if (getParameterStatus(conn))
 					return;
@@ -192,7 +194,7 @@ pqParseInput3(PGconn *conn)
 			 */
 			switch (id)
 			{
-				case 'C':		/* command complete */
+				case PQMSG_RESP_COMMAND_COMPLETE:
 					if (pqGets(&conn->workBuffer, conn))
 						return;
 					if (!pgHavePendingResult(conn))
@@ -210,12 +212,12 @@ pqParseInput3(PGconn *conn)
 								CMDSTATUS_LEN);
 					conn->asyncStatus = PGASYNC_READY;
 					break;
-				case 'E':		/* error return */
+				case PQMSG_RESP_ERROR:
 					if (pqGetErrorNotice3(conn, true))
 						return;
 					conn->asyncStatus = PGASYNC_READY;
 					break;
-				case 'Z':		/* sync response, backend is ready for new
+				case PQMSG_RESP_READY_FOR_QUERY:		/* sync response, backend is ready for new
 								 * query */
 					if (getReadyForQuery(conn))
 						return;
@@ -246,7 +248,7 @@ pqParseInput3(PGconn *conn)
 						conn->asyncStatus = PGASYNC_IDLE;
 					}
 					break;
-				case 'I':		/* empty query */
+				case PQMSG_RESP_EMPTY_QUERY:
 					if (!pgHavePendingResult(conn))
 					{
 						conn->result = PQmakeEmptyPGresult(conn,
@@ -259,7 +261,7 @@ pqParseInput3(PGconn *conn)
 					}
 					conn->asyncStatus = PGASYNC_READY;
 					break;
-				case '1':		/* Parse Complete */
+				case PQMSG_RESP_PARSE_COMPLETE:
 					/* If we're doing PQprepare, we're done; else ignore */
 					if (conn->cmd_queue_head &&
 						conn->cmd_queue_head->queryclass == PGQUERY_PREPARE)
@@ -277,10 +279,10 @@ pqParseInput3(PGconn *conn)
 						conn->asyncStatus = PGASYNC_READY;
 					}
 					break;
-				case '2':		/* Bind Complete */
+				case PQMSG_RESP_BIND_COMPLETE:
 					/* Nothing to do for this message type */
 					break;
-				case '3':		/* Close Complete */
+				case PQMSG_RESP_CLOSE_COMPLETE:
 					/* If we're doing PQsendClose, we're done; else ignore */
 					if (conn->cmd_queue_head &&
 						conn->cmd_queue_head->queryclass == PGQUERY_CLOSE)
@@ -298,11 +300,11 @@ pqParseInput3(PGconn *conn)
 						conn->asyncStatus = PGASYNC_READY;
 					}
 					break;
-				case 'S':		/* parameter status */
+				case PQMSG_RESP_PARAMETER_STATUS:
 					if (getParameterStatus(conn))
 						return;
 					break;
-				case 'K':		/* secret key data from the backend */
+				case PQMSG_REQ_BACKEND_KEY_DATA:		/* secret key data from the backend */
 
 					/*
 					 * This is expected only during backend startup, but it's
@@ -314,7 +316,7 @@ pqParseInput3(PGconn *conn)
 					if (pqGetInt(&(conn->be_key), 4, conn))
 						return;
 					break;
-				case 'T':		/* Row Description */
+				case PQMSG_RESP_ROW_DESCRIPTION:
 					if (conn->error_result ||
 						(conn->result != NULL &&
 						 conn->result->resultStatus == PGRES_FATAL_ERROR))
@@ -346,7 +348,7 @@ pqParseInput3(PGconn *conn)
 						return;
 					}
 					break;
-				case 'n':		/* No Data */
+				case PQMSG_RESP_NO_DATA:
 
 					/*
 					 * NoData indicates that we will not be seeing a
@@ -374,11 +376,11 @@ pqParseInput3(PGconn *conn)
 						conn->asyncStatus = PGASYNC_READY;
 					}
 					break;
-				case 't':		/* Parameter Description */
+				case PQMSG_RESP_PARAMETER_DESCRIPTION:
 					if (getParamDescriptions(conn, msgLength))
 						return;
 					break;
-				case 'D':		/* Data Row */
+				case PQMSG_RESP_DATA_ROW:
 					if (conn->result != NULL &&
 						conn->result->resultStatus == PGRES_TUPLES_OK)
 					{
@@ -405,24 +407,24 @@ pqParseInput3(PGconn *conn)
 						conn->inCursor += msgLength;
 					}
 					break;
-				case 'G':		/* Start Copy In */
+				case PQMSG_RESP_COPY_IN:
 					if (getCopyStart(conn, PGRES_COPY_IN))
 						return;
 					conn->asyncStatus = PGASYNC_COPY_IN;
 					break;
-				case 'H':		/* Start Copy Out */
+				case PQMSG_RESP_COPY_OUT:
 					if (getCopyStart(conn, PGRES_COPY_OUT))
 						return;
 					conn->asyncStatus = PGASYNC_COPY_OUT;
 					conn->copy_already_done = 0;
 					break;
-				case 'W':		/* Start Copy Both */
+				case PQMSG_RESP_COPY_BOTH:
 					if (getCopyStart(conn, PGRES_COPY_BOTH))
 						return;
 					conn->asyncStatus = PGASYNC_COPY_BOTH;
 					conn->copy_already_done = 0;
 					break;
-				case 'd':		/* Copy Data */
+				case PQMSG_REQ_COPY_DATA:
 
 					/*
 					 * If we see Copy Data, just silently drop it.  This would
@@ -431,7 +433,7 @@ pqParseInput3(PGconn *conn)
 					 */
 					conn->inCursor += msgLength;
 					break;
-				case 'c':		/* Copy Done */
+				case PQMSG_REQ_COPY_DONE:
 
 					/*
 					 * If we see Copy Done, just silently drop it.  This is
@@ -1692,21 +1694,21 @@ getCopyDataMessage(PGconn *conn)
 		 */
 		switch (id)
 		{
-			case 'A':			/* NOTIFY */
+			case PQMSG_RESP_NOTIFY:
 				if (getNotify(conn))
 					return 0;
 				break;
-			case 'N':			/* NOTICE */
+			case PQMSG_RESP_NOTICE:
 				if (pqGetErrorNotice3(conn, false))
 					return 0;
 				break;
-			case 'S':			/* ParameterStatus */
+			case PQMSG_RESP_PARAMETER_STATUS:
 				if (getParameterStatus(conn))
 					return 0;
 				break;
-			case 'd':			/* Copy Data, pass it back to caller */
+			case PQMSG_REQ_COPY_DATA:			/* Copy Data, pass it back to caller */
 				return msgLength;
-			case 'c':
+			case PQMSG_REQ_COPY_DONE:
 
 				/*
 				 * If this is a CopyDone message, exit COPY_OUT mode and let
@@ -1929,7 +1931,7 @@ pqEndcopy3(PGconn *conn)
 	if (conn->asyncStatus == PGASYNC_COPY_IN ||
 		conn->asyncStatus == PGASYNC_COPY_BOTH)
 	{
-		if (pqPutMsgStart('c', conn) < 0 ||
+		if (pqPutMsgStart(PQMSG_REQ_COPY_DONE, conn) < 0 ||
 			pqPutMsgEnd(conn) < 0)
 			return 1;
 
@@ -1940,7 +1942,7 @@ pqEndcopy3(PGconn *conn)
 		if (conn->cmd_queue_head &&
 			conn->cmd_queue_head->queryclass != PGQUERY_SIMPLE)
 		{
-			if (pqPutMsgStart('S', conn) < 0 ||
+			if (pqPutMsgStart(PQMSG_REQ_SYNC_DATA, conn) < 0 ||
 				pqPutMsgEnd(conn) < 0)
 				return 1;
 		}
@@ -2023,7 +2025,7 @@ pqFunctionCall3(PGconn *conn, Oid fnid,
 
 	/* PQfn already validated connection state */
 
-	if (pqPutMsgStart('F', conn) < 0 || /* function call msg */
+	if (pqPutMsgStart(PQMSG_REQ_FUNCTION_CALL, conn) < 0 || /* function call msg */
 		pqPutInt(fnid, 4, conn) < 0 ||	/* function id */
 		pqPutInt(1, 2, conn) < 0 || /* # of format codes */
 		pqPutInt(1, 2, conn) < 0 || /* format code: BINARY */
diff --git a/src/interfaces/libpq/fe-trace.c b/src/interfaces/libpq/fe-trace.c
index 402784f40e..19eaa38019 100644
--- a/src/interfaces/libpq/fe-trace.c
+++ b/src/interfaces/libpq/fe-trace.c
@@ -28,6 +28,7 @@
 #include "libpq-fe.h"
 #include "libpq-int.h"
 #include "port/pg_bswap.h"
+#include "protocol.h"
 
 
 /* Enable tracing */
@@ -215,7 +216,7 @@ pqTraceOutputNchar(FILE *pfdebug, int len, const char *data, int *cursor)
  * Output functions by protocol message type
  */
 
-/* NotificationResponse */
+/* PQMSG_RESP_NOTIFY */
 static void
 pqTraceOutputA(FILE *f, const char *message, int *cursor, bool regress)
 {
@@ -225,7 +226,7 @@ pqTraceOutputA(FILE *f, const char *message, int *cursor, bool regress)
 	pqTraceOutputString(f, message, cursor, false);
 }
 
-/* Bind */
+/* PQMSG_REQ_BIND */
 static void
 pqTraceOutputB(FILE *f, const char *message, int *cursor)
 {
@@ -256,7 +257,7 @@ pqTraceOutputB(FILE *f, const char *message, int *cursor)
 		pqTraceOutputInt16(f, message, cursor);
 }
 
-/* Close(F) or CommandComplete(B) */
+/* PQMSG_REQ_CLOSE or PQMSG_RESP_COMMAND_COMPLETE */
 static void
 pqTraceOutputC(FILE *f, bool toServer, const char *message, int *cursor)
 {
@@ -273,7 +274,7 @@ pqTraceOutputC(FILE *f, bool toServer, const char *message, int *cursor)
 	}
 }
 
-/* Describe(F) or DataRow(B) */
+/* PQMSG_REQ_DESCRIBE or PQMSG_RESP_DATA_ROW */
 static void
 pqTraceOutputD(FILE *f, bool toServer, const char *message, int *cursor)
 {
@@ -322,7 +323,7 @@ pqTraceOutputNR(FILE *f, const char *type, const char *message, int *cursor,
 	}
 }
 
-/* Execute(F) or ErrorResponse(B) */
+/* PQMSG_REQ_EXECUTE or PQMSG_RESP_ERROR */
 static void
 pqTraceOutputE(FILE *f, bool toServer, const char *message, int *cursor, bool regress)
 {
@@ -336,7 +337,7 @@ pqTraceOutputE(FILE *f, bool toServer, const char *message, int *cursor, bool re
 		pqTraceOutputNR(f, "ErrorResponse", message, cursor, regress);
 }
 
-/* CopyFail */
+/* PQMSG_REQ_COPY_FAIL */
 static void
 pqTraceOutputf(FILE *f, const char *message, int *cursor)
 {
@@ -344,7 +345,7 @@ pqTraceOutputf(FILE *f, const char *message, int *cursor)
 	pqTraceOutputString(f, message, cursor, false);
 }
 
-/* FunctionCall */
+/* PQMSG_REQ_FUNCTION_CALL */
 static void
 pqTraceOutputF(FILE *f, const char *message, int *cursor, bool regress)
 {
@@ -371,7 +372,7 @@ pqTraceOutputF(FILE *f, const char *message, int *cursor, bool regress)
 	pqTraceOutputInt16(f, message, cursor);
 }
 
-/* CopyInResponse */
+/* PQMSG_RESP_COPY_IN */
 static void
 pqTraceOutputG(FILE *f, const char *message, int *cursor)
 {
@@ -385,7 +386,7 @@ pqTraceOutputG(FILE *f, const char *message, int *cursor)
 		pqTraceOutputInt16(f, message, cursor);
 }
 
-/* CopyOutResponse */
+/* PQMSG_RESP_COPY_OUT */
 static void
 pqTraceOutputH(FILE *f, const char *message, int *cursor)
 {
@@ -399,7 +400,7 @@ pqTraceOutputH(FILE *f, const char *message, int *cursor)
 		pqTraceOutputInt16(f, message, cursor);
 }
 
-/* BackendKeyData */
+/* PQMSG_REQ_BACKEND_KEY_DATA */
 static void
 pqTraceOutputK(FILE *f, const char *message, int *cursor, bool regress)
 {
@@ -408,7 +409,7 @@ pqTraceOutputK(FILE *f, const char *message, int *cursor, bool regress)
 	pqTraceOutputInt32(f, message, cursor, regress);
 }
 
-/* Parse */
+/* PQMSG_REQ_PARSE */
 static void
 pqTraceOutputP(FILE *f, const char *message, int *cursor, bool regress)
 {
@@ -423,7 +424,7 @@ pqTraceOutputP(FILE *f, const char *message, int *cursor, bool regress)
 		pqTraceOutputInt32(f, message, cursor, regress);
 }
 
-/* Query */
+/* PQMSG_REQ_SIMPLE_QUERY */
 static void
 pqTraceOutputQ(FILE *f, const char *message, int *cursor)
 {
@@ -431,7 +432,7 @@ pqTraceOutputQ(FILE *f, const char *message, int *cursor)
 	pqTraceOutputString(f, message, cursor, false);
 }
 
-/* Authentication */
+/* PQMSG_REQ_AUTHENTICATION */
 static void
 pqTraceOutputR(FILE *f, const char *message, int *cursor)
 {
@@ -439,7 +440,7 @@ pqTraceOutputR(FILE *f, const char *message, int *cursor)
 	pqTraceOutputInt32(f, message, cursor, false);
 }
 
-/* ParameterStatus */
+/* PQMSG_RESP_PARAMETER_STATUS */
 static void
 pqTraceOutputS(FILE *f, const char *message, int *cursor)
 {
@@ -448,7 +449,7 @@ pqTraceOutputS(FILE *f, const char *message, int *cursor)
 	pqTraceOutputString(f, message, cursor, false);
 }
 
-/* ParameterDescription */
+/* PQMSG_RESP_PARAMETER_DESCRIPTION */
 static void
 pqTraceOutputt(FILE *f, const char *message, int *cursor, bool regress)
 {
@@ -461,7 +462,7 @@ pqTraceOutputt(FILE *f, const char *message, int *cursor, bool regress)
 		pqTraceOutputInt32(f, message, cursor, regress);
 }
 
-/* RowDescription */
+/* PQMSG_RESP_ROW_DESCRIPTION */
 static void
 pqTraceOutputT(FILE *f, const char *message, int *cursor, bool regress)
 {
@@ -482,7 +483,7 @@ pqTraceOutputT(FILE *f, const char *message, int *cursor, bool regress)
 	}
 }
 
-/* NegotiateProtocolVersion */
+/* PQMSG_RESP_NEGOTIATE_PROTOCOL */
 static void
 pqTraceOutputv(FILE *f, const char *message, int *cursor)
 {
@@ -491,7 +492,7 @@ pqTraceOutputv(FILE *f, const char *message, int *cursor)
 	pqTraceOutputInt32(f, message, cursor, false);
 }
 
-/* FunctionCallResponse */
+/* PQMSG_RESP_FUNCTION_CALL */
 static void
 pqTraceOutputV(FILE *f, const char *message, int *cursor)
 {
@@ -503,7 +504,7 @@ pqTraceOutputV(FILE *f, const char *message, int *cursor)
 		pqTraceOutputNchar(f, len, message, cursor);
 }
 
-/* CopyBothResponse */
+/* PQMSG_RESP_COPY_BOTH */
 static void
 pqTraceOutputW(FILE *f, const char *message, int *cursor, int length)
 {
@@ -514,7 +515,7 @@ pqTraceOutputW(FILE *f, const char *message, int *cursor, int length)
 		pqTraceOutputInt16(f, message, cursor);
 }
 
-/* ReadyForQuery */
+/* PQMSG_RESP_READY_FOR_QUERY */
 static void
 pqTraceOutputZ(FILE *f, const char *message, int *cursor)
 {
@@ -562,110 +563,110 @@ pqTraceOutputMessage(PGconn *conn, const char *message, bool toServer)
 
 	switch (id)
 	{
-		case '1':
+		case PQMSG_RESP_PARSE_COMPLETE:
 			fprintf(conn->Pfdebug, "ParseComplete");
 			/* No message content */
 			break;
-		case '2':
+		case PQMSG_RESP_BIND_COMPLETE:
 			fprintf(conn->Pfdebug, "BindComplete");
 			/* No message content */
 			break;
-		case '3':
+		case PQMSG_RESP_CLOSE_COMPLETE:
 			fprintf(conn->Pfdebug, "CloseComplete");
 			/* No message content */
 			break;
-		case 'A':				/* Notification Response */
+		case PQMSG_RESP_NOTIFY:
 			pqTraceOutputA(conn->Pfdebug, message, &logCursor, regress);
 			break;
-		case 'B':				/* Bind */
+		case PQMSG_REQ_BIND:
 			pqTraceOutputB(conn->Pfdebug, message, &logCursor);
 			break;
-		case 'c':
+		case PQMSG_REQ_COPY_DONE:
 			fprintf(conn->Pfdebug, "CopyDone");
 			/* No message content */
 			break;
-		case 'C':				/* Close(F) or Command Complete(B) */
+		case PQMSG_RESP_COMMAND_COMPLETE:				/* Close(F) or Command Complete(B) */
 			pqTraceOutputC(conn->Pfdebug, toServer, message, &logCursor);
 			break;
-		case 'd':				/* Copy Data */
+		case PQMSG_REQ_COPY_DATA:
 			/* Drop COPY data to reduce the overhead of logging. */
 			break;
-		case 'D':				/* Describe(F) or Data Row(B) */
+		case PQMSG_REQ_DESCRIBE:		/* Describe(F) or Data Row(B) */
 			pqTraceOutputD(conn->Pfdebug, toServer, message, &logCursor);
 			break;
-		case 'E':				/* Execute(F) or Error Response(B) */
+		case PQMSG_REQ_EXECUTE:		/* Execute(F) or Error Response(B) */
 			pqTraceOutputE(conn->Pfdebug, toServer, message, &logCursor,
 						   regress);
 			break;
-		case 'f':				/* Copy Fail */
+		case PQMSG_REQ_COPY_FAIL:
 			pqTraceOutputf(conn->Pfdebug, message, &logCursor);
 			break;
-		case 'F':				/* Function Call */
+		case PQMSG_REQ_FUNCTION_CALL:
 			pqTraceOutputF(conn->Pfdebug, message, &logCursor, regress);
 			break;
-		case 'G':				/* Start Copy In */
+		case PQMSG_RESP_COPY_IN:
 			pqTraceOutputG(conn->Pfdebug, message, &logCursor);
 			break;
-		case 'H':				/* Flush(F) or Start Copy Out(B) */
+		case PQMSG_REQ_FLUSH_DATA:	/* Flush(F) or Start Copy Out(B) */
 			if (!toServer)
 				pqTraceOutputH(conn->Pfdebug, message, &logCursor);
 			else
 				fprintf(conn->Pfdebug, "Flush");	/* no message content */
 			break;
-		case 'I':
+		case PQMSG_RESP_EMPTY_QUERY:
 			fprintf(conn->Pfdebug, "EmptyQueryResponse");
 			/* No message content */
 			break;
-		case 'K':				/* secret key data from the backend */
+		case PQMSG_REQ_BACKEND_KEY_DATA:		/* secret key data from the backend */
 			pqTraceOutputK(conn->Pfdebug, message, &logCursor, regress);
 			break;
-		case 'n':
+		case PQMSG_RESP_NO_DATA:
 			fprintf(conn->Pfdebug, "NoData");
 			/* No message content */
 			break;
-		case 'N':
+		case PQMSG_RESP_NOTICE:
 			pqTraceOutputNR(conn->Pfdebug, "NoticeResponse", message,
 							&logCursor, regress);
 			break;
-		case 'P':				/* Parse */
+		case PQMSG_REQ_PARSE:
 			pqTraceOutputP(conn->Pfdebug, message, &logCursor, regress);
 			break;
-		case 'Q':				/* Query */
+		case PQMSG_REQ_SIMPLE_QUERY:
 			pqTraceOutputQ(conn->Pfdebug, message, &logCursor);
 			break;
-		case 'R':				/* Authentication */
+		case PQMSG_REQ_AUTHENTICATION:
 			pqTraceOutputR(conn->Pfdebug, message, &logCursor);
 			break;
-		case 's':
+		case PQMSG_RESP_PORTAL_SUSPENDED:
 			fprintf(conn->Pfdebug, "PortalSuspended");
 			/* No message content */
 			break;
-		case 'S':				/* Parameter Status(B) or Sync(F) */
+		case PQMSG_REQ_SYNC_DATA:	/* Parameter Status(B) or Sync(F) */
 			if (!toServer)
 				pqTraceOutputS(conn->Pfdebug, message, &logCursor);
 			else
 				fprintf(conn->Pfdebug, "Sync"); /* no message content */
 			break;
-		case 't':				/* Parameter Description */
+		case PQMSG_RESP_PARAMETER_DESCRIPTION:
 			pqTraceOutputt(conn->Pfdebug, message, &logCursor, regress);
 			break;
-		case 'T':				/* Row Description */
+		case PQMSG_RESP_ROW_DESCRIPTION:
 			pqTraceOutputT(conn->Pfdebug, message, &logCursor, regress);
 			break;
-		case 'v':				/* Negotiate Protocol Version */
+		case PQMSG_RESP_NEGOTIATE_PROTOCOL:
 			pqTraceOutputv(conn->Pfdebug, message, &logCursor);
 			break;
-		case 'V':				/* Function Call response */
+		case PQMSG_RESP_FUNCTION_CALL:
 			pqTraceOutputV(conn->Pfdebug, message, &logCursor);
 			break;
-		case 'W':				/* Start Copy Both */
+		case PQMSG_RESP_COPY_BOTH:
 			pqTraceOutputW(conn->Pfdebug, message, &logCursor, length);
 			break;
-		case 'X':
+		case PQMSG_REQ_TERMINATE:
 			fprintf(conn->Pfdebug, "Terminate");
 			/* No message content */
 			break;
-		case 'Z':				/* Ready For Query */
+		case PQMSG_RESP_READY_FOR_QUERY:
 			pqTraceOutputZ(conn->Pfdebug, message, &logCursor);
 			break;
 		default:
-- 
2.37.1 (Apple Git-137.1)

From 1b613d3c96991a1616786d1ca4b1f5365b68f74a Mon Sep 17 00:00:00 2001
From: Dave Cramer <davecramer@gmail.com>
Date: Thu, 20 Apr 2023 15:40:03 -0400
Subject: [PATCH] Created protocol.h Protocol.h has defines for every protocol
 message both backend and frontend Instead of using hardcoded values for each
 protocol message use defines to make code easier to read

Use commands for documenting fe-trace.c
More commands instead of characters

Change name to
PqMsg<name>Request|Response
---
 src/backend/access/common/printsimple.c       |   5 +-
 src/backend/access/transam/parallel.c         |  17 +--
 src/backend/backup/basebackup_copy.c          |  21 ++--
 src/backend/commands/async.c                  |   3 +-
 src/backend/commands/copyfromparse.c          |  23 ++--
 src/backend/commands/copyto.c                 |   7 +-
 src/backend/libpq/auth-sasl.c                 |   3 +-
 src/backend/libpq/auth.c                      |   9 +-
 src/backend/postmaster/postmaster.c           |   3 +-
 src/backend/replication/walsender.c           |  19 ++--
 src/backend/tcop/dest.c                       |   9 +-
 src/backend/tcop/fastpath.c                   |   3 +-
 src/backend/tcop/postgres.c                   |  77 ++++++-------
 src/backend/utils/activity/backend_progress.c |   3 +-
 src/backend/utils/error/elog.c                |   3 +-
 src/backend/utils/misc/guc.c                  |   3 +-
 src/include/protocol.h                        |  60 +++++++++++
 src/interfaces/libpq/fe-auth.c                |   3 +-
 src/interfaces/libpq/fe-connect.c             |  15 +--
 src/interfaces/libpq/fe-exec.c                |  53 ++++-----
 src/interfaces/libpq/fe-protocol3.c           |  66 ++++++------
 src/interfaces/libpq/fe-trace.c               | 101 +++++++++---------
 22 files changed, 294 insertions(+), 212 deletions(-)
 create mode 100644 src/include/protocol.h

diff --git a/src/backend/access/common/printsimple.c b/src/backend/access/common/printsimple.c
index ef818228ac..54052b1bb7 100644
--- a/src/backend/access/common/printsimple.c
+++ b/src/backend/access/common/printsimple.c
@@ -21,6 +21,7 @@
 #include "access/printsimple.h"
 #include "catalog/pg_type.h"
 #include "libpq/pqformat.h"
+#include "protocol.h"
 #include "utils/builtins.h"
 
 /*
@@ -32,7 +33,7 @@ printsimple_startup(DestReceiver *self, int operation, TupleDesc tupdesc)
 	StringInfoData buf;
 	int			i;
 
-	pq_beginmessage(&buf, 'T'); /* RowDescription */
+	pq_beginmessage(&buf, PqMsgRowDescriptionResponse);
 	pq_sendint16(&buf, tupdesc->natts);
 
 	for (i = 0; i < tupdesc->natts; ++i)
@@ -65,7 +66,7 @@ printsimple(TupleTableSlot *slot, DestReceiver *self)
 	slot_getallattrs(slot);
 
 	/* Prepare and send message */
-	pq_beginmessage(&buf, 'D');
+	pq_beginmessage(&buf, PqMsgDataRowResponse);
 	pq_sendint16(&buf, tupdesc->natts);
 
 	for (i = 0; i < tupdesc->natts; ++i)
diff --git a/src/backend/access/transam/parallel.c b/src/backend/access/transam/parallel.c
index 1738aecf1f..dd7c8290f4 100644
--- a/src/backend/access/transam/parallel.c
+++ b/src/backend/access/transam/parallel.c
@@ -33,6 +33,7 @@
 #include "miscadmin.h"
 #include "optimizer/optimizer.h"
 #include "pgstat.h"
+#include "protocol.h"
 #include "storage/ipc.h"
 #include "storage/predicate.h"
 #include "storage/sinval.h"
@@ -1127,7 +1128,7 @@ HandleParallelMessage(ParallelContext *pcxt, int i, StringInfo msg)
 
 	switch (msgtype)
 	{
-		case 'K':				/* BackendKeyData */
+		case PqMsgBackendKeyDataRequest:
 			{
 				int32		pid = pq_getmsgint(msg, 4);
 
@@ -1137,8 +1138,8 @@ HandleParallelMessage(ParallelContext *pcxt, int i, StringInfo msg)
 				break;
 			}
 
-		case 'E':				/* ErrorResponse */
-		case 'N':				/* NoticeResponse */
+		case PqMsgErrorResponse:
+		case PqMsgNoticeResponse:
 			{
 				ErrorData	edata;
 				ErrorContextCallback *save_error_context_stack;
@@ -1183,7 +1184,7 @@ HandleParallelMessage(ParallelContext *pcxt, int i, StringInfo msg)
 				break;
 			}
 
-		case 'A':				/* NotifyResponse */
+		case PqMsgNotifyResponse:
 			{
 				/* Propagate NotifyResponse. */
 				int32		pid;
@@ -1200,7 +1201,7 @@ HandleParallelMessage(ParallelContext *pcxt, int i, StringInfo msg)
 				break;
 			}
 
-		case 'P':				/* Parallel progress reporting */
+		case PqMsgParallelProgressResponse:
 			{
 				/*
 				 * Only incremental progress reporting is currently supported.
@@ -1217,7 +1218,7 @@ HandleParallelMessage(ParallelContext *pcxt, int i, StringInfo msg)
 				break;
 			}
 
-		case 'X':				/* Terminate, indicating clean exit */
+		case PqMsgTerminateRequest:	/* Terminate, indicating clean exit */
 			{
 				shm_mq_detach(pcxt->worker[i].error_mqh);
 				pcxt->worker[i].error_mqh = NULL;
@@ -1372,7 +1373,7 @@ ParallelWorkerMain(Datum main_arg)
 	 * protocol message is defined, but it won't actually be used for anything
 	 * in this case.
 	 */
-	pq_beginmessage(&msgbuf, 'K');
+	pq_beginmessage(&msgbuf, PqMsgBackendKeyDataRequest);
 	pq_sendint32(&msgbuf, (int32) MyProcPid);
 	pq_sendint32(&msgbuf, (int32) MyCancelKey);
 	pq_endmessage(&msgbuf);
@@ -1550,7 +1551,7 @@ ParallelWorkerMain(Datum main_arg)
 	DetachSession();
 
 	/* Report success. */
-	pq_putmessage('X', NULL, 0);
+	pq_putmessage(PqMsgTerminateRequest, NULL, 0);
 }
 
 /*
diff --git a/src/backend/backup/basebackup_copy.c b/src/backend/backup/basebackup_copy.c
index 1db80cde1b..e44189b171 100644
--- a/src/backend/backup/basebackup_copy.c
+++ b/src/backend/backup/basebackup_copy.c
@@ -32,6 +32,7 @@
 #include "executor/executor.h"
 #include "libpq/libpq.h"
 #include "libpq/pqformat.h"
+#include "protocol.h"
 #include "tcop/dest.h"
 #include "utils/builtins.h"
 #include "utils/timestamp.h"
@@ -152,7 +153,7 @@ bbsink_copystream_begin_backup(bbsink *sink)
 	SendTablespaceList(state->tablespaces);
 
 	/* Send a CommandComplete message */
-	pq_puttextmessage('C', "SELECT");
+	pq_puttextmessage(PqMsgCommandCompleteResponse, "SELECT");
 
 	/* Begin COPY stream. This will be used for all archives + manifest. */
 	SendCopyOutResponse();
@@ -169,7 +170,7 @@ bbsink_copystream_begin_archive(bbsink *sink, const char *archive_name)
 	StringInfoData buf;
 
 	ti = list_nth(state->tablespaces, state->tablespace_num);
-	pq_beginmessage(&buf, 'd'); /* CopyData */
+	pq_beginmessage(&buf, PqMsgCopyDataRequest);
 	pq_sendbyte(&buf, 'n');		/* New archive */
 	pq_sendstring(&buf, archive_name);
 	pq_sendstring(&buf, ti->path == NULL ? "" : ti->path);
@@ -220,8 +221,8 @@ bbsink_copystream_archive_contents(bbsink *sink, size_t len)
 		{
 			mysink->last_progress_report_time = now;
 
-			pq_beginmessage(&buf, 'd'); /* CopyData */
-			pq_sendbyte(&buf, 'p'); /* Progress report */
+			pq_beginmessage(&buf, PqMsgCopyDataRequest);
+			pq_sendbyte(&buf, PqMsgCopyProgressRequest);
 			pq_sendint64(&buf, state->bytes_done);
 			pq_endmessage(&buf);
 			pq_flush_if_writable();
@@ -246,8 +247,8 @@ bbsink_copystream_end_archive(bbsink *sink)
 
 	mysink->bytes_done_at_last_time_check = state->bytes_done;
 	mysink->last_progress_report_time = GetCurrentTimestamp();
-	pq_beginmessage(&buf, 'd'); /* CopyData */
-	pq_sendbyte(&buf, 'p');		/* Progress report */
+	pq_beginmessage(&buf, PqMsgCopyDataRequest);
+	pq_sendbyte(&buf, PqMsgCopyProgressRequest);
 	pq_sendint64(&buf, state->bytes_done);
 	pq_endmessage(&buf);
 	pq_flush_if_writable();
@@ -261,7 +262,7 @@ bbsink_copystream_begin_manifest(bbsink *sink)
 {
 	StringInfoData buf;
 
-	pq_beginmessage(&buf, 'd'); /* CopyData */
+	pq_beginmessage(&buf, PqMsgCopyDataRequest);
 	pq_sendbyte(&buf, 'm');		/* Manifest */
 	pq_endmessage(&buf);
 }
@@ -318,7 +319,7 @@ SendCopyOutResponse(void)
 {
 	StringInfoData buf;
 
-	pq_beginmessage(&buf, 'H');
+	pq_beginmessage(&buf, PqMsgCopyOutResponse);
 	pq_sendbyte(&buf, 0);		/* overall format */
 	pq_sendint16(&buf, 0);		/* natts */
 	pq_endmessage(&buf);
@@ -330,7 +331,7 @@ SendCopyOutResponse(void)
 static void
 SendCopyDone(void)
 {
-	pq_putemptymessage('c');
+	pq_putemptymessage(PqMsgCopyDoneRequest);
 }
 
 /*
@@ -368,7 +369,7 @@ SendXlogRecPtrResult(XLogRecPtr ptr, TimeLineID tli)
 	end_tup_output(tstate);
 
 	/* Send a CommandComplete message */
-	pq_puttextmessage('C', "SELECT");
+	pq_puttextmessage(PqMsgCommandCompleteResponse, "SELECT");
 }
 
 /*
diff --git a/src/backend/commands/async.c b/src/backend/commands/async.c
index ef909cf4e0..27e2401601 100644
--- a/src/backend/commands/async.c
+++ b/src/backend/commands/async.c
@@ -141,6 +141,7 @@
 #include "libpq/libpq.h"
 #include "libpq/pqformat.h"
 #include "miscadmin.h"
+#include "protocol.h"
 #include "storage/ipc.h"
 #include "storage/lmgr.h"
 #include "storage/proc.h"
@@ -2281,7 +2282,7 @@ NotifyMyFrontEnd(const char *channel, const char *payload, int32 srcPid)
 	{
 		StringInfoData buf;
 
-		pq_beginmessage(&buf, 'A');
+		pq_beginmessage(&buf, PqMsgNotifyResponse);
 		pq_sendint32(&buf, srcPid);
 		pq_sendstring(&buf, channel);
 		pq_sendstring(&buf, payload);
diff --git a/src/backend/commands/copyfromparse.c b/src/backend/commands/copyfromparse.c
index 232768a6e1..7e90e2da5d 100644
--- a/src/backend/commands/copyfromparse.c
+++ b/src/backend/commands/copyfromparse.c
@@ -72,6 +72,7 @@
 #include "miscadmin.h"
 #include "pgstat.h"
 #include "port/pg_bswap.h"
+#include "protocol.h"
 #include "utils/builtins.h"
 #include "utils/memutils.h"
 #include "utils/rel.h"
@@ -174,7 +175,7 @@ ReceiveCopyBegin(CopyFromState cstate)
 	int16		format = (cstate->opts.binary ? 1 : 0);
 	int			i;
 
-	pq_beginmessage(&buf, 'G');
+	pq_beginmessage(&buf, PqMsgCopyInResponse);
 	pq_sendbyte(&buf, format);	/* overall format */
 	pq_sendint16(&buf, natts);
 	for (i = 0; i < natts; i++)
@@ -279,13 +280,13 @@ CopyGetData(CopyFromState cstate, void *databuf, int minread, int maxread)
 					/* Validate message type and set packet size limit */
 					switch (mtype)
 					{
-						case 'd':	/* CopyData */
+						case PqMsgCopyDataRequest:
 							maxmsglen = PQ_LARGE_MESSAGE_LIMIT;
 							break;
-						case 'c':	/* CopyDone */
-						case 'f':	/* CopyFail */
-						case 'H':	/* Flush */
-						case 'S':	/* Sync */
+						case PqMsgCopyDoneRequest:
+						case PqMsgCopyFailRequest:
+						case PqMsgFlushDataRequest:
+						case PqMsgSyncDataRequest:
 							maxmsglen = PQ_SMALL_MESSAGE_LIMIT;
 							break;
 						default:
@@ -305,20 +306,20 @@ CopyGetData(CopyFromState cstate, void *databuf, int minread, int maxread)
 					/* ... and process it */
 					switch (mtype)
 					{
-						case 'd':	/* CopyData */
+						case PqMsgCopyDataRequest:
 							break;
-						case 'c':	/* CopyDone */
+						case PqMsgCopyDoneRequest:
 							/* COPY IN correctly terminated by frontend */
 							cstate->raw_reached_eof = true;
 							return bytesread;
-						case 'f':	/* CopyFail */
+						case PqMsgCopyFailRequest:
 							ereport(ERROR,
 									(errcode(ERRCODE_QUERY_CANCELED),
 									 errmsg("COPY from stdin failed: %s",
 											pq_getmsgstring(cstate->fe_msgbuf))));
 							break;
-						case 'H':	/* Flush */
-						case 'S':	/* Sync */
+						case PqMsgFlushDataRequest:
+						case PqMsgSyncDataRequest:
 
 							/*
 							 * Ignore Flush/Sync for the convenience of client
diff --git a/src/backend/commands/copyto.c b/src/backend/commands/copyto.c
index 9e4b2437a5..2b832bed1e 100644
--- a/src/backend/commands/copyto.c
+++ b/src/backend/commands/copyto.c
@@ -34,6 +34,7 @@
 #include "miscadmin.h"
 #include "optimizer/optimizer.h"
 #include "pgstat.h"
+#include "protocol.h"
 #include "rewrite/rewriteHandler.h"
 #include "storage/fd.h"
 #include "tcop/tcopprot.h"
@@ -144,7 +145,7 @@ SendCopyBegin(CopyToState cstate)
 	int16		format = (cstate->opts.binary ? 1 : 0);
 	int			i;
 
-	pq_beginmessage(&buf, 'H');
+	pq_beginmessage(&buf, PqMsgCopyOutResponse);
 	pq_sendbyte(&buf, format);	/* overall format */
 	pq_sendint16(&buf, natts);
 	for (i = 0; i < natts; i++)
@@ -159,7 +160,7 @@ SendCopyEnd(CopyToState cstate)
 	/* Shouldn't have any unsent data */
 	Assert(cstate->fe_msgbuf->len == 0);
 	/* Send Copy Done message */
-	pq_putemptymessage('c');
+	pq_putemptymessage(PqMsgCopyDoneRequest);
 }
 
 /*----------
@@ -247,7 +248,7 @@ CopySendEndOfRow(CopyToState cstate)
 				CopySendChar(cstate, '\n');
 
 			/* Dump the accumulated row as one CopyData message */
-			(void) pq_putmessage('d', fe_msgbuf->data, fe_msgbuf->len);
+			(void) pq_putmessage(PqMsgCopyDataRequest, fe_msgbuf->data, fe_msgbuf->len);
 			break;
 		case COPY_CALLBACK:
 			cstate->data_dest_cb(fe_msgbuf->data, fe_msgbuf->len);
diff --git a/src/backend/libpq/auth-sasl.c b/src/backend/libpq/auth-sasl.c
index 684680897b..7327b7316c 100644
--- a/src/backend/libpq/auth-sasl.c
+++ b/src/backend/libpq/auth-sasl.c
@@ -19,6 +19,7 @@
 #include "libpq/libpq.h"
 #include "libpq/pqformat.h"
 #include "libpq/sasl.h"
+#include "protocol.h"
 
 /*
  * Maximum accepted size of SASL messages.
@@ -87,7 +88,7 @@ CheckSASLAuth(const pg_be_sasl_mech *mech, Port *port, char *shadow_pass,
 	{
 		pq_startmsgread();
 		mtype = pq_getbyte();
-		if (mtype != 'p')
+		if (mtype != PqMsgPasswordResponse)
 		{
 			/* Only log error if client didn't disconnect. */
 			if (mtype != EOF)
diff --git a/src/backend/libpq/auth.c b/src/backend/libpq/auth.c
index 315a24bb3f..fb572cfe8c 100644
--- a/src/backend/libpq/auth.c
+++ b/src/backend/libpq/auth.c
@@ -34,6 +34,7 @@
 #include "libpq/scram.h"
 #include "miscadmin.h"
 #include "port/pg_bswap.h"
+#include "protocol.h"
 #include "postmaster/postmaster.h"
 #include "replication/walsender.h"
 #include "storage/ipc.h"
@@ -665,7 +666,7 @@ sendAuthRequest(Port *port, AuthRequest areq, const char *extradata, int extrale
 
 	CHECK_FOR_INTERRUPTS();
 
-	pq_beginmessage(&buf, 'R');
+	pq_beginmessage(&buf, PqMsgAuthenticationRequest);
 	pq_sendint32(&buf, (int32) areq);
 	if (extralen > 0)
 		pq_sendbytes(&buf, extradata, extralen);
@@ -698,7 +699,7 @@ recv_password_packet(Port *port)
 
 	/* Expect 'p' message type */
 	mtype = pq_getbyte();
-	if (mtype != 'p')
+	if (mtype != PqMsgPasswordResponse)
 	{
 		/*
 		 * If the client just disconnects without offering a password, don't
@@ -961,7 +962,7 @@ pg_GSS_recvauth(Port *port)
 		CHECK_FOR_INTERRUPTS();
 
 		mtype = pq_getbyte();
-		if (mtype != 'p')
+		if (mtype != PqMsgPasswordResponse)
 		{
 			/* Only log error if client didn't disconnect. */
 			if (mtype != EOF)
@@ -1232,7 +1233,7 @@ pg_SSPI_recvauth(Port *port)
 	{
 		pq_startmsgread();
 		mtype = pq_getbyte();
-		if (mtype != 'p')
+		if (mtype != PqMsgPasswordResponse)
 		{
 			if (sspictx != NULL)
 			{
diff --git a/src/backend/postmaster/postmaster.c b/src/backend/postmaster/postmaster.c
index 9c8ec779f9..d4c8c40596 100644
--- a/src/backend/postmaster/postmaster.c
+++ b/src/backend/postmaster/postmaster.c
@@ -114,6 +114,7 @@
 #include "postmaster/pgarch.h"
 #include "postmaster/postmaster.h"
 #include "postmaster/syslogger.h"
+#include "protocol.h"
 #include "replication/logicallauncher.h"
 #include "replication/walsender.h"
 #include "storage/fd.h"
@@ -2357,7 +2358,7 @@ SendNegotiateProtocolVersion(List *unrecognized_protocol_options)
 	StringInfoData buf;
 	ListCell   *lc;
 
-	pq_beginmessage(&buf, 'v'); /* NegotiateProtocolVersion */
+	pq_beginmessage(&buf, PqMsgNegotiateProtocolResponse);
 	pq_sendint32(&buf, PG_PROTOCOL_LATEST);
 	pq_sendint32(&buf, list_length(unrecognized_protocol_options));
 	foreach(lc, unrecognized_protocol_options)
diff --git a/src/backend/replication/walsender.c b/src/backend/replication/walsender.c
index d27ef2985d..99f357519a 100644
--- a/src/backend/replication/walsender.c
+++ b/src/backend/replication/walsender.c
@@ -69,6 +69,7 @@
 #include "nodes/replnodes.h"
 #include "pgstat.h"
 #include "postmaster/interrupt.h"
+#include "protocol.h"
 #include "replication/decode.h"
 #include "replication/logical.h"
 #include "replication/slot.h"
@@ -603,7 +604,7 @@ SendTimeLineHistory(TimeLineHistoryCmd *cmd)
 	dest->rStartup(dest, CMD_SELECT, tupdesc);
 
 	/* Send a DataRow message */
-	pq_beginmessage(&buf, 'D');
+	pq_beginmessage(&buf, PqMsgDataRowResponse);
 	pq_sendint16(&buf, 2);		/* # of columns */
 	len = strlen(histfname);
 	pq_sendint32(&buf, len);	/* col1 len */
@@ -801,7 +802,7 @@ StartReplication(StartReplicationCmd *cmd)
 		WalSndSetState(WALSNDSTATE_CATCHUP);
 
 		/* Send a CopyBothResponse message, and start streaming */
-		pq_beginmessage(&buf, 'W');
+		pq_beginmessage(&buf, PqMsgCopyBothResponse);
 		pq_sendbyte(&buf, 0);
 		pq_sendint16(&buf, 0);
 		pq_endmessage(&buf);
@@ -1294,7 +1295,7 @@ StartLogicalReplication(StartReplicationCmd *cmd)
 	WalSndSetState(WALSNDSTATE_CATCHUP);
 
 	/* Send a CopyBothResponse message, and start streaming */
-	pq_beginmessage(&buf, 'W');
+	pq_beginmessage(&buf, PqMsgCopyBothResponse);
 	pq_sendbyte(&buf, 0);
 	pq_sendint16(&buf, 0);
 	pq_endmessage(&buf);
@@ -1923,11 +1924,11 @@ ProcessRepliesIfAny(void)
 		/* Validate message type and set packet size limit */
 		switch (firstchar)
 		{
-			case 'd':
+			case PqMsgCopyDataRequest:
 				maxmsglen = PQ_LARGE_MESSAGE_LIMIT;
 				break;
-			case 'c':
-			case 'X':
+			case PqMsgCopyDoneRequest:
+			case PqMsgTerminateRequest:
 				maxmsglen = PQ_SMALL_MESSAGE_LIMIT;
 				break;
 			default:
@@ -1955,7 +1956,7 @@ ProcessRepliesIfAny(void)
 				/*
 				 * 'd' means a standby reply wrapped in a CopyData packet.
 				 */
-			case 'd':
+			case PqMsgCopyDataRequest:
 				ProcessStandbyMessage();
 				received = true;
 				break;
@@ -1964,7 +1965,7 @@ ProcessRepliesIfAny(void)
 				 * CopyDone means the standby requested to finish streaming.
 				 * Reply with CopyDone, if we had not sent that already.
 				 */
-			case 'c':
+			case PqMsgCopyDoneRequest:
 				if (!streamingDoneSending)
 				{
 					pq_putmessage_noblock('c', NULL, 0);
@@ -1978,7 +1979,7 @@ ProcessRepliesIfAny(void)
 				/*
 				 * 'X' means that the standby is closing down the socket.
 				 */
-			case 'X':
+			case PqMsgTerminateRequest:
 				proc_exit(0);
 
 			default:
diff --git a/src/backend/tcop/dest.c b/src/backend/tcop/dest.c
index c0406e2ee5..2f95678b41 100644
--- a/src/backend/tcop/dest.c
+++ b/src/backend/tcop/dest.c
@@ -39,6 +39,7 @@
 #include "executor/tstoreReceiver.h"
 #include "libpq/libpq.h"
 #include "libpq/pqformat.h"
+#include "protocol.h"
 #include "utils/portal.h"
 
 
@@ -176,7 +177,7 @@ EndCommand(const QueryCompletion *qc, CommandDest dest, bool force_undecorated_o
 
 			len = BuildQueryCompletionString(completionTag, qc,
 											 force_undecorated_output);
-			pq_putmessage('C', completionTag, len + 1);
+			pq_putmessage(PqMsgCloseRequest, completionTag, len + 1);
 
 		case DestNone:
 		case DestDebug:
@@ -200,7 +201,7 @@ EndCommand(const QueryCompletion *qc, CommandDest dest, bool force_undecorated_o
 void
 EndReplicationCommand(const char *commandTag)
 {
-	pq_putmessage('C', commandTag, strlen(commandTag) + 1);
+	pq_putmessage(PqMsgCloseRequest, commandTag, strlen(commandTag) + 1);
 }
 
 /* ----------------
@@ -220,7 +221,7 @@ NullCommand(CommandDest dest)
 		case DestRemoteSimple:
 
 			/* Tell the FE that we saw an empty query string */
-			pq_putemptymessage('I');
+			pq_putemptymessage(PqMsgEmptyQueryResponse);
 			break;
 
 		case DestNone:
@@ -258,7 +259,7 @@ ReadyForQuery(CommandDest dest)
 			{
 				StringInfoData buf;
 
-				pq_beginmessage(&buf, 'Z');
+				pq_beginmessage(&buf, PqMsgReadyForQueryResponse);
 				pq_sendbyte(&buf, TransactionBlockStatusCode());
 				pq_endmessage(&buf);
 			}
diff --git a/src/backend/tcop/fastpath.c b/src/backend/tcop/fastpath.c
index 2f70ebd5fa..51d5957b7b 100644
--- a/src/backend/tcop/fastpath.c
+++ b/src/backend/tcop/fastpath.c
@@ -27,6 +27,7 @@
 #include "mb/pg_wchar.h"
 #include "miscadmin.h"
 #include "port/pg_bswap.h"
+#include "protocol.h"
 #include "tcop/fastpath.h"
 #include "tcop/tcopprot.h"
 #include "utils/acl.h"
@@ -69,7 +70,7 @@ SendFunctionResult(Datum retval, bool isnull, Oid rettype, int16 format)
 {
 	StringInfoData buf;
 
-	pq_beginmessage(&buf, 'V');
+	pq_beginmessage(&buf, PqMsgFunctionCallResponse);
 
 	if (isnull)
 	{
diff --git a/src/backend/tcop/postgres.c b/src/backend/tcop/postgres.c
index 36cc99ec9c..849e51f4d1 100644
--- a/src/backend/tcop/postgres.c
+++ b/src/backend/tcop/postgres.c
@@ -55,6 +55,7 @@
 #include "postmaster/autovacuum.h"
 #include "postmaster/interrupt.h"
 #include "postmaster/postmaster.h"
+#include "protocol.h"
 #include "replication/logicallauncher.h"
 #include "replication/logicalworker.h"
 #include "replication/slot.h"
@@ -402,37 +403,37 @@ SocketBackend(StringInfo inBuf)
 	 */
 	switch (qtype)
 	{
-		case 'Q':				/* simple query */
+		case PqMsgSimpleQueryRequest:
 			maxmsglen = PQ_LARGE_MESSAGE_LIMIT;
 			doing_extended_query_message = false;
 			break;
 
-		case 'F':				/* fastpath function call */
+		case PqMsgFunctionCallRequest:
 			maxmsglen = PQ_LARGE_MESSAGE_LIMIT;
 			doing_extended_query_message = false;
 			break;
 
-		case 'X':				/* terminate */
+		case PqMsgTerminateRequest:
 			maxmsglen = PQ_SMALL_MESSAGE_LIMIT;
 			doing_extended_query_message = false;
 			ignore_till_sync = false;
 			break;
 
-		case 'B':				/* bind */
-		case 'P':				/* parse */
+		case PqMsgBindRequest :
+		case PqMsgParseRequest:
 			maxmsglen = PQ_LARGE_MESSAGE_LIMIT;
 			doing_extended_query_message = true;
 			break;
 
-		case 'C':				/* close */
-		case 'D':				/* describe */
-		case 'E':				/* execute */
-		case 'H':				/* flush */
+		case PqMsgCloseRequest:
+		case PqMsgDescribeRequest:
+		case PqMsgExecuteRequest:
+		case PqMsgFlushDataRequest:
 			maxmsglen = PQ_SMALL_MESSAGE_LIMIT;
 			doing_extended_query_message = true;
 			break;
 
-		case 'S':				/* sync */
+		case PqMsgSyncDataRequest:
 			maxmsglen = PQ_SMALL_MESSAGE_LIMIT;
 			/* stop any active skip-till-Sync */
 			ignore_till_sync = false;
@@ -440,13 +441,13 @@ SocketBackend(StringInfo inBuf)
 			doing_extended_query_message = false;
 			break;
 
-		case 'd':				/* copy data */
+		case PqMsgCopyDataRequest:
 			maxmsglen = PQ_LARGE_MESSAGE_LIMIT;
 			doing_extended_query_message = false;
 			break;
 
-		case 'c':				/* copy done */
-		case 'f':				/* copy fail */
+		case PqMsgCopyDoneRequest:
+		case PqMsgCopyFailRequest:
 			maxmsglen = PQ_SMALL_MESSAGE_LIMIT;
 			doing_extended_query_message = false;
 			break;
@@ -1589,7 +1590,7 @@ exec_parse_message(const char *query_string,	/* string to execute */
 	 * Send ParseComplete.
 	 */
 	if (whereToSendOutput == DestRemote)
-		pq_putemptymessage('1');
+		pq_putemptymessage(PqMsgParseCompleteResponse);
 
 	/*
 	 * Emit duration logging if appropriate.
@@ -2047,7 +2048,7 @@ exec_bind_message(StringInfo input_message)
 	 * Send BindComplete.
 	 */
 	if (whereToSendOutput == DestRemote)
-		pq_putemptymessage('2');
+		pq_putemptymessage(PqMsgBindCompleteResponse);
 
 	/*
 	 * Emit duration logging if appropriate.
@@ -2290,7 +2291,7 @@ exec_execute_message(const char *portal_name, long max_rows)
 	{
 		/* Portal run not complete, so send PortalSuspended */
 		if (whereToSendOutput == DestRemote)
-			pq_putemptymessage('s');
+			pq_putemptymessage(PqMsgPortalSuspendedResponse);
 
 		/*
 		 * Set XACT_FLAGS_PIPELINING whenever we suspend an Execute message,
@@ -2683,7 +2684,7 @@ exec_describe_statement_message(const char *stmt_name)
 								  NULL);
 	}
 	else
-		pq_putemptymessage('n');	/* NoData */
+		pq_putemptymessage(PqMsgNoDataResponse);
 }
 
 /*
@@ -2736,7 +2737,7 @@ exec_describe_portal_message(const char *portal_name)
 								  FetchPortalTargetList(portal),
 								  portal->formats);
 	else
-		pq_putemptymessage('n');	/* NoData */
+		pq_putemptymessage(PqMsgNoDataResponse);
 }
 
 
@@ -4239,7 +4240,7 @@ PostgresMain(const char *dbname, const char *username)
 	{
 		StringInfoData buf;
 
-		pq_beginmessage(&buf, 'K');
+		pq_beginmessage(&buf, PqMsgBackendKeyDataRequest);
 		pq_sendint32(&buf, (int32) MyProcPid);
 		pq_sendint32(&buf, (int32) MyCancelKey);
 		pq_endmessage(&buf);
@@ -4618,7 +4619,7 @@ PostgresMain(const char *dbname, const char *username)
 
 		switch (firstchar)
 		{
-			case 'Q':			/* simple query */
+			case PqMsgSimpleQueryRequest:
 				{
 					const char *query_string;
 
@@ -4642,7 +4643,7 @@ PostgresMain(const char *dbname, const char *username)
 				}
 				break;
 
-			case 'P':			/* parse */
+			case PqMsgParseRequest:
 				{
 					const char *stmt_name;
 					const char *query_string;
@@ -4672,7 +4673,7 @@ PostgresMain(const char *dbname, const char *username)
 				}
 				break;
 
-			case 'B':			/* bind */
+			case PqMsgBindRequest:
 				forbidden_in_wal_sender(firstchar);
 
 				/* Set statement_timestamp() */
@@ -4687,7 +4688,7 @@ PostgresMain(const char *dbname, const char *username)
 				/* exec_bind_message does valgrind_report_error_query */
 				break;
 
-			case 'E':			/* execute */
+			case PqMsgExecuteRequest:
 				{
 					const char *portal_name;
 					int			max_rows;
@@ -4707,7 +4708,7 @@ PostgresMain(const char *dbname, const char *username)
 				}
 				break;
 
-			case 'F':			/* fastpath function call */
+			case PqMsgFunctionCallRequest:
 				forbidden_in_wal_sender(firstchar);
 
 				/* Set statement_timestamp() */
@@ -4742,7 +4743,7 @@ PostgresMain(const char *dbname, const char *username)
 				send_ready_for_query = true;
 				break;
 
-			case 'C':			/* close */
+			case PqMsgCloseRequest:
 				{
 					int			close_type;
 					const char *close_target;
@@ -4755,7 +4756,7 @@ PostgresMain(const char *dbname, const char *username)
 
 					switch (close_type)
 					{
-						case 'S':
+						case PqMsgPreparedSubCommand:
 							if (close_target[0] != '\0')
 								DropPreparedStatement(close_target, false);
 							else
@@ -4764,7 +4765,7 @@ PostgresMain(const char *dbname, const char *username)
 								drop_unnamed_stmt();
 							}
 							break;
-						case 'P':
+						case PqMsgPortalSubCommand:
 							{
 								Portal		portal;
 
@@ -4782,13 +4783,13 @@ PostgresMain(const char *dbname, const char *username)
 					}
 
 					if (whereToSendOutput == DestRemote)
-						pq_putemptymessage('3');	/* CloseComplete */
+						pq_putemptymessage(PqMsgCloseCompleteResponse);
 
 					valgrind_report_error_query("CLOSE message");
 				}
 				break;
 
-			case 'D':			/* describe */
+			case PqMsgDescribeRequest:
 				{
 					int			describe_type;
 					const char *describe_target;
@@ -4804,10 +4805,10 @@ PostgresMain(const char *dbname, const char *username)
 
 					switch (describe_type)
 					{
-						case 'S':
+						case PqMsgPreparedSubCommand:
 							exec_describe_statement_message(describe_target);
 							break;
-						case 'P':
+						case PqMsgPortalSubCommand:
 							exec_describe_portal_message(describe_target);
 							break;
 						default:
@@ -4822,13 +4823,13 @@ PostgresMain(const char *dbname, const char *username)
 				}
 				break;
 
-			case 'H':			/* flush */
+			case PqMsgFlushDataRequest:
 				pq_getmsgend(&input_message);
 				if (whereToSendOutput == DestRemote)
 					pq_flush();
 				break;
 
-			case 'S':			/* sync */
+			case PqMsgSyncDataRequest:
 				pq_getmsgend(&input_message);
 				finish_xact_command();
 				valgrind_report_error_query("SYNC message");
@@ -4847,7 +4848,7 @@ PostgresMain(const char *dbname, const char *username)
 
 				/* FALLTHROUGH */
 
-			case 'X':
+			case PqMsgTerminateRequest:
 
 				/*
 				 * Reset whereToSendOutput to prevent ereport from attempting
@@ -4865,9 +4866,9 @@ PostgresMain(const char *dbname, const char *username)
 				 */
 				proc_exit(0);
 
-			case 'd':			/* copy data */
-			case 'c':			/* copy done */
-			case 'f':			/* copy fail */
+			case PqMsgCopyDataRequest:
+			case PqMsgCopyDoneRequest:
+			case PqMsgCopyFailRequest:
 
 				/*
 				 * Accept but ignore these messages, per protocol spec; we
@@ -4897,7 +4898,7 @@ forbidden_in_wal_sender(char firstchar)
 {
 	if (am_walsender)
 	{
-		if (firstchar == 'F')
+		if (firstchar == PqMsgFunctionCallRequest)
 			ereport(ERROR,
 					(errcode(ERRCODE_PROTOCOL_VIOLATION),
 					 errmsg("fastpath function calls not supported in a replication connection")));
diff --git a/src/backend/utils/activity/backend_progress.c b/src/backend/utils/activity/backend_progress.c
index 67447ef03a..1059677857 100644
--- a/src/backend/utils/activity/backend_progress.c
+++ b/src/backend/utils/activity/backend_progress.c
@@ -10,6 +10,7 @@
  */
 #include "postgres.h"
 
+#include "protocol.h"
 #include "access/parallel.h"
 #include "libpq/pqformat.h"
 #include "port/atomics.h"		/* for memory barriers */
@@ -102,7 +103,7 @@ pgstat_progress_parallel_incr_param(int index, int64 incr)
 
 		initStringInfo(&progress_message);
 
-		pq_beginmessage(&progress_message, 'P');
+		pq_beginmessage(&progress_message, PqMsgParallelProgressResponse);
 		pq_sendint32(&progress_message, index);
 		pq_sendint64(&progress_message, incr);
 		pq_endmessage(&progress_message);
diff --git a/src/backend/utils/error/elog.c b/src/backend/utils/error/elog.c
index 5898100acb..7c1dc4e69b 100644
--- a/src/backend/utils/error/elog.c
+++ b/src/backend/utils/error/elog.c
@@ -77,6 +77,7 @@
 #include "postmaster/bgworker.h"
 #include "postmaster/postmaster.h"
 #include "postmaster/syslogger.h"
+#include "protocol.h"
 #include "storage/ipc.h"
 #include "storage/proc.h"
 #include "tcop/tcopprot.h"
@@ -3465,7 +3466,7 @@ send_message_to_frontend(ErrorData *edata)
 		char		tbuf[12];
 
 		/* 'N' (Notice) is for nonfatal conditions, 'E' is for errors */
-		pq_beginmessage(&msgbuf, (edata->elevel < ERROR) ? 'N' : 'E');
+		pq_beginmessage(&msgbuf, (edata->elevel < ERROR) ? PqMsgNoticeResponse : PqMsgErrorResponse);
 
 		sev = error_severity(edata->elevel);
 		pq_sendbyte(&msgbuf, PG_DIAG_SEVERITY);
diff --git a/src/backend/utils/misc/guc.c b/src/backend/utils/misc/guc.c
index 5308896c87..55b9b8bbed 100644
--- a/src/backend/utils/misc/guc.c
+++ b/src/backend/utils/misc/guc.c
@@ -37,6 +37,7 @@
 #include "libpq/pqformat.h"
 #include "parser/scansup.h"
 #include "port/pg_bitutils.h"
+#include "protocol.h"
 #include "storage/fd.h"
 #include "storage/lwlock.h"
 #include "storage/shmem.h"
@@ -2593,7 +2594,7 @@ ReportGUCOption(struct config_generic *record)
 	{
 		StringInfoData msgbuf;
 
-		pq_beginmessage(&msgbuf, 'S');
+		pq_beginmessage(&msgbuf, PqMsgParameterStatusResponse);
 		pq_sendstring(&msgbuf, record->name);
 		pq_sendstring(&msgbuf, val);
 		pq_endmessage(&msgbuf);
diff --git a/src/include/protocol.h b/src/include/protocol.h
new file mode 100644
index 0000000000..b26f6a04f3
--- /dev/null
+++ b/src/include/protocol.h
@@ -0,0 +1,60 @@
+/*-------------------------------------------------------------------------
+ *
+ * protocol.h
+ *	  Exports from postmaster/postmaster.c.
+ *
+ * Portions Copyright (c) 1996-2023, PostgreSQL Global Development Group
+ *
+ * src/include/protocol.h
+ *
+ *-------------------------------------------------------------------------
+ */
+#ifndef _PROTOCOL_H
+#define _PROTOCOL_H
+
+#define PqMsgBindRequest            'B'
+#define PqMsgCloseRequest           'C'
+#define PqMsgDescribeRequest        'D'
+#define PqMsgExecuteRequest         'E'
+#define PqMsgFunctionCallRequest    'F'
+#define PqMsgFlushDataRequest       'H'
+#define PqMsgBackendKeyDataRequest  'K'
+#define PqMsgParseRequest           'P'
+#define PqMsgAuthenticationRequest  'R'
+#define PqMsgSyncDataRequest        'S'
+#define PqMsgSimpleQueryRequest     'Q'
+#define PqMsgTerminateRequest       'X'
+#define PqMsgCopyFailRequest        'f'
+#define PqMsgCopyDoneRequest        'c'
+#define PqMsgCopyDataRequest        'd'
+#define PqMsgCopyProgressRequest    'p'
+#define PqMsgPreparedSubCommand     'S'
+#define PqMsgPortalSubCommand       'P'
+
+
+/*
+Responses
+*/
+#define PqMsgParseCompleteResponse    '1'
+#define PqMsgBindCompleteResponse     '2'
+#define PqMsgCloseCompleteResponse    '3'
+#define PqMsgNotifyResponse           'A'
+#define PqMsgCommandCompleteResponse  'C'
+#define PqMsgDataRowResponse          'D'
+#define PqMsgErrorResponse            'E'
+#define PqMsgCopyInResponse           'G'
+#define PqMsgCopyOutResponse          'H'
+#define PqMsgEmptyQueryResponse       'I'
+#define PqMsgNoticeResponse           'N'
+#define PqMsgParallelProgressResponse 'P'
+#define PqMsgFunctionCallResponse     'V'
+#define PqMsgParameterStatusResponse  'S'
+#define PqMsgRowDescriptionResponse   'T'
+#define PqMsgCopyBothResponse         'W'
+#define PqMsgReadyForQueryResponse    'Z'
+#define PqMsgNoDataResponse           'n'
+#define PqMsgPasswordResponse         'p'
+#define PqMsgPortalSuspendedResponse  's'
+#define PqMsgParameterDescriptionResponse 't'
+#define PqMsgNegotiateProtocolResponse    'v'
+#endif
\ No newline at end of file
diff --git a/src/interfaces/libpq/fe-auth.c b/src/interfaces/libpq/fe-auth.c
index 887ca5e9e1..0ee48112cb 100644
--- a/src/interfaces/libpq/fe-auth.c
+++ b/src/interfaces/libpq/fe-auth.c
@@ -43,6 +43,7 @@
 #include "fe-auth.h"
 #include "fe-auth-sasl.h"
 #include "libpq-fe.h"
+#include "protocol.h"
 
 #ifdef ENABLE_GSS
 /*
@@ -586,7 +587,7 @@ pg_SASL_init(PGconn *conn, int payloadlen)
 	/*
 	 * Build a SASLInitialResponse message, and send it.
 	 */
-	if (pqPutMsgStart('p', conn))
+	if (pqPutMsgStart(PqMsgPasswordResponse, conn))
 		goto error;
 	if (pqPuts(selected_mechanism, conn))
 		goto error;
diff --git a/src/interfaces/libpq/fe-connect.c b/src/interfaces/libpq/fe-connect.c
index 837c5321aa..b2a90adea4 100644
--- a/src/interfaces/libpq/fe-connect.c
+++ b/src/interfaces/libpq/fe-connect.c
@@ -32,6 +32,7 @@
 #include "mb/pg_wchar.h"
 #include "pg_config_paths.h"
 #include "port/pg_bswap.h"
+#include "protocol.h"
 
 #ifdef WIN32
 #include "win32.h"
@@ -3591,7 +3592,7 @@ keep_going:						/* We will come back to here until there is
 				 * Anything else probably means it's not Postgres on the other
 				 * end at all.
 				 */
-				if (!(beresp == 'R' || beresp == 'v' || beresp == 'E'))
+				if (!(beresp == PqMsgAuthenticationRequest || beresp == PqMsgNegotiateProtocolResponse || beresp == PqMsgErrorResponse))
 				{
 					libpq_append_conn_error(conn, "expected authentication request from server, but received %c",
 											beresp);
@@ -3618,19 +3619,19 @@ keep_going:						/* We will come back to here until there is
 				 * version 14, the server also used the old protocol for
 				 * errors that happened before processing the startup packet.)
 				 */
-				if (beresp == 'R' && (msgLength < 8 || msgLength > 2000))
+				if (beresp == PqMsgAuthenticationRequest && (msgLength < 8 || msgLength > 2000))
 				{
 					libpq_append_conn_error(conn, "received invalid authentication request");
 					goto error_return;
 				}
-				if (beresp == 'v' && (msgLength < 8 || msgLength > 2000))
+				if (beresp == PqMsgNegotiateProtocolResponse && (msgLength < 8 || msgLength > 2000))
 				{
 					libpq_append_conn_error(conn, "received invalid protocol negotiation message");
 					goto error_return;
 				}
 
 #define MAX_ERRLEN 30000
-				if (beresp == 'E' && (msgLength < 8 || msgLength > MAX_ERRLEN))
+				if (beresp == PqMsgErrorResponse && (msgLength < 8 || msgLength > MAX_ERRLEN))
 				{
 					/* Handle error from a pre-3.0 server */
 					conn->inCursor = conn->inStart + 1; /* reread data */
@@ -3693,7 +3694,7 @@ keep_going:						/* We will come back to here until there is
 				}
 
 				/* Handle errors. */
-				if (beresp == 'E')
+				if (beresp == PqMsgErrorResponse)
 				{
 					if (pqGetErrorNotice3(conn, true))
 					{
@@ -3770,7 +3771,7 @@ keep_going:						/* We will come back to here until there is
 
 					goto error_return;
 				}
-				else if (beresp == 'v')
+				else if (beresp == PqMsgNegotiateProtocolResponse)
 				{
 					if (pqGetNegotiateProtocolVersion3(conn))
 					{
@@ -4540,7 +4541,7 @@ sendTerminateConn(PGconn *conn)
 		 * Try to send "close connection" message to backend. Ignore any
 		 * error.
 		 */
-		pqPutMsgStart('X', conn);
+		pqPutMsgStart(PqMsgTerminateRequest, conn);
 		pqPutMsgEnd(conn);
 		(void) pqFlush(conn);
 	}
diff --git a/src/interfaces/libpq/fe-exec.c b/src/interfaces/libpq/fe-exec.c
index a868284ff8..67ed8e3682 100644
--- a/src/interfaces/libpq/fe-exec.c
+++ b/src/interfaces/libpq/fe-exec.c
@@ -27,6 +27,7 @@
 #include "libpq-fe.h"
 #include "libpq-int.h"
 #include "mb/pg_wchar.h"
+#include "protocol.h"
 
 /* keep this in same order as ExecStatusType in libpq-fe.h */
 char	   *const pgresStatus[] = {
@@ -1458,7 +1459,7 @@ PQsendQueryInternal(PGconn *conn, const char *query, bool newQuery)
 
 	/* Send the query message(s) */
 	/* construct the outgoing Query message */
-	if (pqPutMsgStart('Q', conn) < 0 ||
+	if (pqPutMsgStart(PqMsgSimpleQueryRequest, conn) < 0 ||
 		pqPuts(query, conn) < 0 ||
 		pqPutMsgEnd(conn) < 0)
 	{
@@ -1571,7 +1572,7 @@ PQsendPrepare(PGconn *conn,
 		return 0;				/* error msg already set */
 
 	/* construct the Parse message */
-	if (pqPutMsgStart('P', conn) < 0 ||
+	if (pqPutMsgStart(PqMsgParseRequest, conn) < 0 ||
 		pqPuts(stmtName, conn) < 0 ||
 		pqPuts(query, conn) < 0)
 		goto sendFailed;
@@ -1599,7 +1600,7 @@ PQsendPrepare(PGconn *conn,
 	/* Add a Sync, unless in pipeline mode. */
 	if (conn->pipelineStatus == PQ_PIPELINE_OFF)
 	{
-		if (pqPutMsgStart('S', conn) < 0 ||
+		if (pqPutMsgStart(PqMsgSyncDataRequest, conn) < 0 ||
 			pqPutMsgEnd(conn) < 0)
 			goto sendFailed;
 	}
@@ -1784,7 +1785,7 @@ PQsendQueryGuts(PGconn *conn,
 	if (command)
 	{
 		/* construct the Parse message */
-		if (pqPutMsgStart('P', conn) < 0 ||
+		if (pqPutMsgStart(PqMsgParseRequest, conn) < 0 ||
 			pqPuts(stmtName, conn) < 0 ||
 			pqPuts(command, conn) < 0)
 			goto sendFailed;
@@ -1808,7 +1809,7 @@ PQsendQueryGuts(PGconn *conn,
 	}
 
 	/* Construct the Bind message */
-	if (pqPutMsgStart('B', conn) < 0 ||
+	if (pqPutMsgStart(PqMsgBindRequest, conn) < 0 ||
 		pqPuts("", conn) < 0 ||
 		pqPuts(stmtName, conn) < 0)
 		goto sendFailed;
@@ -1874,14 +1875,14 @@ PQsendQueryGuts(PGconn *conn,
 		goto sendFailed;
 
 	/* construct the Describe Portal message */
-	if (pqPutMsgStart('D', conn) < 0 ||
-		pqPutc('P', conn) < 0 ||
+	if (pqPutMsgStart(PqMsgDescribeRequest, conn) < 0 ||
+		pqPutc(PqMsgPortalSubCommand, conn) < 0 ||
 		pqPuts("", conn) < 0 ||
 		pqPutMsgEnd(conn) < 0)
 		goto sendFailed;
 
 	/* construct the Execute message */
-	if (pqPutMsgStart('E', conn) < 0 ||
+	if (pqPutMsgStart(PqMsgExecuteRequest, conn) < 0 ||
 		pqPuts("", conn) < 0 ||
 		pqPutInt(0, 4, conn) < 0 ||
 		pqPutMsgEnd(conn) < 0)
@@ -1890,7 +1891,7 @@ PQsendQueryGuts(PGconn *conn,
 	/* construct the Sync message if not in pipeline mode */
 	if (conn->pipelineStatus == PQ_PIPELINE_OFF)
 	{
-		if (pqPutMsgStart('S', conn) < 0 ||
+		if (pqPutMsgStart(PqMsgSyncDataRequest, conn) < 0 ||
 			pqPutMsgEnd(conn) < 0)
 			goto sendFailed;
 	}
@@ -2422,7 +2423,7 @@ PQdescribePrepared(PGconn *conn, const char *stmt)
 {
 	if (!PQexecStart(conn))
 		return NULL;
-	if (!PQsendTypedCommand(conn, 'D', 'S', stmt))
+	if (!PQsendTypedCommand(conn, PqMsgDescribeRequest, PqMsgPreparedSubCommand, stmt))
 		return NULL;
 	return PQexecFinish(conn);
 }
@@ -2441,7 +2442,7 @@ PQdescribePortal(PGconn *conn, const char *portal)
 {
 	if (!PQexecStart(conn))
 		return NULL;
-	if (!PQsendTypedCommand(conn, 'D', 'P', portal))
+	if (!PQsendTypedCommand(conn, PqMsgDescribeRequest, PqMsgPortalSubCommand, portal))
 		return NULL;
 	return PQexecFinish(conn);
 }
@@ -2456,7 +2457,7 @@ PQdescribePortal(PGconn *conn, const char *portal)
 int
 PQsendDescribePrepared(PGconn *conn, const char *stmt)
 {
-	return PQsendTypedCommand(conn, 'D', 'S', stmt);
+	return PQsendTypedCommand(conn, PqMsgDescribeRequest, PqMsgPreparedSubCommand, stmt);
 }
 
 /*
@@ -2469,7 +2470,7 @@ PQsendDescribePrepared(PGconn *conn, const char *stmt)
 int
 PQsendDescribePortal(PGconn *conn, const char *portal)
 {
-	return PQsendTypedCommand(conn, 'D', 'P', portal);
+	return PQsendTypedCommand(conn, PqMsgDescribeRequest, PqMsgPortalSubCommand, portal);
 }
 
 /*
@@ -2488,7 +2489,7 @@ PQclosePrepared(PGconn *conn, const char *stmt)
 {
 	if (!PQexecStart(conn))
 		return NULL;
-	if (!PQsendTypedCommand(conn, 'C', 'S', stmt))
+	if (!PQsendTypedCommand(conn, PqMsgCloseRequest, PqMsgPreparedSubCommand, stmt))
 		return NULL;
 	return PQexecFinish(conn);
 }
@@ -2506,7 +2507,7 @@ PQclosePortal(PGconn *conn, const char *portal)
 {
 	if (!PQexecStart(conn))
 		return NULL;
-	if (!PQsendTypedCommand(conn, 'C', 'P', portal))
+	if (!PQsendTypedCommand(conn, PqMsgCloseRequest, PqMsgPortalSubCommand, portal))
 		return NULL;
 	return PQexecFinish(conn);
 }
@@ -2521,7 +2522,7 @@ PQclosePortal(PGconn *conn, const char *portal)
 int
 PQsendClosePrepared(PGconn *conn, const char *stmt)
 {
-	return PQsendTypedCommand(conn, 'C', 'S', stmt);
+	return PQsendTypedCommand(conn, PqMsgCloseRequest, PqMsgPreparedSubCommand, stmt);
 }
 
 /*
@@ -2534,7 +2535,7 @@ PQsendClosePrepared(PGconn *conn, const char *stmt)
 int
 PQsendClosePortal(PGconn *conn, const char *portal)
 {
-	return PQsendTypedCommand(conn, 'C', 'P', portal);
+	return PQsendTypedCommand(conn, PqMsgCloseRequest, PqMsgPortalSubCommand, portal);
 }
 
 /*
@@ -2577,17 +2578,17 @@ PQsendTypedCommand(PGconn *conn, char command, char type, const char *target)
 	/* construct the Sync message */
 	if (conn->pipelineStatus == PQ_PIPELINE_OFF)
 	{
-		if (pqPutMsgStart('S', conn) < 0 ||
+		if (pqPutMsgStart(PqMsgSyncDataRequest, conn) < 0 ||
 			pqPutMsgEnd(conn) < 0)
 			goto sendFailed;
 	}
 
 	/* remember if we are doing a Close or a Describe */
-	if (command == 'C')
+	if (command == PqMsgCloseRequest)
 	{
 		entry->queryclass = PGQUERY_CLOSE;
 	}
-	else if (command == 'D')
+	else if (command == PqMsgDescribeRequest)
 	{
 		entry->queryclass = PGQUERY_DESCRIBE;
 	}
@@ -2696,7 +2697,7 @@ PQputCopyData(PGconn *conn, const char *buffer, int nbytes)
 				return pqIsnonblocking(conn) ? 0 : -1;
 		}
 		/* Send the data (too simple to delegate to fe-protocol files) */
-		if (pqPutMsgStart('d', conn) < 0 ||
+		if (pqPutMsgStart(PqMsgCopyDataRequest, conn) < 0 ||
 			pqPutnchar(buffer, nbytes, conn) < 0 ||
 			pqPutMsgEnd(conn) < 0)
 			return -1;
@@ -2731,7 +2732,7 @@ PQputCopyEnd(PGconn *conn, const char *errormsg)
 	if (errormsg)
 	{
 		/* Send COPY FAIL */
-		if (pqPutMsgStart('f', conn) < 0 ||
+		if (pqPutMsgStart(PqMsgCopyFailRequest, conn) < 0 ||
 			pqPuts(errormsg, conn) < 0 ||
 			pqPutMsgEnd(conn) < 0)
 			return -1;
@@ -2739,7 +2740,7 @@ PQputCopyEnd(PGconn *conn, const char *errormsg)
 	else
 	{
 		/* Send COPY DONE */
-		if (pqPutMsgStart('c', conn) < 0 ||
+		if (pqPutMsgStart(PqMsgCopyDoneRequest, conn) < 0 ||
 			pqPutMsgEnd(conn) < 0)
 			return -1;
 	}
@@ -2751,7 +2752,7 @@ PQputCopyEnd(PGconn *conn, const char *errormsg)
 	if (conn->cmd_queue_head &&
 		conn->cmd_queue_head->queryclass != PGQUERY_SIMPLE)
 	{
-		if (pqPutMsgStart('S', conn) < 0 ||
+		if (pqPutMsgStart(PqMsgSyncDataRequest, conn) < 0 ||
 			pqPutMsgEnd(conn) < 0)
 			return -1;
 	}
@@ -3263,7 +3264,7 @@ PQpipelineSync(PGconn *conn)
 	entry->query = NULL;
 
 	/* construct the Sync message */
-	if (pqPutMsgStart('S', conn) < 0 ||
+	if (pqPutMsgStart(PqMsgSyncDataRequest, conn) < 0 ||
 		pqPutMsgEnd(conn) < 0)
 		goto sendFailed;
 
@@ -3311,7 +3312,7 @@ PQsendFlushRequest(PGconn *conn)
 		return 0;
 	}
 
-	if (pqPutMsgStart('H', conn) < 0 ||
+	if (pqPutMsgStart(PqMsgFlushDataRequest, conn) < 0 ||
 		pqPutMsgEnd(conn) < 0)
 	{
 		return 0;
diff --git a/src/interfaces/libpq/fe-protocol3.c b/src/interfaces/libpq/fe-protocol3.c
index 7bc6355d17..c7c1b079ac 100644
--- a/src/interfaces/libpq/fe-protocol3.c
+++ b/src/interfaces/libpq/fe-protocol3.c
@@ -28,14 +28,16 @@
 #include "libpq-int.h"
 #include "mb/pg_wchar.h"
 #include "port/pg_bswap.h"
+#include "protocol.h"
 
 /*
  * This macro lists the backend message types that could be "long" (more
  * than a couple of kilobytes).
  */
 #define VALID_LONG_MESSAGE_TYPE(id) \
-	((id) == 'T' || (id) == 'D' || (id) == 'd' || (id) == 'V' || \
-	 (id) == 'E' || (id) == 'N' || (id) == 'A')
+	((id) == PqMsgRowDescriptionResponse || (id) == PqMsgDataRowResponse || (id) == PqMsgCopyDataRequest || \
+	 (id) == PqMsgFunctionCallResponse ||  (id) == PqMsgErrorResponse || (id) == PqMsgNoticeResponse || \
+	 (id) == PqMsgNotifyResponse)
 
 
 static void handleSyncLoss(PGconn *conn, char id, int msgLength);
@@ -140,12 +142,12 @@ pqParseInput3(PGconn *conn)
 		 * from config file due to SIGHUP), but otherwise we hold off until
 		 * BUSY state.
 		 */
-		if (id == 'A')
+		if (id == PqMsgNotifyResponse)
 		{
 			if (getNotify(conn))
 				return;
 		}
-		else if (id == 'N')
+		else if (id == PqMsgNoticeResponse)
 		{
 			if (pqGetErrorNotice3(conn, false))
 				return;
@@ -165,12 +167,12 @@ pqParseInput3(PGconn *conn)
 			 * it is about to close the connection, so we don't want to just
 			 * discard it...)
 			 */
-			if (id == 'E')
+			if (id == PqMsgErrorResponse)
 			{
 				if (pqGetErrorNotice3(conn, false /* treat as notice */ ))
 					return;
 			}
-			else if (id == 'S')
+			else if (id == PqMsgParameterStatusResponse)
 			{
 				if (getParameterStatus(conn))
 					return;
@@ -192,7 +194,7 @@ pqParseInput3(PGconn *conn)
 			 */
 			switch (id)
 			{
-				case 'C':		/* command complete */
+				case PqMsgCommandCompleteResponse:
 					if (pqGets(&conn->workBuffer, conn))
 						return;
 					if (!pgHavePendingResult(conn))
@@ -210,12 +212,12 @@ pqParseInput3(PGconn *conn)
 								CMDSTATUS_LEN);
 					conn->asyncStatus = PGASYNC_READY;
 					break;
-				case 'E':		/* error return */
+				case PqMsgErrorResponse:
 					if (pqGetErrorNotice3(conn, true))
 						return;
 					conn->asyncStatus = PGASYNC_READY;
 					break;
-				case 'Z':		/* sync response, backend is ready for new
+				case PqMsgReadyForQueryResponse:		/* sync response, backend is ready for new
 								 * query */
 					if (getReadyForQuery(conn))
 						return;
@@ -246,7 +248,7 @@ pqParseInput3(PGconn *conn)
 						conn->asyncStatus = PGASYNC_IDLE;
 					}
 					break;
-				case 'I':		/* empty query */
+				case PqMsgEmptyQueryResponse:
 					if (!pgHavePendingResult(conn))
 					{
 						conn->result = PQmakeEmptyPGresult(conn,
@@ -259,7 +261,7 @@ pqParseInput3(PGconn *conn)
 					}
 					conn->asyncStatus = PGASYNC_READY;
 					break;
-				case '1':		/* Parse Complete */
+				case PqMsgParseCompleteResponse:
 					/* If we're doing PQprepare, we're done; else ignore */
 					if (conn->cmd_queue_head &&
 						conn->cmd_queue_head->queryclass == PGQUERY_PREPARE)
@@ -277,10 +279,10 @@ pqParseInput3(PGconn *conn)
 						conn->asyncStatus = PGASYNC_READY;
 					}
 					break;
-				case '2':		/* Bind Complete */
+				case PqMsgBindCompleteResponse:
 					/* Nothing to do for this message type */
 					break;
-				case '3':		/* Close Complete */
+				case PqMsgCloseCompleteResponse:
 					/* If we're doing PQsendClose, we're done; else ignore */
 					if (conn->cmd_queue_head &&
 						conn->cmd_queue_head->queryclass == PGQUERY_CLOSE)
@@ -298,11 +300,11 @@ pqParseInput3(PGconn *conn)
 						conn->asyncStatus = PGASYNC_READY;
 					}
 					break;
-				case 'S':		/* parameter status */
+				case PqMsgParameterStatusResponse:
 					if (getParameterStatus(conn))
 						return;
 					break;
-				case 'K':		/* secret key data from the backend */
+				case PqMsgBackendKeyDataRequest:		/* secret key data from the backend */
 
 					/*
 					 * This is expected only during backend startup, but it's
@@ -314,7 +316,7 @@ pqParseInput3(PGconn *conn)
 					if (pqGetInt(&(conn->be_key), 4, conn))
 						return;
 					break;
-				case 'T':		/* Row Description */
+				case PqMsgRowDescriptionResponse:
 					if (conn->error_result ||
 						(conn->result != NULL &&
 						 conn->result->resultStatus == PGRES_FATAL_ERROR))
@@ -346,7 +348,7 @@ pqParseInput3(PGconn *conn)
 						return;
 					}
 					break;
-				case 'n':		/* No Data */
+				case PqMsgNoDataResponse:
 
 					/*
 					 * NoData indicates that we will not be seeing a
@@ -374,11 +376,11 @@ pqParseInput3(PGconn *conn)
 						conn->asyncStatus = PGASYNC_READY;
 					}
 					break;
-				case 't':		/* Parameter Description */
+				case PqMsgParameterDescriptionResponse:
 					if (getParamDescriptions(conn, msgLength))
 						return;
 					break;
-				case 'D':		/* Data Row */
+				case PqMsgDataRowResponse:
 					if (conn->result != NULL &&
 						conn->result->resultStatus == PGRES_TUPLES_OK)
 					{
@@ -405,24 +407,24 @@ pqParseInput3(PGconn *conn)
 						conn->inCursor += msgLength;
 					}
 					break;
-				case 'G':		/* Start Copy In */
+				case PqMsgCopyInResponse:
 					if (getCopyStart(conn, PGRES_COPY_IN))
 						return;
 					conn->asyncStatus = PGASYNC_COPY_IN;
 					break;
-				case 'H':		/* Start Copy Out */
+				case PqMsgCopyOutResponse:
 					if (getCopyStart(conn, PGRES_COPY_OUT))
 						return;
 					conn->asyncStatus = PGASYNC_COPY_OUT;
 					conn->copy_already_done = 0;
 					break;
-				case 'W':		/* Start Copy Both */
+				case PqMsgCopyBothResponse:
 					if (getCopyStart(conn, PGRES_COPY_BOTH))
 						return;
 					conn->asyncStatus = PGASYNC_COPY_BOTH;
 					conn->copy_already_done = 0;
 					break;
-				case 'd':		/* Copy Data */
+				case PqMsgCopyDataRequest:
 
 					/*
 					 * If we see Copy Data, just silently drop it.  This would
@@ -431,7 +433,7 @@ pqParseInput3(PGconn *conn)
 					 */
 					conn->inCursor += msgLength;
 					break;
-				case 'c':		/* Copy Done */
+				case PqMsgCopyDoneRequest:
 
 					/*
 					 * If we see Copy Done, just silently drop it.  This is
@@ -1692,21 +1694,21 @@ getCopyDataMessage(PGconn *conn)
 		 */
 		switch (id)
 		{
-			case 'A':			/* NOTIFY */
+			case PqMsgNotifyResponse:
 				if (getNotify(conn))
 					return 0;
 				break;
-			case 'N':			/* NOTICE */
+			case PqMsgNoticeResponse:
 				if (pqGetErrorNotice3(conn, false))
 					return 0;
 				break;
-			case 'S':			/* ParameterStatus */
+			case PqMsgParameterStatusResponse:
 				if (getParameterStatus(conn))
 					return 0;
 				break;
-			case 'd':			/* Copy Data, pass it back to caller */
+			case PqMsgCopyDataRequest:			/* Copy Data, pass it back to caller */
 				return msgLength;
-			case 'c':
+			case PqMsgCopyDoneRequest:
 
 				/*
 				 * If this is a CopyDone message, exit COPY_OUT mode and let
@@ -1929,7 +1931,7 @@ pqEndcopy3(PGconn *conn)
 	if (conn->asyncStatus == PGASYNC_COPY_IN ||
 		conn->asyncStatus == PGASYNC_COPY_BOTH)
 	{
-		if (pqPutMsgStart('c', conn) < 0 ||
+		if (pqPutMsgStart(PqMsgCopyDoneRequest, conn) < 0 ||
 			pqPutMsgEnd(conn) < 0)
 			return 1;
 
@@ -1940,7 +1942,7 @@ pqEndcopy3(PGconn *conn)
 		if (conn->cmd_queue_head &&
 			conn->cmd_queue_head->queryclass != PGQUERY_SIMPLE)
 		{
-			if (pqPutMsgStart('S', conn) < 0 ||
+			if (pqPutMsgStart(PqMsgSyncDataRequest, conn) < 0 ||
 				pqPutMsgEnd(conn) < 0)
 				return 1;
 		}
@@ -2023,7 +2025,7 @@ pqFunctionCall3(PGconn *conn, Oid fnid,
 
 	/* PQfn already validated connection state */
 
-	if (pqPutMsgStart('F', conn) < 0 || /* function call msg */
+	if (pqPutMsgStart(PqMsgFunctionCallRequest, conn) < 0 || /* function call msg */
 		pqPutInt(fnid, 4, conn) < 0 ||	/* function id */
 		pqPutInt(1, 2, conn) < 0 || /* # of format codes */
 		pqPutInt(1, 2, conn) < 0 || /* format code: BINARY */
diff --git a/src/interfaces/libpq/fe-trace.c b/src/interfaces/libpq/fe-trace.c
index 402784f40e..8414418a5c 100644
--- a/src/interfaces/libpq/fe-trace.c
+++ b/src/interfaces/libpq/fe-trace.c
@@ -28,6 +28,7 @@
 #include "libpq-fe.h"
 #include "libpq-int.h"
 #include "port/pg_bswap.h"
+#include "protocol.h"
 
 
 /* Enable tracing */
@@ -215,7 +216,7 @@ pqTraceOutputNchar(FILE *pfdebug, int len, const char *data, int *cursor)
  * Output functions by protocol message type
  */
 
-/* NotificationResponse */
+/* PqMsgNotifyResponse */
 static void
 pqTraceOutputA(FILE *f, const char *message, int *cursor, bool regress)
 {
@@ -225,7 +226,7 @@ pqTraceOutputA(FILE *f, const char *message, int *cursor, bool regress)
 	pqTraceOutputString(f, message, cursor, false);
 }
 
-/* Bind */
+/* PqMsgBindRequest */
 static void
 pqTraceOutputB(FILE *f, const char *message, int *cursor)
 {
@@ -256,7 +257,7 @@ pqTraceOutputB(FILE *f, const char *message, int *cursor)
 		pqTraceOutputInt16(f, message, cursor);
 }
 
-/* Close(F) or CommandComplete(B) */
+/* PqMsgCloseRequest or PqMsgCommandCompleteResponse */
 static void
 pqTraceOutputC(FILE *f, bool toServer, const char *message, int *cursor)
 {
@@ -273,7 +274,7 @@ pqTraceOutputC(FILE *f, bool toServer, const char *message, int *cursor)
 	}
 }
 
-/* Describe(F) or DataRow(B) */
+/* PqMsgDescribeRequest or PqMsgDataRowResponse */
 static void
 pqTraceOutputD(FILE *f, bool toServer, const char *message, int *cursor)
 {
@@ -322,7 +323,7 @@ pqTraceOutputNR(FILE *f, const char *type, const char *message, int *cursor,
 	}
 }
 
-/* Execute(F) or ErrorResponse(B) */
+/* PqMsgExecuteRequest or PqMsgErrorResponse */
 static void
 pqTraceOutputE(FILE *f, bool toServer, const char *message, int *cursor, bool regress)
 {
@@ -336,7 +337,7 @@ pqTraceOutputE(FILE *f, bool toServer, const char *message, int *cursor, bool re
 		pqTraceOutputNR(f, "ErrorResponse", message, cursor, regress);
 }
 
-/* CopyFail */
+/* PqMsgCopyFailRequest */
 static void
 pqTraceOutputf(FILE *f, const char *message, int *cursor)
 {
@@ -344,7 +345,7 @@ pqTraceOutputf(FILE *f, const char *message, int *cursor)
 	pqTraceOutputString(f, message, cursor, false);
 }
 
-/* FunctionCall */
+/* PqMsgFunctionCallRequest */
 static void
 pqTraceOutputF(FILE *f, const char *message, int *cursor, bool regress)
 {
@@ -371,7 +372,7 @@ pqTraceOutputF(FILE *f, const char *message, int *cursor, bool regress)
 	pqTraceOutputInt16(f, message, cursor);
 }
 
-/* CopyInResponse */
+/* PqMsgCopyInResponse */
 static void
 pqTraceOutputG(FILE *f, const char *message, int *cursor)
 {
@@ -385,7 +386,7 @@ pqTraceOutputG(FILE *f, const char *message, int *cursor)
 		pqTraceOutputInt16(f, message, cursor);
 }
 
-/* CopyOutResponse */
+/* PqMsgCopyOutResponse */
 static void
 pqTraceOutputH(FILE *f, const char *message, int *cursor)
 {
@@ -399,7 +400,7 @@ pqTraceOutputH(FILE *f, const char *message, int *cursor)
 		pqTraceOutputInt16(f, message, cursor);
 }
 
-/* BackendKeyData */
+/* PqMsgBackendKeyDataRequest */
 static void
 pqTraceOutputK(FILE *f, const char *message, int *cursor, bool regress)
 {
@@ -408,7 +409,7 @@ pqTraceOutputK(FILE *f, const char *message, int *cursor, bool regress)
 	pqTraceOutputInt32(f, message, cursor, regress);
 }
 
-/* Parse */
+/* PqMsgParseRequest */
 static void
 pqTraceOutputP(FILE *f, const char *message, int *cursor, bool regress)
 {
@@ -423,7 +424,7 @@ pqTraceOutputP(FILE *f, const char *message, int *cursor, bool regress)
 		pqTraceOutputInt32(f, message, cursor, regress);
 }
 
-/* Query */
+/* PqMsgSimpleQueryRequest */
 static void
 pqTraceOutputQ(FILE *f, const char *message, int *cursor)
 {
@@ -431,7 +432,7 @@ pqTraceOutputQ(FILE *f, const char *message, int *cursor)
 	pqTraceOutputString(f, message, cursor, false);
 }
 
-/* Authentication */
+/* PqMsgAuthenticationRequest */
 static void
 pqTraceOutputR(FILE *f, const char *message, int *cursor)
 {
@@ -439,7 +440,7 @@ pqTraceOutputR(FILE *f, const char *message, int *cursor)
 	pqTraceOutputInt32(f, message, cursor, false);
 }
 
-/* ParameterStatus */
+/* PqMsgParameterStatusResponse */
 static void
 pqTraceOutputS(FILE *f, const char *message, int *cursor)
 {
@@ -448,7 +449,7 @@ pqTraceOutputS(FILE *f, const char *message, int *cursor)
 	pqTraceOutputString(f, message, cursor, false);
 }
 
-/* ParameterDescription */
+/* PqMsgParameterDescriptionResponse */
 static void
 pqTraceOutputt(FILE *f, const char *message, int *cursor, bool regress)
 {
@@ -461,7 +462,7 @@ pqTraceOutputt(FILE *f, const char *message, int *cursor, bool regress)
 		pqTraceOutputInt32(f, message, cursor, regress);
 }
 
-/* RowDescription */
+/* PqMsgRowDescriptionResponse */
 static void
 pqTraceOutputT(FILE *f, const char *message, int *cursor, bool regress)
 {
@@ -482,7 +483,7 @@ pqTraceOutputT(FILE *f, const char *message, int *cursor, bool regress)
 	}
 }
 
-/* NegotiateProtocolVersion */
+/* PqMsgNegotiateProtocolResponse */
 static void
 pqTraceOutputv(FILE *f, const char *message, int *cursor)
 {
@@ -491,7 +492,7 @@ pqTraceOutputv(FILE *f, const char *message, int *cursor)
 	pqTraceOutputInt32(f, message, cursor, false);
 }
 
-/* FunctionCallResponse */
+/* PqMsgFunctionCallResponse */
 static void
 pqTraceOutputV(FILE *f, const char *message, int *cursor)
 {
@@ -503,7 +504,7 @@ pqTraceOutputV(FILE *f, const char *message, int *cursor)
 		pqTraceOutputNchar(f, len, message, cursor);
 }
 
-/* CopyBothResponse */
+/* PqMsgCopyBothResponse */
 static void
 pqTraceOutputW(FILE *f, const char *message, int *cursor, int length)
 {
@@ -514,7 +515,7 @@ pqTraceOutputW(FILE *f, const char *message, int *cursor, int length)
 		pqTraceOutputInt16(f, message, cursor);
 }
 
-/* ReadyForQuery */
+/* PqMsgReadyForQueryResponse */
 static void
 pqTraceOutputZ(FILE *f, const char *message, int *cursor)
 {
@@ -562,110 +563,110 @@ pqTraceOutputMessage(PGconn *conn, const char *message, bool toServer)
 
 	switch (id)
 	{
-		case '1':
+		case PqMsgParseCompleteResponse:
 			fprintf(conn->Pfdebug, "ParseComplete");
 			/* No message content */
 			break;
-		case '2':
+		case PqMsgBindCompleteResponse:
 			fprintf(conn->Pfdebug, "BindComplete");
 			/* No message content */
 			break;
-		case '3':
+		case PqMsgCloseCompleteResponse:
 			fprintf(conn->Pfdebug, "CloseComplete");
 			/* No message content */
 			break;
-		case 'A':				/* Notification Response */
+		case PqMsgNotifyResponse:
 			pqTraceOutputA(conn->Pfdebug, message, &logCursor, regress);
 			break;
-		case 'B':				/* Bind */
+		case PqMsgBindRequest:
 			pqTraceOutputB(conn->Pfdebug, message, &logCursor);
 			break;
-		case 'c':
+		case PqMsgCopyDoneRequest:
 			fprintf(conn->Pfdebug, "CopyDone");
 			/* No message content */
 			break;
-		case 'C':				/* Close(F) or Command Complete(B) */
+		case PqMsgCommandCompleteResponse:				/* Close(F) or Command Complete(B) */
 			pqTraceOutputC(conn->Pfdebug, toServer, message, &logCursor);
 			break;
-		case 'd':				/* Copy Data */
+		case PqMsgCopyDataRequest:
 			/* Drop COPY data to reduce the overhead of logging. */
 			break;
-		case 'D':				/* Describe(F) or Data Row(B) */
+		case PqMsgDescribeRequest:		/* Describe(F) or Data Row(B) */
 			pqTraceOutputD(conn->Pfdebug, toServer, message, &logCursor);
 			break;
-		case 'E':				/* Execute(F) or Error Response(B) */
+		case PqMsgExecuteRequest:		/* Execute(F) or Error Response(B) */
 			pqTraceOutputE(conn->Pfdebug, toServer, message, &logCursor,
 						   regress);
 			break;
-		case 'f':				/* Copy Fail */
+		case PqMsgCopyFailRequest:
 			pqTraceOutputf(conn->Pfdebug, message, &logCursor);
 			break;
-		case 'F':				/* Function Call */
+		case PqMsgFunctionCallRequest:
 			pqTraceOutputF(conn->Pfdebug, message, &logCursor, regress);
 			break;
-		case 'G':				/* Start Copy In */
+		case PqMsgCopyInResponse:
 			pqTraceOutputG(conn->Pfdebug, message, &logCursor);
 			break;
-		case 'H':				/* Flush(F) or Start Copy Out(B) */
+		case PqMsgFlushDataRequest:	/* Flush(F) or Start Copy Out(B) */
 			if (!toServer)
 				pqTraceOutputH(conn->Pfdebug, message, &logCursor);
 			else
 				fprintf(conn->Pfdebug, "Flush");	/* no message content */
 			break;
-		case 'I':
+		case PqMsgEmptyQueryResponse:
 			fprintf(conn->Pfdebug, "EmptyQueryResponse");
 			/* No message content */
 			break;
-		case 'K':				/* secret key data from the backend */
+		case PqMsgBackendKeyDataRequest:		/* secret key data from the backend */
 			pqTraceOutputK(conn->Pfdebug, message, &logCursor, regress);
 			break;
-		case 'n':
+		case PqMsgNoDataResponse:
 			fprintf(conn->Pfdebug, "NoData");
 			/* No message content */
 			break;
-		case 'N':
+		case PqMsgNoticeResponse:
 			pqTraceOutputNR(conn->Pfdebug, "NoticeResponse", message,
 							&logCursor, regress);
 			break;
-		case 'P':				/* Parse */
+		case PqMsgParseRequest:
 			pqTraceOutputP(conn->Pfdebug, message, &logCursor, regress);
 			break;
-		case 'Q':				/* Query */
+		case PqMsgSimpleQueryRequest:
 			pqTraceOutputQ(conn->Pfdebug, message, &logCursor);
 			break;
-		case 'R':				/* Authentication */
+		case PqMsgAuthenticationRequest:
 			pqTraceOutputR(conn->Pfdebug, message, &logCursor);
 			break;
-		case 's':
+		case PqMsgPortalSuspendedResponse:
 			fprintf(conn->Pfdebug, "PortalSuspended");
 			/* No message content */
 			break;
-		case 'S':				/* Parameter Status(B) or Sync(F) */
+		case PqMsgSyncDataRequest:	/* Parameter Status(B) or Sync(F) */
 			if (!toServer)
 				pqTraceOutputS(conn->Pfdebug, message, &logCursor);
 			else
 				fprintf(conn->Pfdebug, "Sync"); /* no message content */
 			break;
-		case 't':				/* Parameter Description */
+		case PqMsgParameterDescriptionResponse:
 			pqTraceOutputt(conn->Pfdebug, message, &logCursor, regress);
 			break;
-		case 'T':				/* Row Description */
+		case PqMsgRowDescriptionResponse:
 			pqTraceOutputT(conn->Pfdebug, message, &logCursor, regress);
 			break;
-		case 'v':				/* Negotiate Protocol Version */
+		case PqMsgNegotiateProtocolResponse:
 			pqTraceOutputv(conn->Pfdebug, message, &logCursor);
 			break;
-		case 'V':				/* Function Call response */
+		case PqMsgFunctionCallResponse:
 			pqTraceOutputV(conn->Pfdebug, message, &logCursor);
 			break;
-		case 'W':				/* Start Copy Both */
+		case PqMsgCopyBothResponse:
 			pqTraceOutputW(conn->Pfdebug, message, &logCursor, length);
 			break;
-		case 'X':
+		case PqMsgTerminateRequest:
 			fprintf(conn->Pfdebug, "Terminate");
 			/* No message content */
 			break;
-		case 'Z':				/* Ready For Query */
+		case PqMsgReadyForQueryResponse:
 			pqTraceOutputZ(conn->Pfdebug, message, &logCursor);
 			break;
 		default:
-- 
2.37.1 (Apple Git-137.1)

From 78c2a110a8c940690d1ff363e545acb48adf966e Mon Sep 17 00:00:00 2001
From: Nathan Bossart <nathan@postgresql.org>
Date: Mon, 14 Aug 2023 14:09:30 -0700
Subject: [PATCH v5 1/1] Introduce macros for protocol characters.

Author: Dave Cramer
Reviewed-by: Alvaro Herrera, Tatsuo Ishii, Peter Smith, Robert Haas, Tom Lane, Peter Eisentraut
Discussion: https://postgr.es/m/CADK3HHKbBmK-PKf1bPNFoMC%2BoBt%2BpD9PH8h5nvmBQskEHm-Ehw%40mail.gmail.com
---
 src/backend/access/common/printsimple.c |  5 +-
 src/backend/access/transam/parallel.c   | 14 ++---
 src/backend/backup/basebackup_copy.c    | 16 +++---
 src/backend/commands/async.c            |  2 +-
 src/backend/commands/copyfromparse.c    | 22 ++++----
 src/backend/commands/copyto.c           |  6 +--
 src/backend/libpq/auth-sasl.c           |  2 +-
 src/backend/libpq/auth.c                |  8 +--
 src/backend/postmaster/postmaster.c     |  2 +-
 src/backend/replication/walsender.c     | 18 +++----
 src/backend/tcop/dest.c                 |  8 +--
 src/backend/tcop/fastpath.c             |  2 +-
 src/backend/tcop/postgres.c             | 68 ++++++++++++------------
 src/backend/utils/error/elog.c          |  5 +-
 src/backend/utils/misc/guc.c            |  2 +-
 src/include/libpq/pqcomm.h              | 51 ++++++++++++++++++
 src/interfaces/libpq/fe-auth.c          |  2 +-
 src/interfaces/libpq/fe-connect.c       | 19 ++++---
 src/interfaces/libpq/fe-exec.c          | 50 +++++++++---------
 src/interfaces/libpq/fe-protocol3.c     | 70 +++++++++++++------------
 src/interfaces/libpq/fe-trace.c         | 70 ++++++++++++++-----------
 21 files changed, 258 insertions(+), 184 deletions(-)

diff --git a/src/backend/access/common/printsimple.c b/src/backend/access/common/printsimple.c
index ef818228ac..62de95e1ba 100644
--- a/src/backend/access/common/printsimple.c
+++ b/src/backend/access/common/printsimple.c
@@ -20,6 +20,7 @@
 
 #include "access/printsimple.h"
 #include "catalog/pg_type.h"
+#include "libpq/pqcomm.h"
 #include "libpq/pqformat.h"
 #include "utils/builtins.h"
 
@@ -32,7 +33,7 @@ printsimple_startup(DestReceiver *self, int operation, TupleDesc tupdesc)
 	StringInfoData buf;
 	int			i;
 
-	pq_beginmessage(&buf, 'T'); /* RowDescription */
+	pq_beginmessage(&buf, PqMsg_RowDescription);
 	pq_sendint16(&buf, tupdesc->natts);
 
 	for (i = 0; i < tupdesc->natts; ++i)
@@ -65,7 +66,7 @@ printsimple(TupleTableSlot *slot, DestReceiver *self)
 	slot_getallattrs(slot);
 
 	/* Prepare and send message */
-	pq_beginmessage(&buf, 'D');
+	pq_beginmessage(&buf, PqMsg_DataRow);
 	pq_sendint16(&buf, tupdesc->natts);
 
 	for (i = 0; i < tupdesc->natts; ++i)
diff --git a/src/backend/access/transam/parallel.c b/src/backend/access/transam/parallel.c
index 1738aecf1f..194a1207be 100644
--- a/src/backend/access/transam/parallel.c
+++ b/src/backend/access/transam/parallel.c
@@ -1127,7 +1127,7 @@ HandleParallelMessage(ParallelContext *pcxt, int i, StringInfo msg)
 
 	switch (msgtype)
 	{
-		case 'K':				/* BackendKeyData */
+		case PqMsg_BackendKeyData:
 			{
 				int32		pid = pq_getmsgint(msg, 4);
 
@@ -1137,8 +1137,8 @@ HandleParallelMessage(ParallelContext *pcxt, int i, StringInfo msg)
 				break;
 			}
 
-		case 'E':				/* ErrorResponse */
-		case 'N':				/* NoticeResponse */
+		case PqMsg_ErrorResponse:
+		case PqMsg_NoticeResponse:
 			{
 				ErrorData	edata;
 				ErrorContextCallback *save_error_context_stack;
@@ -1183,7 +1183,7 @@ HandleParallelMessage(ParallelContext *pcxt, int i, StringInfo msg)
 				break;
 			}
 
-		case 'A':				/* NotifyResponse */
+		case PqMsg_NotificationResponse:
 			{
 				/* Propagate NotifyResponse. */
 				int32		pid;
@@ -1217,7 +1217,7 @@ HandleParallelMessage(ParallelContext *pcxt, int i, StringInfo msg)
 				break;
 			}
 
-		case 'X':				/* Terminate, indicating clean exit */
+		case PqMsg_Terminate:
 			{
 				shm_mq_detach(pcxt->worker[i].error_mqh);
 				pcxt->worker[i].error_mqh = NULL;
@@ -1372,7 +1372,7 @@ ParallelWorkerMain(Datum main_arg)
 	 * protocol message is defined, but it won't actually be used for anything
 	 * in this case.
 	 */
-	pq_beginmessage(&msgbuf, 'K');
+	pq_beginmessage(&msgbuf, PqMsg_BackendKeyData);
 	pq_sendint32(&msgbuf, (int32) MyProcPid);
 	pq_sendint32(&msgbuf, (int32) MyCancelKey);
 	pq_endmessage(&msgbuf);
@@ -1550,7 +1550,7 @@ ParallelWorkerMain(Datum main_arg)
 	DetachSession();
 
 	/* Report success. */
-	pq_putmessage('X', NULL, 0);
+	pq_putmessage(PqMsg_Terminate, NULL, 0);
 }
 
 /*
diff --git a/src/backend/backup/basebackup_copy.c b/src/backend/backup/basebackup_copy.c
index 1db80cde1b..fee30c21e1 100644
--- a/src/backend/backup/basebackup_copy.c
+++ b/src/backend/backup/basebackup_copy.c
@@ -152,7 +152,7 @@ bbsink_copystream_begin_backup(bbsink *sink)
 	SendTablespaceList(state->tablespaces);
 
 	/* Send a CommandComplete message */
-	pq_puttextmessage('C', "SELECT");
+	pq_puttextmessage(PqMsg_CommandComplete, "SELECT");
 
 	/* Begin COPY stream. This will be used for all archives + manifest. */
 	SendCopyOutResponse();
@@ -169,7 +169,7 @@ bbsink_copystream_begin_archive(bbsink *sink, const char *archive_name)
 	StringInfoData buf;
 
 	ti = list_nth(state->tablespaces, state->tablespace_num);
-	pq_beginmessage(&buf, 'd'); /* CopyData */
+	pq_beginmessage(&buf, PqMsg_CopyData);
 	pq_sendbyte(&buf, 'n');		/* New archive */
 	pq_sendstring(&buf, archive_name);
 	pq_sendstring(&buf, ti->path == NULL ? "" : ti->path);
@@ -220,7 +220,7 @@ bbsink_copystream_archive_contents(bbsink *sink, size_t len)
 		{
 			mysink->last_progress_report_time = now;
 
-			pq_beginmessage(&buf, 'd'); /* CopyData */
+			pq_beginmessage(&buf, PqMsg_CopyData);
 			pq_sendbyte(&buf, 'p'); /* Progress report */
 			pq_sendint64(&buf, state->bytes_done);
 			pq_endmessage(&buf);
@@ -246,7 +246,7 @@ bbsink_copystream_end_archive(bbsink *sink)
 
 	mysink->bytes_done_at_last_time_check = state->bytes_done;
 	mysink->last_progress_report_time = GetCurrentTimestamp();
-	pq_beginmessage(&buf, 'd'); /* CopyData */
+	pq_beginmessage(&buf, PqMsg_CopyData);
 	pq_sendbyte(&buf, 'p');		/* Progress report */
 	pq_sendint64(&buf, state->bytes_done);
 	pq_endmessage(&buf);
@@ -261,7 +261,7 @@ bbsink_copystream_begin_manifest(bbsink *sink)
 {
 	StringInfoData buf;
 
-	pq_beginmessage(&buf, 'd'); /* CopyData */
+	pq_beginmessage(&buf, PqMsg_CopyData);
 	pq_sendbyte(&buf, 'm');		/* Manifest */
 	pq_endmessage(&buf);
 }
@@ -318,7 +318,7 @@ SendCopyOutResponse(void)
 {
 	StringInfoData buf;
 
-	pq_beginmessage(&buf, 'H');
+	pq_beginmessage(&buf, PqMsg_CopyOutResponse);
 	pq_sendbyte(&buf, 0);		/* overall format */
 	pq_sendint16(&buf, 0);		/* natts */
 	pq_endmessage(&buf);
@@ -330,7 +330,7 @@ SendCopyOutResponse(void)
 static void
 SendCopyDone(void)
 {
-	pq_putemptymessage('c');
+	pq_putemptymessage(PqMsg_CopyDone);
 }
 
 /*
@@ -368,7 +368,7 @@ SendXlogRecPtrResult(XLogRecPtr ptr, TimeLineID tli)
 	end_tup_output(tstate);
 
 	/* Send a CommandComplete message */
-	pq_puttextmessage('C', "SELECT");
+	pq_puttextmessage(PqMsg_CommandComplete, "SELECT");
 }
 
 /*
diff --git a/src/backend/commands/async.c b/src/backend/commands/async.c
index ef909cf4e0..d148d10850 100644
--- a/src/backend/commands/async.c
+++ b/src/backend/commands/async.c
@@ -2281,7 +2281,7 @@ NotifyMyFrontEnd(const char *channel, const char *payload, int32 srcPid)
 	{
 		StringInfoData buf;
 
-		pq_beginmessage(&buf, 'A');
+		pq_beginmessage(&buf, PqMsg_NotificationResponse);
 		pq_sendint32(&buf, srcPid);
 		pq_sendstring(&buf, channel);
 		pq_sendstring(&buf, payload);
diff --git a/src/backend/commands/copyfromparse.c b/src/backend/commands/copyfromparse.c
index 232768a6e1..f553734582 100644
--- a/src/backend/commands/copyfromparse.c
+++ b/src/backend/commands/copyfromparse.c
@@ -174,7 +174,7 @@ ReceiveCopyBegin(CopyFromState cstate)
 	int16		format = (cstate->opts.binary ? 1 : 0);
 	int			i;
 
-	pq_beginmessage(&buf, 'G');
+	pq_beginmessage(&buf, PqMsg_CopyInResponse);
 	pq_sendbyte(&buf, format);	/* overall format */
 	pq_sendint16(&buf, natts);
 	for (i = 0; i < natts; i++)
@@ -279,13 +279,13 @@ CopyGetData(CopyFromState cstate, void *databuf, int minread, int maxread)
 					/* Validate message type and set packet size limit */
 					switch (mtype)
 					{
-						case 'd':	/* CopyData */
+						case PqMsg_CopyData:
 							maxmsglen = PQ_LARGE_MESSAGE_LIMIT;
 							break;
-						case 'c':	/* CopyDone */
-						case 'f':	/* CopyFail */
-						case 'H':	/* Flush */
-						case 'S':	/* Sync */
+						case PqMsg_CopyDone:
+						case PqMsg_CopyFail:
+						case PqMsg_Flush:
+						case PqMsg_Sync:
 							maxmsglen = PQ_SMALL_MESSAGE_LIMIT;
 							break;
 						default:
@@ -305,20 +305,20 @@ CopyGetData(CopyFromState cstate, void *databuf, int minread, int maxread)
 					/* ... and process it */
 					switch (mtype)
 					{
-						case 'd':	/* CopyData */
+						case PqMsg_CopyData:
 							break;
-						case 'c':	/* CopyDone */
+						case PqMsg_CopyDone:
 							/* COPY IN correctly terminated by frontend */
 							cstate->raw_reached_eof = true;
 							return bytesread;
-						case 'f':	/* CopyFail */
+						case PqMsg_CopyFail:
 							ereport(ERROR,
 									(errcode(ERRCODE_QUERY_CANCELED),
 									 errmsg("COPY from stdin failed: %s",
 											pq_getmsgstring(cstate->fe_msgbuf))));
 							break;
-						case 'H':	/* Flush */
-						case 'S':	/* Sync */
+						case PqMsg_Flush:
+						case PqMsg_Sync:
 
 							/*
 							 * Ignore Flush/Sync for the convenience of client
diff --git a/src/backend/commands/copyto.c b/src/backend/commands/copyto.c
index 9e4b2437a5..eaa3172793 100644
--- a/src/backend/commands/copyto.c
+++ b/src/backend/commands/copyto.c
@@ -144,7 +144,7 @@ SendCopyBegin(CopyToState cstate)
 	int16		format = (cstate->opts.binary ? 1 : 0);
 	int			i;
 
-	pq_beginmessage(&buf, 'H');
+	pq_beginmessage(&buf, PqMsg_CopyOutResponse);
 	pq_sendbyte(&buf, format);	/* overall format */
 	pq_sendint16(&buf, natts);
 	for (i = 0; i < natts; i++)
@@ -159,7 +159,7 @@ SendCopyEnd(CopyToState cstate)
 	/* Shouldn't have any unsent data */
 	Assert(cstate->fe_msgbuf->len == 0);
 	/* Send Copy Done message */
-	pq_putemptymessage('c');
+	pq_putemptymessage(PqMsg_CopyDone);
 }
 
 /*----------
@@ -247,7 +247,7 @@ CopySendEndOfRow(CopyToState cstate)
 				CopySendChar(cstate, '\n');
 
 			/* Dump the accumulated row as one CopyData message */
-			(void) pq_putmessage('d', fe_msgbuf->data, fe_msgbuf->len);
+			(void) pq_putmessage(PqMsg_CopyData, fe_msgbuf->data, fe_msgbuf->len);
 			break;
 		case COPY_CALLBACK:
 			cstate->data_dest_cb(fe_msgbuf->data, fe_msgbuf->len);
diff --git a/src/backend/libpq/auth-sasl.c b/src/backend/libpq/auth-sasl.c
index 684680897b..c535bc5383 100644
--- a/src/backend/libpq/auth-sasl.c
+++ b/src/backend/libpq/auth-sasl.c
@@ -87,7 +87,7 @@ CheckSASLAuth(const pg_be_sasl_mech *mech, Port *port, char *shadow_pass,
 	{
 		pq_startmsgread();
 		mtype = pq_getbyte();
-		if (mtype != 'p')
+		if (mtype != PqMsg_SASLResponse)
 		{
 			/* Only log error if client didn't disconnect. */
 			if (mtype != EOF)
diff --git a/src/backend/libpq/auth.c b/src/backend/libpq/auth.c
index 315a24bb3f..0356fe3e45 100644
--- a/src/backend/libpq/auth.c
+++ b/src/backend/libpq/auth.c
@@ -665,7 +665,7 @@ sendAuthRequest(Port *port, AuthRequest areq, const char *extradata, int extrale
 
 	CHECK_FOR_INTERRUPTS();
 
-	pq_beginmessage(&buf, 'R');
+	pq_beginmessage(&buf, PqMsg_AuthenticationRequest);
 	pq_sendint32(&buf, (int32) areq);
 	if (extralen > 0)
 		pq_sendbytes(&buf, extradata, extralen);
@@ -698,7 +698,7 @@ recv_password_packet(Port *port)
 
 	/* Expect 'p' message type */
 	mtype = pq_getbyte();
-	if (mtype != 'p')
+	if (mtype != PqMsg_PasswordMessage)
 	{
 		/*
 		 * If the client just disconnects without offering a password, don't
@@ -961,7 +961,7 @@ pg_GSS_recvauth(Port *port)
 		CHECK_FOR_INTERRUPTS();
 
 		mtype = pq_getbyte();
-		if (mtype != 'p')
+		if (mtype != PqMsg_GSSResponse)
 		{
 			/* Only log error if client didn't disconnect. */
 			if (mtype != EOF)
@@ -1232,7 +1232,7 @@ pg_SSPI_recvauth(Port *port)
 	{
 		pq_startmsgread();
 		mtype = pq_getbyte();
-		if (mtype != 'p')
+		if (mtype != PqMsg_GSSResponse)
 		{
 			if (sspictx != NULL)
 			{
diff --git a/src/backend/postmaster/postmaster.c b/src/backend/postmaster/postmaster.c
index 9c8ec779f9..07d376d77e 100644
--- a/src/backend/postmaster/postmaster.c
+++ b/src/backend/postmaster/postmaster.c
@@ -2357,7 +2357,7 @@ SendNegotiateProtocolVersion(List *unrecognized_protocol_options)
 	StringInfoData buf;
 	ListCell   *lc;
 
-	pq_beginmessage(&buf, 'v'); /* NegotiateProtocolVersion */
+	pq_beginmessage(&buf, PqMsg_NegotiateProtocolVersion);
 	pq_sendint32(&buf, PG_PROTOCOL_LATEST);
 	pq_sendint32(&buf, list_length(unrecognized_protocol_options));
 	foreach(lc, unrecognized_protocol_options)
diff --git a/src/backend/replication/walsender.c b/src/backend/replication/walsender.c
index d27ef2985d..80374c55be 100644
--- a/src/backend/replication/walsender.c
+++ b/src/backend/replication/walsender.c
@@ -603,7 +603,7 @@ SendTimeLineHistory(TimeLineHistoryCmd *cmd)
 	dest->rStartup(dest, CMD_SELECT, tupdesc);
 
 	/* Send a DataRow message */
-	pq_beginmessage(&buf, 'D');
+	pq_beginmessage(&buf, PqMsg_DataRow);
 	pq_sendint16(&buf, 2);		/* # of columns */
 	len = strlen(histfname);
 	pq_sendint32(&buf, len);	/* col1 len */
@@ -801,7 +801,7 @@ StartReplication(StartReplicationCmd *cmd)
 		WalSndSetState(WALSNDSTATE_CATCHUP);
 
 		/* Send a CopyBothResponse message, and start streaming */
-		pq_beginmessage(&buf, 'W');
+		pq_beginmessage(&buf, PqMsg_CopyBothResponse);
 		pq_sendbyte(&buf, 0);
 		pq_sendint16(&buf, 0);
 		pq_endmessage(&buf);
@@ -1294,7 +1294,7 @@ StartLogicalReplication(StartReplicationCmd *cmd)
 	WalSndSetState(WALSNDSTATE_CATCHUP);
 
 	/* Send a CopyBothResponse message, and start streaming */
-	pq_beginmessage(&buf, 'W');
+	pq_beginmessage(&buf, PqMsg_CopyBothResponse);
 	pq_sendbyte(&buf, 0);
 	pq_sendint16(&buf, 0);
 	pq_endmessage(&buf);
@@ -1923,11 +1923,11 @@ ProcessRepliesIfAny(void)
 		/* Validate message type and set packet size limit */
 		switch (firstchar)
 		{
-			case 'd':
+			case PqMsg_CopyData:
 				maxmsglen = PQ_LARGE_MESSAGE_LIMIT;
 				break;
-			case 'c':
-			case 'X':
+			case PqMsg_CopyDone:
+			case PqMsg_Terminate:
 				maxmsglen = PQ_SMALL_MESSAGE_LIMIT;
 				break;
 			default:
@@ -1955,7 +1955,7 @@ ProcessRepliesIfAny(void)
 				/*
 				 * 'd' means a standby reply wrapped in a CopyData packet.
 				 */
-			case 'd':
+			case PqMsg_CopyData:
 				ProcessStandbyMessage();
 				received = true;
 				break;
@@ -1964,7 +1964,7 @@ ProcessRepliesIfAny(void)
 				 * CopyDone means the standby requested to finish streaming.
 				 * Reply with CopyDone, if we had not sent that already.
 				 */
-			case 'c':
+			case PqMsg_CopyDone:
 				if (!streamingDoneSending)
 				{
 					pq_putmessage_noblock('c', NULL, 0);
@@ -1978,7 +1978,7 @@ ProcessRepliesIfAny(void)
 				/*
 				 * 'X' means that the standby is closing down the socket.
 				 */
-			case 'X':
+			case PqMsg_Terminate:
 				proc_exit(0);
 
 			default:
diff --git a/src/backend/tcop/dest.c b/src/backend/tcop/dest.c
index c0406e2ee5..06d1872b9a 100644
--- a/src/backend/tcop/dest.c
+++ b/src/backend/tcop/dest.c
@@ -176,7 +176,7 @@ EndCommand(const QueryCompletion *qc, CommandDest dest, bool force_undecorated_o
 
 			len = BuildQueryCompletionString(completionTag, qc,
 											 force_undecorated_output);
-			pq_putmessage('C', completionTag, len + 1);
+			pq_putmessage(PqMsg_Close, completionTag, len + 1);
 
 		case DestNone:
 		case DestDebug:
@@ -200,7 +200,7 @@ EndCommand(const QueryCompletion *qc, CommandDest dest, bool force_undecorated_o
 void
 EndReplicationCommand(const char *commandTag)
 {
-	pq_putmessage('C', commandTag, strlen(commandTag) + 1);
+	pq_putmessage(PqMsg_Close, commandTag, strlen(commandTag) + 1);
 }
 
 /* ----------------
@@ -220,7 +220,7 @@ NullCommand(CommandDest dest)
 		case DestRemoteSimple:
 
 			/* Tell the FE that we saw an empty query string */
-			pq_putemptymessage('I');
+			pq_putemptymessage(PqMsg_EmptyQueryResponse);
 			break;
 
 		case DestNone:
@@ -258,7 +258,7 @@ ReadyForQuery(CommandDest dest)
 			{
 				StringInfoData buf;
 
-				pq_beginmessage(&buf, 'Z');
+				pq_beginmessage(&buf, PqMsg_ReadyForQuery);
 				pq_sendbyte(&buf, TransactionBlockStatusCode());
 				pq_endmessage(&buf);
 			}
diff --git a/src/backend/tcop/fastpath.c b/src/backend/tcop/fastpath.c
index 2f70ebd5fa..71f161dbe2 100644
--- a/src/backend/tcop/fastpath.c
+++ b/src/backend/tcop/fastpath.c
@@ -69,7 +69,7 @@ SendFunctionResult(Datum retval, bool isnull, Oid rettype, int16 format)
 {
 	StringInfoData buf;
 
-	pq_beginmessage(&buf, 'V');
+	pq_beginmessage(&buf, PqMsg_FunctionCallResponse);
 
 	if (isnull)
 	{
diff --git a/src/backend/tcop/postgres.c b/src/backend/tcop/postgres.c
index 36cc99ec9c..e4756f8be2 100644
--- a/src/backend/tcop/postgres.c
+++ b/src/backend/tcop/postgres.c
@@ -402,37 +402,37 @@ SocketBackend(StringInfo inBuf)
 	 */
 	switch (qtype)
 	{
-		case 'Q':				/* simple query */
+		case PqMsg_Query:
 			maxmsglen = PQ_LARGE_MESSAGE_LIMIT;
 			doing_extended_query_message = false;
 			break;
 
-		case 'F':				/* fastpath function call */
+		case PqMsg_FunctionCall:
 			maxmsglen = PQ_LARGE_MESSAGE_LIMIT;
 			doing_extended_query_message = false;
 			break;
 
-		case 'X':				/* terminate */
+		case PqMsg_Terminate:
 			maxmsglen = PQ_SMALL_MESSAGE_LIMIT;
 			doing_extended_query_message = false;
 			ignore_till_sync = false;
 			break;
 
-		case 'B':				/* bind */
-		case 'P':				/* parse */
+		case PqMsg_Bind:
+		case PqMsg_Parse:
 			maxmsglen = PQ_LARGE_MESSAGE_LIMIT;
 			doing_extended_query_message = true;
 			break;
 
-		case 'C':				/* close */
-		case 'D':				/* describe */
-		case 'E':				/* execute */
-		case 'H':				/* flush */
+		case PqMsg_Close:
+		case PqMsg_Describe:
+		case PqMsg_Execute:
+		case PqMsg_Flush:
 			maxmsglen = PQ_SMALL_MESSAGE_LIMIT;
 			doing_extended_query_message = true;
 			break;
 
-		case 'S':				/* sync */
+		case PqMsg_Sync:
 			maxmsglen = PQ_SMALL_MESSAGE_LIMIT;
 			/* stop any active skip-till-Sync */
 			ignore_till_sync = false;
@@ -440,13 +440,13 @@ SocketBackend(StringInfo inBuf)
 			doing_extended_query_message = false;
 			break;
 
-		case 'd':				/* copy data */
+		case PqMsg_CopyData:
 			maxmsglen = PQ_LARGE_MESSAGE_LIMIT;
 			doing_extended_query_message = false;
 			break;
 
-		case 'c':				/* copy done */
-		case 'f':				/* copy fail */
+		case PqMsg_CopyDone:
+		case PqMsg_CopyFail:
 			maxmsglen = PQ_SMALL_MESSAGE_LIMIT;
 			doing_extended_query_message = false;
 			break;
@@ -1589,7 +1589,7 @@ exec_parse_message(const char *query_string,	/* string to execute */
 	 * Send ParseComplete.
 	 */
 	if (whereToSendOutput == DestRemote)
-		pq_putemptymessage('1');
+		pq_putemptymessage(PqMsg_ParseComplete);
 
 	/*
 	 * Emit duration logging if appropriate.
@@ -2047,7 +2047,7 @@ exec_bind_message(StringInfo input_message)
 	 * Send BindComplete.
 	 */
 	if (whereToSendOutput == DestRemote)
-		pq_putemptymessage('2');
+		pq_putemptymessage(PqMsg_BindComplete);
 
 	/*
 	 * Emit duration logging if appropriate.
@@ -2290,7 +2290,7 @@ exec_execute_message(const char *portal_name, long max_rows)
 	{
 		/* Portal run not complete, so send PortalSuspended */
 		if (whereToSendOutput == DestRemote)
-			pq_putemptymessage('s');
+			pq_putemptymessage(PqMsg_PortalSuspended);
 
 		/*
 		 * Set XACT_FLAGS_PIPELINING whenever we suspend an Execute message,
@@ -2683,7 +2683,7 @@ exec_describe_statement_message(const char *stmt_name)
 								  NULL);
 	}
 	else
-		pq_putemptymessage('n');	/* NoData */
+		pq_putemptymessage(PqMsg_NoData);
 }
 
 /*
@@ -2736,7 +2736,7 @@ exec_describe_portal_message(const char *portal_name)
 								  FetchPortalTargetList(portal),
 								  portal->formats);
 	else
-		pq_putemptymessage('n');	/* NoData */
+		pq_putemptymessage(PqMsg_NoData);
 }
 
 
@@ -4239,7 +4239,7 @@ PostgresMain(const char *dbname, const char *username)
 	{
 		StringInfoData buf;
 
-		pq_beginmessage(&buf, 'K');
+		pq_beginmessage(&buf, PqMsg_BackendKeyData);
 		pq_sendint32(&buf, (int32) MyProcPid);
 		pq_sendint32(&buf, (int32) MyCancelKey);
 		pq_endmessage(&buf);
@@ -4618,7 +4618,7 @@ PostgresMain(const char *dbname, const char *username)
 
 		switch (firstchar)
 		{
-			case 'Q':			/* simple query */
+			case PqMsg_Query:
 				{
 					const char *query_string;
 
@@ -4642,7 +4642,7 @@ PostgresMain(const char *dbname, const char *username)
 				}
 				break;
 
-			case 'P':			/* parse */
+			case PqMsg_Parse:
 				{
 					const char *stmt_name;
 					const char *query_string;
@@ -4672,7 +4672,7 @@ PostgresMain(const char *dbname, const char *username)
 				}
 				break;
 
-			case 'B':			/* bind */
+			case PqMsg_Bind:
 				forbidden_in_wal_sender(firstchar);
 
 				/* Set statement_timestamp() */
@@ -4687,7 +4687,7 @@ PostgresMain(const char *dbname, const char *username)
 				/* exec_bind_message does valgrind_report_error_query */
 				break;
 
-			case 'E':			/* execute */
+			case PqMsg_Execute:
 				{
 					const char *portal_name;
 					int			max_rows;
@@ -4707,7 +4707,7 @@ PostgresMain(const char *dbname, const char *username)
 				}
 				break;
 
-			case 'F':			/* fastpath function call */
+			case PqMsg_FunctionCall:
 				forbidden_in_wal_sender(firstchar);
 
 				/* Set statement_timestamp() */
@@ -4742,7 +4742,7 @@ PostgresMain(const char *dbname, const char *username)
 				send_ready_for_query = true;
 				break;
 
-			case 'C':			/* close */
+			case PqMsg_Close:
 				{
 					int			close_type;
 					const char *close_target;
@@ -4782,13 +4782,13 @@ PostgresMain(const char *dbname, const char *username)
 					}
 
 					if (whereToSendOutput == DestRemote)
-						pq_putemptymessage('3');	/* CloseComplete */
+						pq_putemptymessage(PqMsg_CloseComplete);
 
 					valgrind_report_error_query("CLOSE message");
 				}
 				break;
 
-			case 'D':			/* describe */
+			case PqMsg_Describe:
 				{
 					int			describe_type;
 					const char *describe_target;
@@ -4822,13 +4822,13 @@ PostgresMain(const char *dbname, const char *username)
 				}
 				break;
 
-			case 'H':			/* flush */
+			case PqMsg_Flush:
 				pq_getmsgend(&input_message);
 				if (whereToSendOutput == DestRemote)
 					pq_flush();
 				break;
 
-			case 'S':			/* sync */
+			case PqMsg_Sync:
 				pq_getmsgend(&input_message);
 				finish_xact_command();
 				valgrind_report_error_query("SYNC message");
@@ -4847,7 +4847,7 @@ PostgresMain(const char *dbname, const char *username)
 
 				/* FALLTHROUGH */
 
-			case 'X':
+			case PqMsg_Terminate:
 
 				/*
 				 * Reset whereToSendOutput to prevent ereport from attempting
@@ -4865,9 +4865,9 @@ PostgresMain(const char *dbname, const char *username)
 				 */
 				proc_exit(0);
 
-			case 'd':			/* copy data */
-			case 'c':			/* copy done */
-			case 'f':			/* copy fail */
+			case PqMsg_CopyData:
+			case PqMsg_CopyDone:
+			case PqMsg_CopyFail:
 
 				/*
 				 * Accept but ignore these messages, per protocol spec; we
@@ -4897,7 +4897,7 @@ forbidden_in_wal_sender(char firstchar)
 {
 	if (am_walsender)
 	{
-		if (firstchar == 'F')
+		if (firstchar == PqMsg_FunctionCall)
 			ereport(ERROR,
 					(errcode(ERRCODE_PROTOCOL_VIOLATION),
 					 errmsg("fastpath function calls not supported in a replication connection")));
diff --git a/src/backend/utils/error/elog.c b/src/backend/utils/error/elog.c
index 5898100acb..8e1f3e8521 100644
--- a/src/backend/utils/error/elog.c
+++ b/src/backend/utils/error/elog.c
@@ -3465,7 +3465,10 @@ send_message_to_frontend(ErrorData *edata)
 		char		tbuf[12];
 
 		/* 'N' (Notice) is for nonfatal conditions, 'E' is for errors */
-		pq_beginmessage(&msgbuf, (edata->elevel < ERROR) ? 'N' : 'E');
+		if (edata->elevel < ERROR)
+			pq_beginmessage(&msgbuf, PqMsg_NoticeResponse);
+		else
+			pq_beginmessage(&msgbuf, PqMsg_ErrorResponse);
 
 		sev = error_severity(edata->elevel);
 		pq_sendbyte(&msgbuf, PG_DIAG_SEVERITY);
diff --git a/src/backend/utils/misc/guc.c b/src/backend/utils/misc/guc.c
index 99bb2fdd19..84e7ad4d90 100644
--- a/src/backend/utils/misc/guc.c
+++ b/src/backend/utils/misc/guc.c
@@ -2593,7 +2593,7 @@ ReportGUCOption(struct config_generic *record)
 	{
 		StringInfoData msgbuf;
 
-		pq_beginmessage(&msgbuf, 'S');
+		pq_beginmessage(&msgbuf, PqMsg_ParameterStatus);
 		pq_sendstring(&msgbuf, record->name);
 		pq_sendstring(&msgbuf, val);
 		pq_endmessage(&msgbuf);
diff --git a/src/include/libpq/pqcomm.h b/src/include/libpq/pqcomm.h
index 3da00f7983..f0e90d2604 100644
--- a/src/include/libpq/pqcomm.h
+++ b/src/include/libpq/pqcomm.h
@@ -112,6 +112,57 @@ typedef uint32 PacketLen;
 #define MAX_STARTUP_PACKET_LENGTH 10000
 
 
+/* These are the request codes sent by the frontend. */
+
+#define PqMsg_Bind					'B'
+#define PqMsg_Close					'C'
+#define PqMsg_Describe				'D'
+#define PqMsg_Execute				'E'
+#define PqMsg_FunctionCall			'F'
+#define PqMsg_Flush					'H'
+#define PqMsg_Parse					'P'
+#define PqMsg_Query					'Q'
+#define PqMsg_Sync					'S'
+#define PqMsg_Terminate				'X'
+#define PqMsg_CopyFail				'f'
+#define PqMsg_GSSResponse			'p'
+#define PqMsg_PasswordMessage		'p'
+#define PqMsg_SASLInitialResponse	'p'
+#define PqMsg_SASLResponse			'p'
+
+
+/* These are the response codes sent by the backend. */
+
+#define PqMsg_ParseComplete			'1'
+#define PqMsg_BindComplete			'2'
+#define PqMsg_CloseComplete			'3'
+#define PqMsg_NotificationResponse	'A'
+#define PqMsg_CommandComplete		'C'
+#define PqMsg_DataRow				'D'
+#define PqMsg_ErrorResponse			'E'
+#define PqMsg_CopyInResponse		'G'
+#define PqMsg_CopyOutResponse		'H'
+#define PqMsg_EmptyQueryResponse	'I'
+#define PqMsg_BackendKeyData		'K'
+#define PqMsg_NoticeResponse		'N'
+#define PqMsg_AuthenticationRequest 'R'
+#define PqMsg_ParameterStatus		'S'
+#define PqMsg_RowDescription		'T'
+#define PqMsg_FunctionCallResponse	'V'
+#define PqMsg_CopyBothResponse		'W'
+#define PqMsg_ReadyForQuery			'Z'
+#define PqMsg_NoData				'n'
+#define PqMsg_PortalSuspended		's'
+#define PqMsg_ParameterDescription	't'
+#define PqMsg_NegotiateProtocolVersion 'v'
+
+
+/* These are the codes sent by both the frontend and backend. */
+
+#define PqMsg_CopyDone				'c'
+#define PqMsg_CopyData				'd'
+
+
 /* These are the authentication request codes sent by the backend. */
 
 #define AUTH_REQ_OK			0	/* User is authenticated  */
diff --git a/src/interfaces/libpq/fe-auth.c b/src/interfaces/libpq/fe-auth.c
index 887ca5e9e1..912aa14821 100644
--- a/src/interfaces/libpq/fe-auth.c
+++ b/src/interfaces/libpq/fe-auth.c
@@ -586,7 +586,7 @@ pg_SASL_init(PGconn *conn, int payloadlen)
 	/*
 	 * Build a SASLInitialResponse message, and send it.
 	 */
-	if (pqPutMsgStart('p', conn))
+	if (pqPutMsgStart(PqMsg_SASLInitialResponse, conn))
 		goto error;
 	if (pqPuts(selected_mechanism, conn))
 		goto error;
diff --git a/src/interfaces/libpq/fe-connect.c b/src/interfaces/libpq/fe-connect.c
index 837c5321aa..bf83a9b569 100644
--- a/src/interfaces/libpq/fe-connect.c
+++ b/src/interfaces/libpq/fe-connect.c
@@ -3591,7 +3591,9 @@ keep_going:						/* We will come back to here until there is
 				 * Anything else probably means it's not Postgres on the other
 				 * end at all.
 				 */
-				if (!(beresp == 'R' || beresp == 'v' || beresp == 'E'))
+				if (beresp != PqMsg_AuthenticationRequest &&
+					beresp != PqMsg_ErrorResponse &&
+					beresp != PqMsg_NegotiateProtocolVersion)
 				{
 					libpq_append_conn_error(conn, "expected authentication request from server, but received %c",
 											beresp);
@@ -3618,19 +3620,22 @@ keep_going:						/* We will come back to here until there is
 				 * version 14, the server also used the old protocol for
 				 * errors that happened before processing the startup packet.)
 				 */
-				if (beresp == 'R' && (msgLength < 8 || msgLength > 2000))
+				if (beresp == PqMsg_AuthenticationRequest &&
+					(msgLength < 8 || msgLength > 2000))
 				{
 					libpq_append_conn_error(conn, "received invalid authentication request");
 					goto error_return;
 				}
-				if (beresp == 'v' && (msgLength < 8 || msgLength > 2000))
+				if (beresp == PqMsg_NegotiateProtocolVersion &&
+					(msgLength < 8 || msgLength > 2000))
 				{
 					libpq_append_conn_error(conn, "received invalid protocol negotiation message");
 					goto error_return;
 				}
 
 #define MAX_ERRLEN 30000
-				if (beresp == 'E' && (msgLength < 8 || msgLength > MAX_ERRLEN))
+				if (beresp == PqMsg_ErrorResponse &&
+					(msgLength < 8 || msgLength > MAX_ERRLEN))
 				{
 					/* Handle error from a pre-3.0 server */
 					conn->inCursor = conn->inStart + 1; /* reread data */
@@ -3693,7 +3698,7 @@ keep_going:						/* We will come back to here until there is
 				}
 
 				/* Handle errors. */
-				if (beresp == 'E')
+				if (beresp == PqMsg_ErrorResponse)
 				{
 					if (pqGetErrorNotice3(conn, true))
 					{
@@ -3770,7 +3775,7 @@ keep_going:						/* We will come back to here until there is
 
 					goto error_return;
 				}
-				else if (beresp == 'v')
+				else if (beresp == PqMsg_NegotiateProtocolVersion)
 				{
 					if (pqGetNegotiateProtocolVersion3(conn))
 					{
@@ -4540,7 +4545,7 @@ sendTerminateConn(PGconn *conn)
 		 * Try to send "close connection" message to backend. Ignore any
 		 * error.
 		 */
-		pqPutMsgStart('X', conn);
+		pqPutMsgStart(PqMsg_Terminate, conn);
 		pqPutMsgEnd(conn);
 		(void) pqFlush(conn);
 	}
diff --git a/src/interfaces/libpq/fe-exec.c b/src/interfaces/libpq/fe-exec.c
index a868284ff8..fdb7994779 100644
--- a/src/interfaces/libpq/fe-exec.c
+++ b/src/interfaces/libpq/fe-exec.c
@@ -1458,7 +1458,7 @@ PQsendQueryInternal(PGconn *conn, const char *query, bool newQuery)
 
 	/* Send the query message(s) */
 	/* construct the outgoing Query message */
-	if (pqPutMsgStart('Q', conn) < 0 ||
+	if (pqPutMsgStart(PqMsg_Query, conn) < 0 ||
 		pqPuts(query, conn) < 0 ||
 		pqPutMsgEnd(conn) < 0)
 	{
@@ -1571,7 +1571,7 @@ PQsendPrepare(PGconn *conn,
 		return 0;				/* error msg already set */
 
 	/* construct the Parse message */
-	if (pqPutMsgStart('P', conn) < 0 ||
+	if (pqPutMsgStart(PqMsg_Parse, conn) < 0 ||
 		pqPuts(stmtName, conn) < 0 ||
 		pqPuts(query, conn) < 0)
 		goto sendFailed;
@@ -1599,7 +1599,7 @@ PQsendPrepare(PGconn *conn,
 	/* Add a Sync, unless in pipeline mode. */
 	if (conn->pipelineStatus == PQ_PIPELINE_OFF)
 	{
-		if (pqPutMsgStart('S', conn) < 0 ||
+		if (pqPutMsgStart(PqMsg_Sync, conn) < 0 ||
 			pqPutMsgEnd(conn) < 0)
 			goto sendFailed;
 	}
@@ -1784,7 +1784,7 @@ PQsendQueryGuts(PGconn *conn,
 	if (command)
 	{
 		/* construct the Parse message */
-		if (pqPutMsgStart('P', conn) < 0 ||
+		if (pqPutMsgStart(PqMsg_Parse, conn) < 0 ||
 			pqPuts(stmtName, conn) < 0 ||
 			pqPuts(command, conn) < 0)
 			goto sendFailed;
@@ -1808,7 +1808,7 @@ PQsendQueryGuts(PGconn *conn,
 	}
 
 	/* Construct the Bind message */
-	if (pqPutMsgStart('B', conn) < 0 ||
+	if (pqPutMsgStart(PqMsg_Bind, conn) < 0 ||
 		pqPuts("", conn) < 0 ||
 		pqPuts(stmtName, conn) < 0)
 		goto sendFailed;
@@ -1874,14 +1874,14 @@ PQsendQueryGuts(PGconn *conn,
 		goto sendFailed;
 
 	/* construct the Describe Portal message */
-	if (pqPutMsgStart('D', conn) < 0 ||
+	if (pqPutMsgStart(PqMsg_Describe, conn) < 0 ||
 		pqPutc('P', conn) < 0 ||
 		pqPuts("", conn) < 0 ||
 		pqPutMsgEnd(conn) < 0)
 		goto sendFailed;
 
 	/* construct the Execute message */
-	if (pqPutMsgStart('E', conn) < 0 ||
+	if (pqPutMsgStart(PqMsg_Execute, conn) < 0 ||
 		pqPuts("", conn) < 0 ||
 		pqPutInt(0, 4, conn) < 0 ||
 		pqPutMsgEnd(conn) < 0)
@@ -1890,7 +1890,7 @@ PQsendQueryGuts(PGconn *conn,
 	/* construct the Sync message if not in pipeline mode */
 	if (conn->pipelineStatus == PQ_PIPELINE_OFF)
 	{
-		if (pqPutMsgStart('S', conn) < 0 ||
+		if (pqPutMsgStart(PqMsg_Sync, conn) < 0 ||
 			pqPutMsgEnd(conn) < 0)
 			goto sendFailed;
 	}
@@ -2422,7 +2422,7 @@ PQdescribePrepared(PGconn *conn, const char *stmt)
 {
 	if (!PQexecStart(conn))
 		return NULL;
-	if (!PQsendTypedCommand(conn, 'D', 'S', stmt))
+	if (!PQsendTypedCommand(conn, PqMsg_Describe, 'S', stmt))
 		return NULL;
 	return PQexecFinish(conn);
 }
@@ -2441,7 +2441,7 @@ PQdescribePortal(PGconn *conn, const char *portal)
 {
 	if (!PQexecStart(conn))
 		return NULL;
-	if (!PQsendTypedCommand(conn, 'D', 'P', portal))
+	if (!PQsendTypedCommand(conn, PqMsg_Describe, 'P', portal))
 		return NULL;
 	return PQexecFinish(conn);
 }
@@ -2456,7 +2456,7 @@ PQdescribePortal(PGconn *conn, const char *portal)
 int
 PQsendDescribePrepared(PGconn *conn, const char *stmt)
 {
-	return PQsendTypedCommand(conn, 'D', 'S', stmt);
+	return PQsendTypedCommand(conn, PqMsg_Describe, 'S', stmt);
 }
 
 /*
@@ -2469,7 +2469,7 @@ PQsendDescribePrepared(PGconn *conn, const char *stmt)
 int
 PQsendDescribePortal(PGconn *conn, const char *portal)
 {
-	return PQsendTypedCommand(conn, 'D', 'P', portal);
+	return PQsendTypedCommand(conn, PqMsg_Describe, 'P', portal);
 }
 
 /*
@@ -2488,7 +2488,7 @@ PQclosePrepared(PGconn *conn, const char *stmt)
 {
 	if (!PQexecStart(conn))
 		return NULL;
-	if (!PQsendTypedCommand(conn, 'C', 'S', stmt))
+	if (!PQsendTypedCommand(conn, PqMsg_Close, 'S', stmt))
 		return NULL;
 	return PQexecFinish(conn);
 }
@@ -2506,7 +2506,7 @@ PQclosePortal(PGconn *conn, const char *portal)
 {
 	if (!PQexecStart(conn))
 		return NULL;
-	if (!PQsendTypedCommand(conn, 'C', 'P', portal))
+	if (!PQsendTypedCommand(conn, PqMsg_Close, 'P', portal))
 		return NULL;
 	return PQexecFinish(conn);
 }
@@ -2521,7 +2521,7 @@ PQclosePortal(PGconn *conn, const char *portal)
 int
 PQsendClosePrepared(PGconn *conn, const char *stmt)
 {
-	return PQsendTypedCommand(conn, 'C', 'S', stmt);
+	return PQsendTypedCommand(conn, PqMsg_Close, 'S', stmt);
 }
 
 /*
@@ -2534,7 +2534,7 @@ PQsendClosePrepared(PGconn *conn, const char *stmt)
 int
 PQsendClosePortal(PGconn *conn, const char *portal)
 {
-	return PQsendTypedCommand(conn, 'C', 'P', portal);
+	return PQsendTypedCommand(conn, PqMsg_Close, 'P', portal);
 }
 
 /*
@@ -2577,17 +2577,17 @@ PQsendTypedCommand(PGconn *conn, char command, char type, const char *target)
 	/* construct the Sync message */
 	if (conn->pipelineStatus == PQ_PIPELINE_OFF)
 	{
-		if (pqPutMsgStart('S', conn) < 0 ||
+		if (pqPutMsgStart(PqMsg_Sync, conn) < 0 ||
 			pqPutMsgEnd(conn) < 0)
 			goto sendFailed;
 	}
 
 	/* remember if we are doing a Close or a Describe */
-	if (command == 'C')
+	if (command == PqMsg_Close)
 	{
 		entry->queryclass = PGQUERY_CLOSE;
 	}
-	else if (command == 'D')
+	else if (command == PqMsg_Describe)
 	{
 		entry->queryclass = PGQUERY_DESCRIBE;
 	}
@@ -2696,7 +2696,7 @@ PQputCopyData(PGconn *conn, const char *buffer, int nbytes)
 				return pqIsnonblocking(conn) ? 0 : -1;
 		}
 		/* Send the data (too simple to delegate to fe-protocol files) */
-		if (pqPutMsgStart('d', conn) < 0 ||
+		if (pqPutMsgStart(PqMsg_CopyData, conn) < 0 ||
 			pqPutnchar(buffer, nbytes, conn) < 0 ||
 			pqPutMsgEnd(conn) < 0)
 			return -1;
@@ -2731,7 +2731,7 @@ PQputCopyEnd(PGconn *conn, const char *errormsg)
 	if (errormsg)
 	{
 		/* Send COPY FAIL */
-		if (pqPutMsgStart('f', conn) < 0 ||
+		if (pqPutMsgStart(PqMsg_CopyFail, conn) < 0 ||
 			pqPuts(errormsg, conn) < 0 ||
 			pqPutMsgEnd(conn) < 0)
 			return -1;
@@ -2739,7 +2739,7 @@ PQputCopyEnd(PGconn *conn, const char *errormsg)
 	else
 	{
 		/* Send COPY DONE */
-		if (pqPutMsgStart('c', conn) < 0 ||
+		if (pqPutMsgStart(PqMsg_CopyDone, conn) < 0 ||
 			pqPutMsgEnd(conn) < 0)
 			return -1;
 	}
@@ -2751,7 +2751,7 @@ PQputCopyEnd(PGconn *conn, const char *errormsg)
 	if (conn->cmd_queue_head &&
 		conn->cmd_queue_head->queryclass != PGQUERY_SIMPLE)
 	{
-		if (pqPutMsgStart('S', conn) < 0 ||
+		if (pqPutMsgStart(PqMsg_Sync, conn) < 0 ||
 			pqPutMsgEnd(conn) < 0)
 			return -1;
 	}
@@ -3263,7 +3263,7 @@ PQpipelineSync(PGconn *conn)
 	entry->query = NULL;
 
 	/* construct the Sync message */
-	if (pqPutMsgStart('S', conn) < 0 ||
+	if (pqPutMsgStart(PqMsg_Sync, conn) < 0 ||
 		pqPutMsgEnd(conn) < 0)
 		goto sendFailed;
 
@@ -3311,7 +3311,7 @@ PQsendFlushRequest(PGconn *conn)
 		return 0;
 	}
 
-	if (pqPutMsgStart('H', conn) < 0 ||
+	if (pqPutMsgStart(PqMsg_Flush, conn) < 0 ||
 		pqPutMsgEnd(conn) < 0)
 	{
 		return 0;
diff --git a/src/interfaces/libpq/fe-protocol3.c b/src/interfaces/libpq/fe-protocol3.c
index 7bc6355d17..5613c56b14 100644
--- a/src/interfaces/libpq/fe-protocol3.c
+++ b/src/interfaces/libpq/fe-protocol3.c
@@ -34,8 +34,13 @@
  * than a couple of kilobytes).
  */
 #define VALID_LONG_MESSAGE_TYPE(id) \
-	((id) == 'T' || (id) == 'D' || (id) == 'd' || (id) == 'V' || \
-	 (id) == 'E' || (id) == 'N' || (id) == 'A')
+	((id) == PqMsg_CopyData || \
+	 (id) == PqMsg_DataRow || \
+	 (id) == PqMsg_ErrorResponse || \
+	 (id) == PqMsg_FunctionCallResponse || \
+	 (id) == PqMsg_NoticeResponse || \
+	 (id) == PqMsg_NotificationResponse || \
+	 (id) == PqMsg_RowDescription)
 
 
 static void handleSyncLoss(PGconn *conn, char id, int msgLength);
@@ -140,12 +145,12 @@ pqParseInput3(PGconn *conn)
 		 * from config file due to SIGHUP), but otherwise we hold off until
 		 * BUSY state.
 		 */
-		if (id == 'A')
+		if (id == PqMsg_NotificationResponse)
 		{
 			if (getNotify(conn))
 				return;
 		}
-		else if (id == 'N')
+		else if (id == PqMsg_NoticeResponse)
 		{
 			if (pqGetErrorNotice3(conn, false))
 				return;
@@ -165,12 +170,12 @@ pqParseInput3(PGconn *conn)
 			 * it is about to close the connection, so we don't want to just
 			 * discard it...)
 			 */
-			if (id == 'E')
+			if (id == PqMsg_ErrorResponse)
 			{
 				if (pqGetErrorNotice3(conn, false /* treat as notice */ ))
 					return;
 			}
-			else if (id == 'S')
+			else if (id == PqMsg_ParameterStatus)
 			{
 				if (getParameterStatus(conn))
 					return;
@@ -192,7 +197,7 @@ pqParseInput3(PGconn *conn)
 			 */
 			switch (id)
 			{
-				case 'C':		/* command complete */
+				case PqMsg_CommandComplete:
 					if (pqGets(&conn->workBuffer, conn))
 						return;
 					if (!pgHavePendingResult(conn))
@@ -210,13 +215,12 @@ pqParseInput3(PGconn *conn)
 								CMDSTATUS_LEN);
 					conn->asyncStatus = PGASYNC_READY;
 					break;
-				case 'E':		/* error return */
+				case PqMsg_ErrorResponse:
 					if (pqGetErrorNotice3(conn, true))
 						return;
 					conn->asyncStatus = PGASYNC_READY;
 					break;
-				case 'Z':		/* sync response, backend is ready for new
-								 * query */
+				case PqMsg_ReadyForQuery:
 					if (getReadyForQuery(conn))
 						return;
 					if (conn->pipelineStatus != PQ_PIPELINE_OFF)
@@ -246,7 +250,7 @@ pqParseInput3(PGconn *conn)
 						conn->asyncStatus = PGASYNC_IDLE;
 					}
 					break;
-				case 'I':		/* empty query */
+				case PqMsg_EmptyQueryResponse:
 					if (!pgHavePendingResult(conn))
 					{
 						conn->result = PQmakeEmptyPGresult(conn,
@@ -259,7 +263,7 @@ pqParseInput3(PGconn *conn)
 					}
 					conn->asyncStatus = PGASYNC_READY;
 					break;
-				case '1':		/* Parse Complete */
+				case PqMsg_ParseComplete:
 					/* If we're doing PQprepare, we're done; else ignore */
 					if (conn->cmd_queue_head &&
 						conn->cmd_queue_head->queryclass == PGQUERY_PREPARE)
@@ -277,10 +281,10 @@ pqParseInput3(PGconn *conn)
 						conn->asyncStatus = PGASYNC_READY;
 					}
 					break;
-				case '2':		/* Bind Complete */
+				case PqMsg_BindComplete:
 					/* Nothing to do for this message type */
 					break;
-				case '3':		/* Close Complete */
+				case PqMsg_CloseComplete:
 					/* If we're doing PQsendClose, we're done; else ignore */
 					if (conn->cmd_queue_head &&
 						conn->cmd_queue_head->queryclass == PGQUERY_CLOSE)
@@ -298,11 +302,11 @@ pqParseInput3(PGconn *conn)
 						conn->asyncStatus = PGASYNC_READY;
 					}
 					break;
-				case 'S':		/* parameter status */
+				case PqMsg_ParameterStatus:
 					if (getParameterStatus(conn))
 						return;
 					break;
-				case 'K':		/* secret key data from the backend */
+				case PqMsg_BackendKeyData:
 
 					/*
 					 * This is expected only during backend startup, but it's
@@ -314,7 +318,7 @@ pqParseInput3(PGconn *conn)
 					if (pqGetInt(&(conn->be_key), 4, conn))
 						return;
 					break;
-				case 'T':		/* Row Description */
+				case PqMsg_RowDescription:
 					if (conn->error_result ||
 						(conn->result != NULL &&
 						 conn->result->resultStatus == PGRES_FATAL_ERROR))
@@ -346,7 +350,7 @@ pqParseInput3(PGconn *conn)
 						return;
 					}
 					break;
-				case 'n':		/* No Data */
+				case PqMsg_NoData:
 
 					/*
 					 * NoData indicates that we will not be seeing a
@@ -374,11 +378,11 @@ pqParseInput3(PGconn *conn)
 						conn->asyncStatus = PGASYNC_READY;
 					}
 					break;
-				case 't':		/* Parameter Description */
+				case PqMsg_ParameterDescription:
 					if (getParamDescriptions(conn, msgLength))
 						return;
 					break;
-				case 'D':		/* Data Row */
+				case PqMsg_DataRow:
 					if (conn->result != NULL &&
 						conn->result->resultStatus == PGRES_TUPLES_OK)
 					{
@@ -405,24 +409,24 @@ pqParseInput3(PGconn *conn)
 						conn->inCursor += msgLength;
 					}
 					break;
-				case 'G':		/* Start Copy In */
+				case PqMsg_CopyInResponse:
 					if (getCopyStart(conn, PGRES_COPY_IN))
 						return;
 					conn->asyncStatus = PGASYNC_COPY_IN;
 					break;
-				case 'H':		/* Start Copy Out */
+				case PqMsg_CopyOutResponse:
 					if (getCopyStart(conn, PGRES_COPY_OUT))
 						return;
 					conn->asyncStatus = PGASYNC_COPY_OUT;
 					conn->copy_already_done = 0;
 					break;
-				case 'W':		/* Start Copy Both */
+				case PqMsg_CopyBothResponse:
 					if (getCopyStart(conn, PGRES_COPY_BOTH))
 						return;
 					conn->asyncStatus = PGASYNC_COPY_BOTH;
 					conn->copy_already_done = 0;
 					break;
-				case 'd':		/* Copy Data */
+				case PqMsg_CopyData:
 
 					/*
 					 * If we see Copy Data, just silently drop it.  This would
@@ -431,7 +435,7 @@ pqParseInput3(PGconn *conn)
 					 */
 					conn->inCursor += msgLength;
 					break;
-				case 'c':		/* Copy Done */
+				case PqMsg_CopyDone:
 
 					/*
 					 * If we see Copy Done, just silently drop it.  This is
@@ -1692,21 +1696,21 @@ getCopyDataMessage(PGconn *conn)
 		 */
 		switch (id)
 		{
-			case 'A':			/* NOTIFY */
+			case PqMsg_NotificationResponse:
 				if (getNotify(conn))
 					return 0;
 				break;
-			case 'N':			/* NOTICE */
+			case PqMsg_NoticeResponse:
 				if (pqGetErrorNotice3(conn, false))
 					return 0;
 				break;
-			case 'S':			/* ParameterStatus */
+			case PqMsg_ParameterStatus:
 				if (getParameterStatus(conn))
 					return 0;
 				break;
-			case 'd':			/* Copy Data, pass it back to caller */
+			case PqMsg_CopyData:
 				return msgLength;
-			case 'c':
+			case PqMsg_CopyDone:
 
 				/*
 				 * If this is a CopyDone message, exit COPY_OUT mode and let
@@ -1929,7 +1933,7 @@ pqEndcopy3(PGconn *conn)
 	if (conn->asyncStatus == PGASYNC_COPY_IN ||
 		conn->asyncStatus == PGASYNC_COPY_BOTH)
 	{
-		if (pqPutMsgStart('c', conn) < 0 ||
+		if (pqPutMsgStart(PqMsg_CopyDone, conn) < 0 ||
 			pqPutMsgEnd(conn) < 0)
 			return 1;
 
@@ -1940,7 +1944,7 @@ pqEndcopy3(PGconn *conn)
 		if (conn->cmd_queue_head &&
 			conn->cmd_queue_head->queryclass != PGQUERY_SIMPLE)
 		{
-			if (pqPutMsgStart('S', conn) < 0 ||
+			if (pqPutMsgStart(PqMsg_Sync, conn) < 0 ||
 				pqPutMsgEnd(conn) < 0)
 				return 1;
 		}
@@ -2023,7 +2027,7 @@ pqFunctionCall3(PGconn *conn, Oid fnid,
 
 	/* PQfn already validated connection state */
 
-	if (pqPutMsgStart('F', conn) < 0 || /* function call msg */
+	if (pqPutMsgStart(PqMsg_FunctionCall, conn) < 0 ||
 		pqPutInt(fnid, 4, conn) < 0 ||	/* function id */
 		pqPutInt(1, 2, conn) < 0 || /* # of format codes */
 		pqPutInt(1, 2, conn) < 0 || /* format code: BINARY */
diff --git a/src/interfaces/libpq/fe-trace.c b/src/interfaces/libpq/fe-trace.c
index 402784f40e..b18e3deab6 100644
--- a/src/interfaces/libpq/fe-trace.c
+++ b/src/interfaces/libpq/fe-trace.c
@@ -562,110 +562,120 @@ pqTraceOutputMessage(PGconn *conn, const char *message, bool toServer)
 
 	switch (id)
 	{
-		case '1':
+		case PqMsg_ParseComplete:
 			fprintf(conn->Pfdebug, "ParseComplete");
 			/* No message content */
 			break;
-		case '2':
+		case PqMsg_BindComplete:
 			fprintf(conn->Pfdebug, "BindComplete");
 			/* No message content */
 			break;
-		case '3':
+		case PqMsg_CloseComplete:
 			fprintf(conn->Pfdebug, "CloseComplete");
 			/* No message content */
 			break;
-		case 'A':				/* Notification Response */
+		case PqMsg_NotificationResponse:
 			pqTraceOutputA(conn->Pfdebug, message, &logCursor, regress);
 			break;
-		case 'B':				/* Bind */
+		case PqMsg_Bind:
 			pqTraceOutputB(conn->Pfdebug, message, &logCursor);
 			break;
-		case 'c':
+		case PqMsg_CopyDone:
 			fprintf(conn->Pfdebug, "CopyDone");
 			/* No message content */
 			break;
-		case 'C':				/* Close(F) or Command Complete(B) */
+		case PqMsg_CommandComplete:
+			/* Close(F) and CommandComplete(B) use the same identifier. */
+			Assert(PqMsg_Close == PqMsg_CommandComplete);
 			pqTraceOutputC(conn->Pfdebug, toServer, message, &logCursor);
 			break;
-		case 'd':				/* Copy Data */
+		case PqMsg_CopyData:
 			/* Drop COPY data to reduce the overhead of logging. */
 			break;
-		case 'D':				/* Describe(F) or Data Row(B) */
+		case PqMsg_Describe:
+			/* Describe(F) and DataRow(B) use the same identifier. */
+			Assert(PqMsg_Describe == PqMsg_DataRow);
 			pqTraceOutputD(conn->Pfdebug, toServer, message, &logCursor);
 			break;
-		case 'E':				/* Execute(F) or Error Response(B) */
+		case PqMsg_Execute:
+			/* Execute(F) and ErrorResponse(B) use the same identifier. */
+			Assert(PqMsg_Execute == PqMsg_ErrorResponse);
 			pqTraceOutputE(conn->Pfdebug, toServer, message, &logCursor,
 						   regress);
 			break;
-		case 'f':				/* Copy Fail */
+		case PqMsg_CopyFail:
 			pqTraceOutputf(conn->Pfdebug, message, &logCursor);
 			break;
-		case 'F':				/* Function Call */
+		case PqMsg_FunctionCall:
 			pqTraceOutputF(conn->Pfdebug, message, &logCursor, regress);
 			break;
-		case 'G':				/* Start Copy In */
+		case PqMsg_CopyInResponse:
 			pqTraceOutputG(conn->Pfdebug, message, &logCursor);
 			break;
-		case 'H':				/* Flush(F) or Start Copy Out(B) */
+		case PqMsg_Flush:
+			/* Flush(F) and CopyOutResponse(B) use the same identifier */
+			Assert(PqMsg_CopyOutResponse == PqMsg_Flush);
 			if (!toServer)
 				pqTraceOutputH(conn->Pfdebug, message, &logCursor);
 			else
 				fprintf(conn->Pfdebug, "Flush");	/* no message content */
 			break;
-		case 'I':
+		case PqMsg_EmptyQueryResponse:
 			fprintf(conn->Pfdebug, "EmptyQueryResponse");
 			/* No message content */
 			break;
-		case 'K':				/* secret key data from the backend */
+		case PqMsg_BackendKeyData:
 			pqTraceOutputK(conn->Pfdebug, message, &logCursor, regress);
 			break;
-		case 'n':
+		case PqMsg_NoData:
 			fprintf(conn->Pfdebug, "NoData");
 			/* No message content */
 			break;
-		case 'N':
+		case PqMsg_NoticeResponse:
 			pqTraceOutputNR(conn->Pfdebug, "NoticeResponse", message,
 							&logCursor, regress);
 			break;
-		case 'P':				/* Parse */
+		case PqMsg_Parse:
 			pqTraceOutputP(conn->Pfdebug, message, &logCursor, regress);
 			break;
-		case 'Q':				/* Query */
+		case PqMsg_Query:
 			pqTraceOutputQ(conn->Pfdebug, message, &logCursor);
 			break;
-		case 'R':				/* Authentication */
+		case PqMsg_AuthenticationRequest:
 			pqTraceOutputR(conn->Pfdebug, message, &logCursor);
 			break;
-		case 's':
+		case PqMsg_PortalSuspended:
 			fprintf(conn->Pfdebug, "PortalSuspended");
 			/* No message content */
 			break;
-		case 'S':				/* Parameter Status(B) or Sync(F) */
+		case PqMsg_Sync:
+			/* Parameter Status(B) and Sync(F) use the same identifier */
+			Assert(PqMsg_ParameterStatus == PqMsg_Sync);
 			if (!toServer)
 				pqTraceOutputS(conn->Pfdebug, message, &logCursor);
 			else
 				fprintf(conn->Pfdebug, "Sync"); /* no message content */
 			break;
-		case 't':				/* Parameter Description */
+		case PqMsg_ParameterDescription:
 			pqTraceOutputt(conn->Pfdebug, message, &logCursor, regress);
 			break;
-		case 'T':				/* Row Description */
+		case PqMsg_RowDescription:
 			pqTraceOutputT(conn->Pfdebug, message, &logCursor, regress);
 			break;
-		case 'v':				/* Negotiate Protocol Version */
+		case PqMsg_NegotiateProtocolVersion:
 			pqTraceOutputv(conn->Pfdebug, message, &logCursor);
 			break;
-		case 'V':				/* Function Call response */
+		case PqMsg_FunctionCallResponse:
 			pqTraceOutputV(conn->Pfdebug, message, &logCursor);
 			break;
-		case 'W':				/* Start Copy Both */
+		case PqMsg_CopyBothResponse:
 			pqTraceOutputW(conn->Pfdebug, message, &logCursor, length);
 			break;
-		case 'X':
+		case PqMsg_Terminate:
 			fprintf(conn->Pfdebug, "Terminate");
 			/* No message content */
 			break;
-		case 'Z':				/* Ready For Query */
+		case PqMsg_ReadyForQuery:
 			pqTraceOutputZ(conn->Pfdebug, message, &logCursor);
 			break;
 		default:
-- 
2.25.1

From fc54d1655fe662abe99a4605bdff2d1b65f0dc2a Mon Sep 17 00:00:00 2001
From: Nathan Bossart <nathan@postgresql.org>
Date: Wed, 16 Aug 2023 12:08:29 -0700
Subject: [PATCH v6 1/1] Introduce macros for protocol characters.

Author: Dave Cramer
Reviewed-by: Alvaro Herrera, Tatsuo Ishii, Peter Smith, Robert Haas, Tom Lane, Peter Eisentraut
Discussion: https://postgr.es/m/CADK3HHKbBmK-PKf1bPNFoMC%2BoBt%2BpD9PH8h5nvmBQskEHm-Ehw%40mail.gmail.com
---
 src/backend/access/common/printsimple.c |  5 +-
 src/backend/access/transam/parallel.c   | 14 ++--
 src/backend/backup/basebackup_copy.c    | 16 ++---
 src/backend/commands/async.c            |  2 +-
 src/backend/commands/copyfromparse.c    | 22 +++----
 src/backend/commands/copyto.c           |  6 +-
 src/backend/libpq/auth-sasl.c           |  2 +-
 src/backend/libpq/auth.c                |  8 +--
 src/backend/postmaster/postmaster.c     |  2 +-
 src/backend/replication/walsender.c     | 18 +++---
 src/backend/tcop/dest.c                 |  8 +--
 src/backend/tcop/fastpath.c             |  2 +-
 src/backend/tcop/postgres.c             | 68 ++++++++++----------
 src/backend/utils/error/elog.c          |  5 +-
 src/backend/utils/misc/guc.c            |  2 +-
 src/include/Makefile                    |  3 +-
 src/include/libpq/pqcomm.h              | 23 ++-----
 src/include/libpq/protocol.h            | 85 +++++++++++++++++++++++++
 src/include/meson.build                 |  1 +
 src/interfaces/libpq/fe-auth.c          |  2 +-
 src/interfaces/libpq/fe-connect.c       | 19 ++++--
 src/interfaces/libpq/fe-exec.c          | 50 +++++++--------
 src/interfaces/libpq/fe-protocol3.c     | 70 ++++++++++----------
 src/interfaces/libpq/fe-trace.c         | 70 +++++++++++---------
 24 files changed, 301 insertions(+), 202 deletions(-)
 create mode 100644 src/include/libpq/protocol.h

diff --git a/src/backend/access/common/printsimple.c b/src/backend/access/common/printsimple.c
index ef818228ac..675b744db2 100644
--- a/src/backend/access/common/printsimple.c
+++ b/src/backend/access/common/printsimple.c
@@ -20,6 +20,7 @@
 
 #include "access/printsimple.h"
 #include "catalog/pg_type.h"
+#include "libpq/protocol.h"
 #include "libpq/pqformat.h"
 #include "utils/builtins.h"
 
@@ -32,7 +33,7 @@ printsimple_startup(DestReceiver *self, int operation, TupleDesc tupdesc)
 	StringInfoData buf;
 	int			i;
 
-	pq_beginmessage(&buf, 'T'); /* RowDescription */
+	pq_beginmessage(&buf, PqMsg_RowDescription);
 	pq_sendint16(&buf, tupdesc->natts);
 
 	for (i = 0; i < tupdesc->natts; ++i)
@@ -65,7 +66,7 @@ printsimple(TupleTableSlot *slot, DestReceiver *self)
 	slot_getallattrs(slot);
 
 	/* Prepare and send message */
-	pq_beginmessage(&buf, 'D');
+	pq_beginmessage(&buf, PqMsg_DataRow);
 	pq_sendint16(&buf, tupdesc->natts);
 
 	for (i = 0; i < tupdesc->natts; ++i)
diff --git a/src/backend/access/transam/parallel.c b/src/backend/access/transam/parallel.c
index 1738aecf1f..194a1207be 100644
--- a/src/backend/access/transam/parallel.c
+++ b/src/backend/access/transam/parallel.c
@@ -1127,7 +1127,7 @@ HandleParallelMessage(ParallelContext *pcxt, int i, StringInfo msg)
 
 	switch (msgtype)
 	{
-		case 'K':				/* BackendKeyData */
+		case PqMsg_BackendKeyData:
 			{
 				int32		pid = pq_getmsgint(msg, 4);
 
@@ -1137,8 +1137,8 @@ HandleParallelMessage(ParallelContext *pcxt, int i, StringInfo msg)
 				break;
 			}
 
-		case 'E':				/* ErrorResponse */
-		case 'N':				/* NoticeResponse */
+		case PqMsg_ErrorResponse:
+		case PqMsg_NoticeResponse:
 			{
 				ErrorData	edata;
 				ErrorContextCallback *save_error_context_stack;
@@ -1183,7 +1183,7 @@ HandleParallelMessage(ParallelContext *pcxt, int i, StringInfo msg)
 				break;
 			}
 
-		case 'A':				/* NotifyResponse */
+		case PqMsg_NotificationResponse:
 			{
 				/* Propagate NotifyResponse. */
 				int32		pid;
@@ -1217,7 +1217,7 @@ HandleParallelMessage(ParallelContext *pcxt, int i, StringInfo msg)
 				break;
 			}
 
-		case 'X':				/* Terminate, indicating clean exit */
+		case PqMsg_Terminate:
 			{
 				shm_mq_detach(pcxt->worker[i].error_mqh);
 				pcxt->worker[i].error_mqh = NULL;
@@ -1372,7 +1372,7 @@ ParallelWorkerMain(Datum main_arg)
 	 * protocol message is defined, but it won't actually be used for anything
 	 * in this case.
 	 */
-	pq_beginmessage(&msgbuf, 'K');
+	pq_beginmessage(&msgbuf, PqMsg_BackendKeyData);
 	pq_sendint32(&msgbuf, (int32) MyProcPid);
 	pq_sendint32(&msgbuf, (int32) MyCancelKey);
 	pq_endmessage(&msgbuf);
@@ -1550,7 +1550,7 @@ ParallelWorkerMain(Datum main_arg)
 	DetachSession();
 
 	/* Report success. */
-	pq_putmessage('X', NULL, 0);
+	pq_putmessage(PqMsg_Terminate, NULL, 0);
 }
 
 /*
diff --git a/src/backend/backup/basebackup_copy.c b/src/backend/backup/basebackup_copy.c
index 1db80cde1b..fee30c21e1 100644
--- a/src/backend/backup/basebackup_copy.c
+++ b/src/backend/backup/basebackup_copy.c
@@ -152,7 +152,7 @@ bbsink_copystream_begin_backup(bbsink *sink)
 	SendTablespaceList(state->tablespaces);
 
 	/* Send a CommandComplete message */
-	pq_puttextmessage('C', "SELECT");
+	pq_puttextmessage(PqMsg_CommandComplete, "SELECT");
 
 	/* Begin COPY stream. This will be used for all archives + manifest. */
 	SendCopyOutResponse();
@@ -169,7 +169,7 @@ bbsink_copystream_begin_archive(bbsink *sink, const char *archive_name)
 	StringInfoData buf;
 
 	ti = list_nth(state->tablespaces, state->tablespace_num);
-	pq_beginmessage(&buf, 'd'); /* CopyData */
+	pq_beginmessage(&buf, PqMsg_CopyData);
 	pq_sendbyte(&buf, 'n');		/* New archive */
 	pq_sendstring(&buf, archive_name);
 	pq_sendstring(&buf, ti->path == NULL ? "" : ti->path);
@@ -220,7 +220,7 @@ bbsink_copystream_archive_contents(bbsink *sink, size_t len)
 		{
 			mysink->last_progress_report_time = now;
 
-			pq_beginmessage(&buf, 'd'); /* CopyData */
+			pq_beginmessage(&buf, PqMsg_CopyData);
 			pq_sendbyte(&buf, 'p'); /* Progress report */
 			pq_sendint64(&buf, state->bytes_done);
 			pq_endmessage(&buf);
@@ -246,7 +246,7 @@ bbsink_copystream_end_archive(bbsink *sink)
 
 	mysink->bytes_done_at_last_time_check = state->bytes_done;
 	mysink->last_progress_report_time = GetCurrentTimestamp();
-	pq_beginmessage(&buf, 'd'); /* CopyData */
+	pq_beginmessage(&buf, PqMsg_CopyData);
 	pq_sendbyte(&buf, 'p');		/* Progress report */
 	pq_sendint64(&buf, state->bytes_done);
 	pq_endmessage(&buf);
@@ -261,7 +261,7 @@ bbsink_copystream_begin_manifest(bbsink *sink)
 {
 	StringInfoData buf;
 
-	pq_beginmessage(&buf, 'd'); /* CopyData */
+	pq_beginmessage(&buf, PqMsg_CopyData);
 	pq_sendbyte(&buf, 'm');		/* Manifest */
 	pq_endmessage(&buf);
 }
@@ -318,7 +318,7 @@ SendCopyOutResponse(void)
 {
 	StringInfoData buf;
 
-	pq_beginmessage(&buf, 'H');
+	pq_beginmessage(&buf, PqMsg_CopyOutResponse);
 	pq_sendbyte(&buf, 0);		/* overall format */
 	pq_sendint16(&buf, 0);		/* natts */
 	pq_endmessage(&buf);
@@ -330,7 +330,7 @@ SendCopyOutResponse(void)
 static void
 SendCopyDone(void)
 {
-	pq_putemptymessage('c');
+	pq_putemptymessage(PqMsg_CopyDone);
 }
 
 /*
@@ -368,7 +368,7 @@ SendXlogRecPtrResult(XLogRecPtr ptr, TimeLineID tli)
 	end_tup_output(tstate);
 
 	/* Send a CommandComplete message */
-	pq_puttextmessage('C', "SELECT");
+	pq_puttextmessage(PqMsg_CommandComplete, "SELECT");
 }
 
 /*
diff --git a/src/backend/commands/async.c b/src/backend/commands/async.c
index ef909cf4e0..d148d10850 100644
--- a/src/backend/commands/async.c
+++ b/src/backend/commands/async.c
@@ -2281,7 +2281,7 @@ NotifyMyFrontEnd(const char *channel, const char *payload, int32 srcPid)
 	{
 		StringInfoData buf;
 
-		pq_beginmessage(&buf, 'A');
+		pq_beginmessage(&buf, PqMsg_NotificationResponse);
 		pq_sendint32(&buf, srcPid);
 		pq_sendstring(&buf, channel);
 		pq_sendstring(&buf, payload);
diff --git a/src/backend/commands/copyfromparse.c b/src/backend/commands/copyfromparse.c
index 232768a6e1..f553734582 100644
--- a/src/backend/commands/copyfromparse.c
+++ b/src/backend/commands/copyfromparse.c
@@ -174,7 +174,7 @@ ReceiveCopyBegin(CopyFromState cstate)
 	int16		format = (cstate->opts.binary ? 1 : 0);
 	int			i;
 
-	pq_beginmessage(&buf, 'G');
+	pq_beginmessage(&buf, PqMsg_CopyInResponse);
 	pq_sendbyte(&buf, format);	/* overall format */
 	pq_sendint16(&buf, natts);
 	for (i = 0; i < natts; i++)
@@ -279,13 +279,13 @@ CopyGetData(CopyFromState cstate, void *databuf, int minread, int maxread)
 					/* Validate message type and set packet size limit */
 					switch (mtype)
 					{
-						case 'd':	/* CopyData */
+						case PqMsg_CopyData:
 							maxmsglen = PQ_LARGE_MESSAGE_LIMIT;
 							break;
-						case 'c':	/* CopyDone */
-						case 'f':	/* CopyFail */
-						case 'H':	/* Flush */
-						case 'S':	/* Sync */
+						case PqMsg_CopyDone:
+						case PqMsg_CopyFail:
+						case PqMsg_Flush:
+						case PqMsg_Sync:
 							maxmsglen = PQ_SMALL_MESSAGE_LIMIT;
 							break;
 						default:
@@ -305,20 +305,20 @@ CopyGetData(CopyFromState cstate, void *databuf, int minread, int maxread)
 					/* ... and process it */
 					switch (mtype)
 					{
-						case 'd':	/* CopyData */
+						case PqMsg_CopyData:
 							break;
-						case 'c':	/* CopyDone */
+						case PqMsg_CopyDone:
 							/* COPY IN correctly terminated by frontend */
 							cstate->raw_reached_eof = true;
 							return bytesread;
-						case 'f':	/* CopyFail */
+						case PqMsg_CopyFail:
 							ereport(ERROR,
 									(errcode(ERRCODE_QUERY_CANCELED),
 									 errmsg("COPY from stdin failed: %s",
 											pq_getmsgstring(cstate->fe_msgbuf))));
 							break;
-						case 'H':	/* Flush */
-						case 'S':	/* Sync */
+						case PqMsg_Flush:
+						case PqMsg_Sync:
 
 							/*
 							 * Ignore Flush/Sync for the convenience of client
diff --git a/src/backend/commands/copyto.c b/src/backend/commands/copyto.c
index 9e4b2437a5..eaa3172793 100644
--- a/src/backend/commands/copyto.c
+++ b/src/backend/commands/copyto.c
@@ -144,7 +144,7 @@ SendCopyBegin(CopyToState cstate)
 	int16		format = (cstate->opts.binary ? 1 : 0);
 	int			i;
 
-	pq_beginmessage(&buf, 'H');
+	pq_beginmessage(&buf, PqMsg_CopyOutResponse);
 	pq_sendbyte(&buf, format);	/* overall format */
 	pq_sendint16(&buf, natts);
 	for (i = 0; i < natts; i++)
@@ -159,7 +159,7 @@ SendCopyEnd(CopyToState cstate)
 	/* Shouldn't have any unsent data */
 	Assert(cstate->fe_msgbuf->len == 0);
 	/* Send Copy Done message */
-	pq_putemptymessage('c');
+	pq_putemptymessage(PqMsg_CopyDone);
 }
 
 /*----------
@@ -247,7 +247,7 @@ CopySendEndOfRow(CopyToState cstate)
 				CopySendChar(cstate, '\n');
 
 			/* Dump the accumulated row as one CopyData message */
-			(void) pq_putmessage('d', fe_msgbuf->data, fe_msgbuf->len);
+			(void) pq_putmessage(PqMsg_CopyData, fe_msgbuf->data, fe_msgbuf->len);
 			break;
 		case COPY_CALLBACK:
 			cstate->data_dest_cb(fe_msgbuf->data, fe_msgbuf->len);
diff --git a/src/backend/libpq/auth-sasl.c b/src/backend/libpq/auth-sasl.c
index 684680897b..c535bc5383 100644
--- a/src/backend/libpq/auth-sasl.c
+++ b/src/backend/libpq/auth-sasl.c
@@ -87,7 +87,7 @@ CheckSASLAuth(const pg_be_sasl_mech *mech, Port *port, char *shadow_pass,
 	{
 		pq_startmsgread();
 		mtype = pq_getbyte();
-		if (mtype != 'p')
+		if (mtype != PqMsg_SASLResponse)
 		{
 			/* Only log error if client didn't disconnect. */
 			if (mtype != EOF)
diff --git a/src/backend/libpq/auth.c b/src/backend/libpq/auth.c
index 315a24bb3f..0356fe3e45 100644
--- a/src/backend/libpq/auth.c
+++ b/src/backend/libpq/auth.c
@@ -665,7 +665,7 @@ sendAuthRequest(Port *port, AuthRequest areq, const char *extradata, int extrale
 
 	CHECK_FOR_INTERRUPTS();
 
-	pq_beginmessage(&buf, 'R');
+	pq_beginmessage(&buf, PqMsg_AuthenticationRequest);
 	pq_sendint32(&buf, (int32) areq);
 	if (extralen > 0)
 		pq_sendbytes(&buf, extradata, extralen);
@@ -698,7 +698,7 @@ recv_password_packet(Port *port)
 
 	/* Expect 'p' message type */
 	mtype = pq_getbyte();
-	if (mtype != 'p')
+	if (mtype != PqMsg_PasswordMessage)
 	{
 		/*
 		 * If the client just disconnects without offering a password, don't
@@ -961,7 +961,7 @@ pg_GSS_recvauth(Port *port)
 		CHECK_FOR_INTERRUPTS();
 
 		mtype = pq_getbyte();
-		if (mtype != 'p')
+		if (mtype != PqMsg_GSSResponse)
 		{
 			/* Only log error if client didn't disconnect. */
 			if (mtype != EOF)
@@ -1232,7 +1232,7 @@ pg_SSPI_recvauth(Port *port)
 	{
 		pq_startmsgread();
 		mtype = pq_getbyte();
-		if (mtype != 'p')
+		if (mtype != PqMsg_GSSResponse)
 		{
 			if (sspictx != NULL)
 			{
diff --git a/src/backend/postmaster/postmaster.c b/src/backend/postmaster/postmaster.c
index 9c8ec779f9..07d376d77e 100644
--- a/src/backend/postmaster/postmaster.c
+++ b/src/backend/postmaster/postmaster.c
@@ -2357,7 +2357,7 @@ SendNegotiateProtocolVersion(List *unrecognized_protocol_options)
 	StringInfoData buf;
 	ListCell   *lc;
 
-	pq_beginmessage(&buf, 'v'); /* NegotiateProtocolVersion */
+	pq_beginmessage(&buf, PqMsg_NegotiateProtocolVersion);
 	pq_sendint32(&buf, PG_PROTOCOL_LATEST);
 	pq_sendint32(&buf, list_length(unrecognized_protocol_options));
 	foreach(lc, unrecognized_protocol_options)
diff --git a/src/backend/replication/walsender.c b/src/backend/replication/walsender.c
index d27ef2985d..80374c55be 100644
--- a/src/backend/replication/walsender.c
+++ b/src/backend/replication/walsender.c
@@ -603,7 +603,7 @@ SendTimeLineHistory(TimeLineHistoryCmd *cmd)
 	dest->rStartup(dest, CMD_SELECT, tupdesc);
 
 	/* Send a DataRow message */
-	pq_beginmessage(&buf, 'D');
+	pq_beginmessage(&buf, PqMsg_DataRow);
 	pq_sendint16(&buf, 2);		/* # of columns */
 	len = strlen(histfname);
 	pq_sendint32(&buf, len);	/* col1 len */
@@ -801,7 +801,7 @@ StartReplication(StartReplicationCmd *cmd)
 		WalSndSetState(WALSNDSTATE_CATCHUP);
 
 		/* Send a CopyBothResponse message, and start streaming */
-		pq_beginmessage(&buf, 'W');
+		pq_beginmessage(&buf, PqMsg_CopyBothResponse);
 		pq_sendbyte(&buf, 0);
 		pq_sendint16(&buf, 0);
 		pq_endmessage(&buf);
@@ -1294,7 +1294,7 @@ StartLogicalReplication(StartReplicationCmd *cmd)
 	WalSndSetState(WALSNDSTATE_CATCHUP);
 
 	/* Send a CopyBothResponse message, and start streaming */
-	pq_beginmessage(&buf, 'W');
+	pq_beginmessage(&buf, PqMsg_CopyBothResponse);
 	pq_sendbyte(&buf, 0);
 	pq_sendint16(&buf, 0);
 	pq_endmessage(&buf);
@@ -1923,11 +1923,11 @@ ProcessRepliesIfAny(void)
 		/* Validate message type and set packet size limit */
 		switch (firstchar)
 		{
-			case 'd':
+			case PqMsg_CopyData:
 				maxmsglen = PQ_LARGE_MESSAGE_LIMIT;
 				break;
-			case 'c':
-			case 'X':
+			case PqMsg_CopyDone:
+			case PqMsg_Terminate:
 				maxmsglen = PQ_SMALL_MESSAGE_LIMIT;
 				break;
 			default:
@@ -1955,7 +1955,7 @@ ProcessRepliesIfAny(void)
 				/*
 				 * 'd' means a standby reply wrapped in a CopyData packet.
 				 */
-			case 'd':
+			case PqMsg_CopyData:
 				ProcessStandbyMessage();
 				received = true;
 				break;
@@ -1964,7 +1964,7 @@ ProcessRepliesIfAny(void)
 				 * CopyDone means the standby requested to finish streaming.
 				 * Reply with CopyDone, if we had not sent that already.
 				 */
-			case 'c':
+			case PqMsg_CopyDone:
 				if (!streamingDoneSending)
 				{
 					pq_putmessage_noblock('c', NULL, 0);
@@ -1978,7 +1978,7 @@ ProcessRepliesIfAny(void)
 				/*
 				 * 'X' means that the standby is closing down the socket.
 				 */
-			case 'X':
+			case PqMsg_Terminate:
 				proc_exit(0);
 
 			default:
diff --git a/src/backend/tcop/dest.c b/src/backend/tcop/dest.c
index c0406e2ee5..06d1872b9a 100644
--- a/src/backend/tcop/dest.c
+++ b/src/backend/tcop/dest.c
@@ -176,7 +176,7 @@ EndCommand(const QueryCompletion *qc, CommandDest dest, bool force_undecorated_o
 
 			len = BuildQueryCompletionString(completionTag, qc,
 											 force_undecorated_output);
-			pq_putmessage('C', completionTag, len + 1);
+			pq_putmessage(PqMsg_Close, completionTag, len + 1);
 
 		case DestNone:
 		case DestDebug:
@@ -200,7 +200,7 @@ EndCommand(const QueryCompletion *qc, CommandDest dest, bool force_undecorated_o
 void
 EndReplicationCommand(const char *commandTag)
 {
-	pq_putmessage('C', commandTag, strlen(commandTag) + 1);
+	pq_putmessage(PqMsg_Close, commandTag, strlen(commandTag) + 1);
 }
 
 /* ----------------
@@ -220,7 +220,7 @@ NullCommand(CommandDest dest)
 		case DestRemoteSimple:
 
 			/* Tell the FE that we saw an empty query string */
-			pq_putemptymessage('I');
+			pq_putemptymessage(PqMsg_EmptyQueryResponse);
 			break;
 
 		case DestNone:
@@ -258,7 +258,7 @@ ReadyForQuery(CommandDest dest)
 			{
 				StringInfoData buf;
 
-				pq_beginmessage(&buf, 'Z');
+				pq_beginmessage(&buf, PqMsg_ReadyForQuery);
 				pq_sendbyte(&buf, TransactionBlockStatusCode());
 				pq_endmessage(&buf);
 			}
diff --git a/src/backend/tcop/fastpath.c b/src/backend/tcop/fastpath.c
index 2f70ebd5fa..71f161dbe2 100644
--- a/src/backend/tcop/fastpath.c
+++ b/src/backend/tcop/fastpath.c
@@ -69,7 +69,7 @@ SendFunctionResult(Datum retval, bool isnull, Oid rettype, int16 format)
 {
 	StringInfoData buf;
 
-	pq_beginmessage(&buf, 'V');
+	pq_beginmessage(&buf, PqMsg_FunctionCallResponse);
 
 	if (isnull)
 	{
diff --git a/src/backend/tcop/postgres.c b/src/backend/tcop/postgres.c
index 36cc99ec9c..e4756f8be2 100644
--- a/src/backend/tcop/postgres.c
+++ b/src/backend/tcop/postgres.c
@@ -402,37 +402,37 @@ SocketBackend(StringInfo inBuf)
 	 */
 	switch (qtype)
 	{
-		case 'Q':				/* simple query */
+		case PqMsg_Query:
 			maxmsglen = PQ_LARGE_MESSAGE_LIMIT;
 			doing_extended_query_message = false;
 			break;
 
-		case 'F':				/* fastpath function call */
+		case PqMsg_FunctionCall:
 			maxmsglen = PQ_LARGE_MESSAGE_LIMIT;
 			doing_extended_query_message = false;
 			break;
 
-		case 'X':				/* terminate */
+		case PqMsg_Terminate:
 			maxmsglen = PQ_SMALL_MESSAGE_LIMIT;
 			doing_extended_query_message = false;
 			ignore_till_sync = false;
 			break;
 
-		case 'B':				/* bind */
-		case 'P':				/* parse */
+		case PqMsg_Bind:
+		case PqMsg_Parse:
 			maxmsglen = PQ_LARGE_MESSAGE_LIMIT;
 			doing_extended_query_message = true;
 			break;
 
-		case 'C':				/* close */
-		case 'D':				/* describe */
-		case 'E':				/* execute */
-		case 'H':				/* flush */
+		case PqMsg_Close:
+		case PqMsg_Describe:
+		case PqMsg_Execute:
+		case PqMsg_Flush:
 			maxmsglen = PQ_SMALL_MESSAGE_LIMIT;
 			doing_extended_query_message = true;
 			break;
 
-		case 'S':				/* sync */
+		case PqMsg_Sync:
 			maxmsglen = PQ_SMALL_MESSAGE_LIMIT;
 			/* stop any active skip-till-Sync */
 			ignore_till_sync = false;
@@ -440,13 +440,13 @@ SocketBackend(StringInfo inBuf)
 			doing_extended_query_message = false;
 			break;
 
-		case 'd':				/* copy data */
+		case PqMsg_CopyData:
 			maxmsglen = PQ_LARGE_MESSAGE_LIMIT;
 			doing_extended_query_message = false;
 			break;
 
-		case 'c':				/* copy done */
-		case 'f':				/* copy fail */
+		case PqMsg_CopyDone:
+		case PqMsg_CopyFail:
 			maxmsglen = PQ_SMALL_MESSAGE_LIMIT;
 			doing_extended_query_message = false;
 			break;
@@ -1589,7 +1589,7 @@ exec_parse_message(const char *query_string,	/* string to execute */
 	 * Send ParseComplete.
 	 */
 	if (whereToSendOutput == DestRemote)
-		pq_putemptymessage('1');
+		pq_putemptymessage(PqMsg_ParseComplete);
 
 	/*
 	 * Emit duration logging if appropriate.
@@ -2047,7 +2047,7 @@ exec_bind_message(StringInfo input_message)
 	 * Send BindComplete.
 	 */
 	if (whereToSendOutput == DestRemote)
-		pq_putemptymessage('2');
+		pq_putemptymessage(PqMsg_BindComplete);
 
 	/*
 	 * Emit duration logging if appropriate.
@@ -2290,7 +2290,7 @@ exec_execute_message(const char *portal_name, long max_rows)
 	{
 		/* Portal run not complete, so send PortalSuspended */
 		if (whereToSendOutput == DestRemote)
-			pq_putemptymessage('s');
+			pq_putemptymessage(PqMsg_PortalSuspended);
 
 		/*
 		 * Set XACT_FLAGS_PIPELINING whenever we suspend an Execute message,
@@ -2683,7 +2683,7 @@ exec_describe_statement_message(const char *stmt_name)
 								  NULL);
 	}
 	else
-		pq_putemptymessage('n');	/* NoData */
+		pq_putemptymessage(PqMsg_NoData);
 }
 
 /*
@@ -2736,7 +2736,7 @@ exec_describe_portal_message(const char *portal_name)
 								  FetchPortalTargetList(portal),
 								  portal->formats);
 	else
-		pq_putemptymessage('n');	/* NoData */
+		pq_putemptymessage(PqMsg_NoData);
 }
 
 
@@ -4239,7 +4239,7 @@ PostgresMain(const char *dbname, const char *username)
 	{
 		StringInfoData buf;
 
-		pq_beginmessage(&buf, 'K');
+		pq_beginmessage(&buf, PqMsg_BackendKeyData);
 		pq_sendint32(&buf, (int32) MyProcPid);
 		pq_sendint32(&buf, (int32) MyCancelKey);
 		pq_endmessage(&buf);
@@ -4618,7 +4618,7 @@ PostgresMain(const char *dbname, const char *username)
 
 		switch (firstchar)
 		{
-			case 'Q':			/* simple query */
+			case PqMsg_Query:
 				{
 					const char *query_string;
 
@@ -4642,7 +4642,7 @@ PostgresMain(const char *dbname, const char *username)
 				}
 				break;
 
-			case 'P':			/* parse */
+			case PqMsg_Parse:
 				{
 					const char *stmt_name;
 					const char *query_string;
@@ -4672,7 +4672,7 @@ PostgresMain(const char *dbname, const char *username)
 				}
 				break;
 
-			case 'B':			/* bind */
+			case PqMsg_Bind:
 				forbidden_in_wal_sender(firstchar);
 
 				/* Set statement_timestamp() */
@@ -4687,7 +4687,7 @@ PostgresMain(const char *dbname, const char *username)
 				/* exec_bind_message does valgrind_report_error_query */
 				break;
 
-			case 'E':			/* execute */
+			case PqMsg_Execute:
 				{
 					const char *portal_name;
 					int			max_rows;
@@ -4707,7 +4707,7 @@ PostgresMain(const char *dbname, const char *username)
 				}
 				break;
 
-			case 'F':			/* fastpath function call */
+			case PqMsg_FunctionCall:
 				forbidden_in_wal_sender(firstchar);
 
 				/* Set statement_timestamp() */
@@ -4742,7 +4742,7 @@ PostgresMain(const char *dbname, const char *username)
 				send_ready_for_query = true;
 				break;
 
-			case 'C':			/* close */
+			case PqMsg_Close:
 				{
 					int			close_type;
 					const char *close_target;
@@ -4782,13 +4782,13 @@ PostgresMain(const char *dbname, const char *username)
 					}
 
 					if (whereToSendOutput == DestRemote)
-						pq_putemptymessage('3');	/* CloseComplete */
+						pq_putemptymessage(PqMsg_CloseComplete);
 
 					valgrind_report_error_query("CLOSE message");
 				}
 				break;
 
-			case 'D':			/* describe */
+			case PqMsg_Describe:
 				{
 					int			describe_type;
 					const char *describe_target;
@@ -4822,13 +4822,13 @@ PostgresMain(const char *dbname, const char *username)
 				}
 				break;
 
-			case 'H':			/* flush */
+			case PqMsg_Flush:
 				pq_getmsgend(&input_message);
 				if (whereToSendOutput == DestRemote)
 					pq_flush();
 				break;
 
-			case 'S':			/* sync */
+			case PqMsg_Sync:
 				pq_getmsgend(&input_message);
 				finish_xact_command();
 				valgrind_report_error_query("SYNC message");
@@ -4847,7 +4847,7 @@ PostgresMain(const char *dbname, const char *username)
 
 				/* FALLTHROUGH */
 
-			case 'X':
+			case PqMsg_Terminate:
 
 				/*
 				 * Reset whereToSendOutput to prevent ereport from attempting
@@ -4865,9 +4865,9 @@ PostgresMain(const char *dbname, const char *username)
 				 */
 				proc_exit(0);
 
-			case 'd':			/* copy data */
-			case 'c':			/* copy done */
-			case 'f':			/* copy fail */
+			case PqMsg_CopyData:
+			case PqMsg_CopyDone:
+			case PqMsg_CopyFail:
 
 				/*
 				 * Accept but ignore these messages, per protocol spec; we
@@ -4897,7 +4897,7 @@ forbidden_in_wal_sender(char firstchar)
 {
 	if (am_walsender)
 	{
-		if (firstchar == 'F')
+		if (firstchar == PqMsg_FunctionCall)
 			ereport(ERROR,
 					(errcode(ERRCODE_PROTOCOL_VIOLATION),
 					 errmsg("fastpath function calls not supported in a replication connection")));
diff --git a/src/backend/utils/error/elog.c b/src/backend/utils/error/elog.c
index 5898100acb..8e1f3e8521 100644
--- a/src/backend/utils/error/elog.c
+++ b/src/backend/utils/error/elog.c
@@ -3465,7 +3465,10 @@ send_message_to_frontend(ErrorData *edata)
 		char		tbuf[12];
 
 		/* 'N' (Notice) is for nonfatal conditions, 'E' is for errors */
-		pq_beginmessage(&msgbuf, (edata->elevel < ERROR) ? 'N' : 'E');
+		if (edata->elevel < ERROR)
+			pq_beginmessage(&msgbuf, PqMsg_NoticeResponse);
+		else
+			pq_beginmessage(&msgbuf, PqMsg_ErrorResponse);
 
 		sev = error_severity(edata->elevel);
 		pq_sendbyte(&msgbuf, PG_DIAG_SEVERITY);
diff --git a/src/backend/utils/misc/guc.c b/src/backend/utils/misc/guc.c
index 99bb2fdd19..84e7ad4d90 100644
--- a/src/backend/utils/misc/guc.c
+++ b/src/backend/utils/misc/guc.c
@@ -2593,7 +2593,7 @@ ReportGUCOption(struct config_generic *record)
 	{
 		StringInfoData msgbuf;
 
-		pq_beginmessage(&msgbuf, 'S');
+		pq_beginmessage(&msgbuf, PqMsg_ParameterStatus);
 		pq_sendstring(&msgbuf, record->name);
 		pq_sendstring(&msgbuf, val);
 		pq_endmessage(&msgbuf);
diff --git a/src/include/Makefile b/src/include/Makefile
index 5d213187e2..2d5242561c 100644
--- a/src/include/Makefile
+++ b/src/include/Makefile
@@ -40,6 +40,7 @@ install: all installdirs
 	$(INSTALL_DATA) $(srcdir)/port.h         '$(DESTDIR)$(includedir_internal)'
 	$(INSTALL_DATA) $(srcdir)/postgres_fe.h  '$(DESTDIR)$(includedir_internal)'
 	$(INSTALL_DATA) $(srcdir)/libpq/pqcomm.h '$(DESTDIR)$(includedir_internal)/libpq'
+	$(INSTALL_DATA) $(srcdir)/libpq/protocol.h '$(DESTDIR)$(includedir_internal)/libpq'
 # These headers are needed for server-side development
 	$(INSTALL_DATA) pg_config.h     '$(DESTDIR)$(includedir_server)'
 	$(INSTALL_DATA) pg_config_ext.h '$(DESTDIR)$(includedir_server)'
@@ -65,7 +66,7 @@ installdirs:
 
 uninstall:
 	rm -f $(addprefix '$(DESTDIR)$(includedir)'/, pg_config.h pg_config_ext.h pg_config_os.h pg_config_manual.h postgres_ext.h libpq/libpq-fs.h)
-	rm -f $(addprefix '$(DESTDIR)$(includedir_internal)'/, c.h port.h postgres_fe.h libpq/pqcomm.h)
+	rm -f $(addprefix '$(DESTDIR)$(includedir_internal)'/, c.h port.h postgres_fe.h libpq/pqcomm.h libpq/protocol.h)
 # heuristic...
 	rm -rf $(addprefix '$(DESTDIR)$(includedir_server)'/, $(SUBDIRS) *.h)
 
diff --git a/src/include/libpq/pqcomm.h b/src/include/libpq/pqcomm.h
index 3da00f7983..46a0946b8b 100644
--- a/src/include/libpq/pqcomm.h
+++ b/src/include/libpq/pqcomm.h
@@ -21,6 +21,12 @@
 #include <netdb.h>
 #include <netinet/in.h>
 
+/*
+ * The definitions for the request/response codes are kept in a separate file
+ * for ease of use in third party programs.
+ */
+#include "libpq/protocol.h"
+
 typedef struct
 {
 	struct sockaddr_storage addr;
@@ -112,23 +118,6 @@ typedef uint32 PacketLen;
 #define MAX_STARTUP_PACKET_LENGTH 10000
 
 
-/* These are the authentication request codes sent by the backend. */
-
-#define AUTH_REQ_OK			0	/* User is authenticated  */
-#define AUTH_REQ_KRB4		1	/* Kerberos V4. Not supported any more. */
-#define AUTH_REQ_KRB5		2	/* Kerberos V5. Not supported any more. */
-#define AUTH_REQ_PASSWORD	3	/* Password */
-#define AUTH_REQ_CRYPT		4	/* crypt password. Not supported any more. */
-#define AUTH_REQ_MD5		5	/* md5 password */
-/* 6 is available.  It was used for SCM creds, not supported any more. */
-#define AUTH_REQ_GSS		7	/* GSSAPI without wrap() */
-#define AUTH_REQ_GSS_CONT	8	/* Continue GSS exchanges */
-#define AUTH_REQ_SSPI		9	/* SSPI negotiate without wrap() */
-#define AUTH_REQ_SASL	   10	/* Begin SASL authentication */
-#define AUTH_REQ_SASL_CONT 11	/* Continue SASL authentication */
-#define AUTH_REQ_SASL_FIN  12	/* Final SASL message */
-#define AUTH_REQ_MAX	   AUTH_REQ_SASL_FIN	/* maximum AUTH_REQ_* value */
-
 typedef uint32 AuthRequest;
 
 
diff --git a/src/include/libpq/protocol.h b/src/include/libpq/protocol.h
new file mode 100644
index 0000000000..cc46f4b586
--- /dev/null
+++ b/src/include/libpq/protocol.h
@@ -0,0 +1,85 @@
+/*-------------------------------------------------------------------------
+ *
+ * protocol.h
+ *		Definitions of the request/response codes for the wire protocol.
+ *
+ *
+ * Portions Copyright (c) 1996-2023, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ *
+ * src/include/libpq/protocol.h
+ *
+ *-------------------------------------------------------------------------
+ */
+#ifndef PROTOCOL_H
+#define PROTOCOL_H
+
+/* These are the request codes sent by the frontend. */
+
+#define PqMsg_Bind					'B'
+#define PqMsg_Close					'C'
+#define PqMsg_Describe				'D'
+#define PqMsg_Execute				'E'
+#define PqMsg_FunctionCall			'F'
+#define PqMsg_Flush					'H'
+#define PqMsg_Parse					'P'
+#define PqMsg_Query					'Q'
+#define PqMsg_Sync					'S'
+#define PqMsg_Terminate				'X'
+#define PqMsg_CopyFail				'f'
+#define PqMsg_GSSResponse			'p'
+#define PqMsg_PasswordMessage		'p'
+#define PqMsg_SASLInitialResponse	'p'
+#define PqMsg_SASLResponse			'p'
+
+
+/* These are the response codes sent by the backend. */
+
+#define PqMsg_ParseComplete			'1'
+#define PqMsg_BindComplete			'2'
+#define PqMsg_CloseComplete			'3'
+#define PqMsg_NotificationResponse	'A'
+#define PqMsg_CommandComplete		'C'
+#define PqMsg_DataRow				'D'
+#define PqMsg_ErrorResponse			'E'
+#define PqMsg_CopyInResponse		'G'
+#define PqMsg_CopyOutResponse		'H'
+#define PqMsg_EmptyQueryResponse	'I'
+#define PqMsg_BackendKeyData		'K'
+#define PqMsg_NoticeResponse		'N'
+#define PqMsg_AuthenticationRequest 'R'
+#define PqMsg_ParameterStatus		'S'
+#define PqMsg_RowDescription		'T'
+#define PqMsg_FunctionCallResponse	'V'
+#define PqMsg_CopyBothResponse		'W'
+#define PqMsg_ReadyForQuery			'Z'
+#define PqMsg_NoData				'n'
+#define PqMsg_PortalSuspended		's'
+#define PqMsg_ParameterDescription	't'
+#define PqMsg_NegotiateProtocolVersion 'v'
+
+
+/* These are the codes sent by both the frontend and backend. */
+
+#define PqMsg_CopyDone				'c'
+#define PqMsg_CopyData				'd'
+
+
+/* These are the authentication request codes sent by the backend. */
+
+#define AUTH_REQ_OK			0	/* User is authenticated  */
+#define AUTH_REQ_KRB4		1	/* Kerberos V4. Not supported any more. */
+#define AUTH_REQ_KRB5		2	/* Kerberos V5. Not supported any more. */
+#define AUTH_REQ_PASSWORD	3	/* Password */
+#define AUTH_REQ_CRYPT		4	/* crypt password. Not supported any more. */
+#define AUTH_REQ_MD5		5	/* md5 password */
+/* 6 is available.  It was used for SCM creds, not supported any more. */
+#define AUTH_REQ_GSS		7	/* GSSAPI without wrap() */
+#define AUTH_REQ_GSS_CONT	8	/* Continue GSS exchanges */
+#define AUTH_REQ_SSPI		9	/* SSPI negotiate without wrap() */
+#define AUTH_REQ_SASL	   10	/* Begin SASL authentication */
+#define AUTH_REQ_SASL_CONT 11	/* Continue SASL authentication */
+#define AUTH_REQ_SASL_FIN  12	/* Final SASL message */
+#define AUTH_REQ_MAX	   AUTH_REQ_SASL_FIN	/* maximum AUTH_REQ_* value */
+
+#endif							/* PROTOCOL_H */
diff --git a/src/include/meson.build b/src/include/meson.build
index d7e1ecd4c9..d50897c9fd 100644
--- a/src/include/meson.build
+++ b/src/include/meson.build
@@ -94,6 +94,7 @@ install_headers(
 
 install_headers(
   'libpq/pqcomm.h',
+  'libpq/protocol.h',
   install_dir: dir_include_internal / 'libpq',
 )
 
diff --git a/src/interfaces/libpq/fe-auth.c b/src/interfaces/libpq/fe-auth.c
index 887ca5e9e1..912aa14821 100644
--- a/src/interfaces/libpq/fe-auth.c
+++ b/src/interfaces/libpq/fe-auth.c
@@ -586,7 +586,7 @@ pg_SASL_init(PGconn *conn, int payloadlen)
 	/*
 	 * Build a SASLInitialResponse message, and send it.
 	 */
-	if (pqPutMsgStart('p', conn))
+	if (pqPutMsgStart(PqMsg_SASLInitialResponse, conn))
 		goto error;
 	if (pqPuts(selected_mechanism, conn))
 		goto error;
diff --git a/src/interfaces/libpq/fe-connect.c b/src/interfaces/libpq/fe-connect.c
index 837c5321aa..bf83a9b569 100644
--- a/src/interfaces/libpq/fe-connect.c
+++ b/src/interfaces/libpq/fe-connect.c
@@ -3591,7 +3591,9 @@ keep_going:						/* We will come back to here until there is
 				 * Anything else probably means it's not Postgres on the other
 				 * end at all.
 				 */
-				if (!(beresp == 'R' || beresp == 'v' || beresp == 'E'))
+				if (beresp != PqMsg_AuthenticationRequest &&
+					beresp != PqMsg_ErrorResponse &&
+					beresp != PqMsg_NegotiateProtocolVersion)
 				{
 					libpq_append_conn_error(conn, "expected authentication request from server, but received %c",
 											beresp);
@@ -3618,19 +3620,22 @@ keep_going:						/* We will come back to here until there is
 				 * version 14, the server also used the old protocol for
 				 * errors that happened before processing the startup packet.)
 				 */
-				if (beresp == 'R' && (msgLength < 8 || msgLength > 2000))
+				if (beresp == PqMsg_AuthenticationRequest &&
+					(msgLength < 8 || msgLength > 2000))
 				{
 					libpq_append_conn_error(conn, "received invalid authentication request");
 					goto error_return;
 				}
-				if (beresp == 'v' && (msgLength < 8 || msgLength > 2000))
+				if (beresp == PqMsg_NegotiateProtocolVersion &&
+					(msgLength < 8 || msgLength > 2000))
 				{
 					libpq_append_conn_error(conn, "received invalid protocol negotiation message");
 					goto error_return;
 				}
 
 #define MAX_ERRLEN 30000
-				if (beresp == 'E' && (msgLength < 8 || msgLength > MAX_ERRLEN))
+				if (beresp == PqMsg_ErrorResponse &&
+					(msgLength < 8 || msgLength > MAX_ERRLEN))
 				{
 					/* Handle error from a pre-3.0 server */
 					conn->inCursor = conn->inStart + 1; /* reread data */
@@ -3693,7 +3698,7 @@ keep_going:						/* We will come back to here until there is
 				}
 
 				/* Handle errors. */
-				if (beresp == 'E')
+				if (beresp == PqMsg_ErrorResponse)
 				{
 					if (pqGetErrorNotice3(conn, true))
 					{
@@ -3770,7 +3775,7 @@ keep_going:						/* We will come back to here until there is
 
 					goto error_return;
 				}
-				else if (beresp == 'v')
+				else if (beresp == PqMsg_NegotiateProtocolVersion)
 				{
 					if (pqGetNegotiateProtocolVersion3(conn))
 					{
@@ -4540,7 +4545,7 @@ sendTerminateConn(PGconn *conn)
 		 * Try to send "close connection" message to backend. Ignore any
 		 * error.
 		 */
-		pqPutMsgStart('X', conn);
+		pqPutMsgStart(PqMsg_Terminate, conn);
 		pqPutMsgEnd(conn);
 		(void) pqFlush(conn);
 	}
diff --git a/src/interfaces/libpq/fe-exec.c b/src/interfaces/libpq/fe-exec.c
index a868284ff8..fdb7994779 100644
--- a/src/interfaces/libpq/fe-exec.c
+++ b/src/interfaces/libpq/fe-exec.c
@@ -1458,7 +1458,7 @@ PQsendQueryInternal(PGconn *conn, const char *query, bool newQuery)
 
 	/* Send the query message(s) */
 	/* construct the outgoing Query message */
-	if (pqPutMsgStart('Q', conn) < 0 ||
+	if (pqPutMsgStart(PqMsg_Query, conn) < 0 ||
 		pqPuts(query, conn) < 0 ||
 		pqPutMsgEnd(conn) < 0)
 	{
@@ -1571,7 +1571,7 @@ PQsendPrepare(PGconn *conn,
 		return 0;				/* error msg already set */
 
 	/* construct the Parse message */
-	if (pqPutMsgStart('P', conn) < 0 ||
+	if (pqPutMsgStart(PqMsg_Parse, conn) < 0 ||
 		pqPuts(stmtName, conn) < 0 ||
 		pqPuts(query, conn) < 0)
 		goto sendFailed;
@@ -1599,7 +1599,7 @@ PQsendPrepare(PGconn *conn,
 	/* Add a Sync, unless in pipeline mode. */
 	if (conn->pipelineStatus == PQ_PIPELINE_OFF)
 	{
-		if (pqPutMsgStart('S', conn) < 0 ||
+		if (pqPutMsgStart(PqMsg_Sync, conn) < 0 ||
 			pqPutMsgEnd(conn) < 0)
 			goto sendFailed;
 	}
@@ -1784,7 +1784,7 @@ PQsendQueryGuts(PGconn *conn,
 	if (command)
 	{
 		/* construct the Parse message */
-		if (pqPutMsgStart('P', conn) < 0 ||
+		if (pqPutMsgStart(PqMsg_Parse, conn) < 0 ||
 			pqPuts(stmtName, conn) < 0 ||
 			pqPuts(command, conn) < 0)
 			goto sendFailed;
@@ -1808,7 +1808,7 @@ PQsendQueryGuts(PGconn *conn,
 	}
 
 	/* Construct the Bind message */
-	if (pqPutMsgStart('B', conn) < 0 ||
+	if (pqPutMsgStart(PqMsg_Bind, conn) < 0 ||
 		pqPuts("", conn) < 0 ||
 		pqPuts(stmtName, conn) < 0)
 		goto sendFailed;
@@ -1874,14 +1874,14 @@ PQsendQueryGuts(PGconn *conn,
 		goto sendFailed;
 
 	/* construct the Describe Portal message */
-	if (pqPutMsgStart('D', conn) < 0 ||
+	if (pqPutMsgStart(PqMsg_Describe, conn) < 0 ||
 		pqPutc('P', conn) < 0 ||
 		pqPuts("", conn) < 0 ||
 		pqPutMsgEnd(conn) < 0)
 		goto sendFailed;
 
 	/* construct the Execute message */
-	if (pqPutMsgStart('E', conn) < 0 ||
+	if (pqPutMsgStart(PqMsg_Execute, conn) < 0 ||
 		pqPuts("", conn) < 0 ||
 		pqPutInt(0, 4, conn) < 0 ||
 		pqPutMsgEnd(conn) < 0)
@@ -1890,7 +1890,7 @@ PQsendQueryGuts(PGconn *conn,
 	/* construct the Sync message if not in pipeline mode */
 	if (conn->pipelineStatus == PQ_PIPELINE_OFF)
 	{
-		if (pqPutMsgStart('S', conn) < 0 ||
+		if (pqPutMsgStart(PqMsg_Sync, conn) < 0 ||
 			pqPutMsgEnd(conn) < 0)
 			goto sendFailed;
 	}
@@ -2422,7 +2422,7 @@ PQdescribePrepared(PGconn *conn, const char *stmt)
 {
 	if (!PQexecStart(conn))
 		return NULL;
-	if (!PQsendTypedCommand(conn, 'D', 'S', stmt))
+	if (!PQsendTypedCommand(conn, PqMsg_Describe, 'S', stmt))
 		return NULL;
 	return PQexecFinish(conn);
 }
@@ -2441,7 +2441,7 @@ PQdescribePortal(PGconn *conn, const char *portal)
 {
 	if (!PQexecStart(conn))
 		return NULL;
-	if (!PQsendTypedCommand(conn, 'D', 'P', portal))
+	if (!PQsendTypedCommand(conn, PqMsg_Describe, 'P', portal))
 		return NULL;
 	return PQexecFinish(conn);
 }
@@ -2456,7 +2456,7 @@ PQdescribePortal(PGconn *conn, const char *portal)
 int
 PQsendDescribePrepared(PGconn *conn, const char *stmt)
 {
-	return PQsendTypedCommand(conn, 'D', 'S', stmt);
+	return PQsendTypedCommand(conn, PqMsg_Describe, 'S', stmt);
 }
 
 /*
@@ -2469,7 +2469,7 @@ PQsendDescribePrepared(PGconn *conn, const char *stmt)
 int
 PQsendDescribePortal(PGconn *conn, const char *portal)
 {
-	return PQsendTypedCommand(conn, 'D', 'P', portal);
+	return PQsendTypedCommand(conn, PqMsg_Describe, 'P', portal);
 }
 
 /*
@@ -2488,7 +2488,7 @@ PQclosePrepared(PGconn *conn, const char *stmt)
 {
 	if (!PQexecStart(conn))
 		return NULL;
-	if (!PQsendTypedCommand(conn, 'C', 'S', stmt))
+	if (!PQsendTypedCommand(conn, PqMsg_Close, 'S', stmt))
 		return NULL;
 	return PQexecFinish(conn);
 }
@@ -2506,7 +2506,7 @@ PQclosePortal(PGconn *conn, const char *portal)
 {
 	if (!PQexecStart(conn))
 		return NULL;
-	if (!PQsendTypedCommand(conn, 'C', 'P', portal))
+	if (!PQsendTypedCommand(conn, PqMsg_Close, 'P', portal))
 		return NULL;
 	return PQexecFinish(conn);
 }
@@ -2521,7 +2521,7 @@ PQclosePortal(PGconn *conn, const char *portal)
 int
 PQsendClosePrepared(PGconn *conn, const char *stmt)
 {
-	return PQsendTypedCommand(conn, 'C', 'S', stmt);
+	return PQsendTypedCommand(conn, PqMsg_Close, 'S', stmt);
 }
 
 /*
@@ -2534,7 +2534,7 @@ PQsendClosePrepared(PGconn *conn, const char *stmt)
 int
 PQsendClosePortal(PGconn *conn, const char *portal)
 {
-	return PQsendTypedCommand(conn, 'C', 'P', portal);
+	return PQsendTypedCommand(conn, PqMsg_Close, 'P', portal);
 }
 
 /*
@@ -2577,17 +2577,17 @@ PQsendTypedCommand(PGconn *conn, char command, char type, const char *target)
 	/* construct the Sync message */
 	if (conn->pipelineStatus == PQ_PIPELINE_OFF)
 	{
-		if (pqPutMsgStart('S', conn) < 0 ||
+		if (pqPutMsgStart(PqMsg_Sync, conn) < 0 ||
 			pqPutMsgEnd(conn) < 0)
 			goto sendFailed;
 	}
 
 	/* remember if we are doing a Close or a Describe */
-	if (command == 'C')
+	if (command == PqMsg_Close)
 	{
 		entry->queryclass = PGQUERY_CLOSE;
 	}
-	else if (command == 'D')
+	else if (command == PqMsg_Describe)
 	{
 		entry->queryclass = PGQUERY_DESCRIBE;
 	}
@@ -2696,7 +2696,7 @@ PQputCopyData(PGconn *conn, const char *buffer, int nbytes)
 				return pqIsnonblocking(conn) ? 0 : -1;
 		}
 		/* Send the data (too simple to delegate to fe-protocol files) */
-		if (pqPutMsgStart('d', conn) < 0 ||
+		if (pqPutMsgStart(PqMsg_CopyData, conn) < 0 ||
 			pqPutnchar(buffer, nbytes, conn) < 0 ||
 			pqPutMsgEnd(conn) < 0)
 			return -1;
@@ -2731,7 +2731,7 @@ PQputCopyEnd(PGconn *conn, const char *errormsg)
 	if (errormsg)
 	{
 		/* Send COPY FAIL */
-		if (pqPutMsgStart('f', conn) < 0 ||
+		if (pqPutMsgStart(PqMsg_CopyFail, conn) < 0 ||
 			pqPuts(errormsg, conn) < 0 ||
 			pqPutMsgEnd(conn) < 0)
 			return -1;
@@ -2739,7 +2739,7 @@ PQputCopyEnd(PGconn *conn, const char *errormsg)
 	else
 	{
 		/* Send COPY DONE */
-		if (pqPutMsgStart('c', conn) < 0 ||
+		if (pqPutMsgStart(PqMsg_CopyDone, conn) < 0 ||
 			pqPutMsgEnd(conn) < 0)
 			return -1;
 	}
@@ -2751,7 +2751,7 @@ PQputCopyEnd(PGconn *conn, const char *errormsg)
 	if (conn->cmd_queue_head &&
 		conn->cmd_queue_head->queryclass != PGQUERY_SIMPLE)
 	{
-		if (pqPutMsgStart('S', conn) < 0 ||
+		if (pqPutMsgStart(PqMsg_Sync, conn) < 0 ||
 			pqPutMsgEnd(conn) < 0)
 			return -1;
 	}
@@ -3263,7 +3263,7 @@ PQpipelineSync(PGconn *conn)
 	entry->query = NULL;
 
 	/* construct the Sync message */
-	if (pqPutMsgStart('S', conn) < 0 ||
+	if (pqPutMsgStart(PqMsg_Sync, conn) < 0 ||
 		pqPutMsgEnd(conn) < 0)
 		goto sendFailed;
 
@@ -3311,7 +3311,7 @@ PQsendFlushRequest(PGconn *conn)
 		return 0;
 	}
 
-	if (pqPutMsgStart('H', conn) < 0 ||
+	if (pqPutMsgStart(PqMsg_Flush, conn) < 0 ||
 		pqPutMsgEnd(conn) < 0)
 	{
 		return 0;
diff --git a/src/interfaces/libpq/fe-protocol3.c b/src/interfaces/libpq/fe-protocol3.c
index 7bc6355d17..5613c56b14 100644
--- a/src/interfaces/libpq/fe-protocol3.c
+++ b/src/interfaces/libpq/fe-protocol3.c
@@ -34,8 +34,13 @@
  * than a couple of kilobytes).
  */
 #define VALID_LONG_MESSAGE_TYPE(id) \
-	((id) == 'T' || (id) == 'D' || (id) == 'd' || (id) == 'V' || \
-	 (id) == 'E' || (id) == 'N' || (id) == 'A')
+	((id) == PqMsg_CopyData || \
+	 (id) == PqMsg_DataRow || \
+	 (id) == PqMsg_ErrorResponse || \
+	 (id) == PqMsg_FunctionCallResponse || \
+	 (id) == PqMsg_NoticeResponse || \
+	 (id) == PqMsg_NotificationResponse || \
+	 (id) == PqMsg_RowDescription)
 
 
 static void handleSyncLoss(PGconn *conn, char id, int msgLength);
@@ -140,12 +145,12 @@ pqParseInput3(PGconn *conn)
 		 * from config file due to SIGHUP), but otherwise we hold off until
 		 * BUSY state.
 		 */
-		if (id == 'A')
+		if (id == PqMsg_NotificationResponse)
 		{
 			if (getNotify(conn))
 				return;
 		}
-		else if (id == 'N')
+		else if (id == PqMsg_NoticeResponse)
 		{
 			if (pqGetErrorNotice3(conn, false))
 				return;
@@ -165,12 +170,12 @@ pqParseInput3(PGconn *conn)
 			 * it is about to close the connection, so we don't want to just
 			 * discard it...)
 			 */
-			if (id == 'E')
+			if (id == PqMsg_ErrorResponse)
 			{
 				if (pqGetErrorNotice3(conn, false /* treat as notice */ ))
 					return;
 			}
-			else if (id == 'S')
+			else if (id == PqMsg_ParameterStatus)
 			{
 				if (getParameterStatus(conn))
 					return;
@@ -192,7 +197,7 @@ pqParseInput3(PGconn *conn)
 			 */
 			switch (id)
 			{
-				case 'C':		/* command complete */
+				case PqMsg_CommandComplete:
 					if (pqGets(&conn->workBuffer, conn))
 						return;
 					if (!pgHavePendingResult(conn))
@@ -210,13 +215,12 @@ pqParseInput3(PGconn *conn)
 								CMDSTATUS_LEN);
 					conn->asyncStatus = PGASYNC_READY;
 					break;
-				case 'E':		/* error return */
+				case PqMsg_ErrorResponse:
 					if (pqGetErrorNotice3(conn, true))
 						return;
 					conn->asyncStatus = PGASYNC_READY;
 					break;
-				case 'Z':		/* sync response, backend is ready for new
-								 * query */
+				case PqMsg_ReadyForQuery:
 					if (getReadyForQuery(conn))
 						return;
 					if (conn->pipelineStatus != PQ_PIPELINE_OFF)
@@ -246,7 +250,7 @@ pqParseInput3(PGconn *conn)
 						conn->asyncStatus = PGASYNC_IDLE;
 					}
 					break;
-				case 'I':		/* empty query */
+				case PqMsg_EmptyQueryResponse:
 					if (!pgHavePendingResult(conn))
 					{
 						conn->result = PQmakeEmptyPGresult(conn,
@@ -259,7 +263,7 @@ pqParseInput3(PGconn *conn)
 					}
 					conn->asyncStatus = PGASYNC_READY;
 					break;
-				case '1':		/* Parse Complete */
+				case PqMsg_ParseComplete:
 					/* If we're doing PQprepare, we're done; else ignore */
 					if (conn->cmd_queue_head &&
 						conn->cmd_queue_head->queryclass == PGQUERY_PREPARE)
@@ -277,10 +281,10 @@ pqParseInput3(PGconn *conn)
 						conn->asyncStatus = PGASYNC_READY;
 					}
 					break;
-				case '2':		/* Bind Complete */
+				case PqMsg_BindComplete:
 					/* Nothing to do for this message type */
 					break;
-				case '3':		/* Close Complete */
+				case PqMsg_CloseComplete:
 					/* If we're doing PQsendClose, we're done; else ignore */
 					if (conn->cmd_queue_head &&
 						conn->cmd_queue_head->queryclass == PGQUERY_CLOSE)
@@ -298,11 +302,11 @@ pqParseInput3(PGconn *conn)
 						conn->asyncStatus = PGASYNC_READY;
 					}
 					break;
-				case 'S':		/* parameter status */
+				case PqMsg_ParameterStatus:
 					if (getParameterStatus(conn))
 						return;
 					break;
-				case 'K':		/* secret key data from the backend */
+				case PqMsg_BackendKeyData:
 
 					/*
 					 * This is expected only during backend startup, but it's
@@ -314,7 +318,7 @@ pqParseInput3(PGconn *conn)
 					if (pqGetInt(&(conn->be_key), 4, conn))
 						return;
 					break;
-				case 'T':		/* Row Description */
+				case PqMsg_RowDescription:
 					if (conn->error_result ||
 						(conn->result != NULL &&
 						 conn->result->resultStatus == PGRES_FATAL_ERROR))
@@ -346,7 +350,7 @@ pqParseInput3(PGconn *conn)
 						return;
 					}
 					break;
-				case 'n':		/* No Data */
+				case PqMsg_NoData:
 
 					/*
 					 * NoData indicates that we will not be seeing a
@@ -374,11 +378,11 @@ pqParseInput3(PGconn *conn)
 						conn->asyncStatus = PGASYNC_READY;
 					}
 					break;
-				case 't':		/* Parameter Description */
+				case PqMsg_ParameterDescription:
 					if (getParamDescriptions(conn, msgLength))
 						return;
 					break;
-				case 'D':		/* Data Row */
+				case PqMsg_DataRow:
 					if (conn->result != NULL &&
 						conn->result->resultStatus == PGRES_TUPLES_OK)
 					{
@@ -405,24 +409,24 @@ pqParseInput3(PGconn *conn)
 						conn->inCursor += msgLength;
 					}
 					break;
-				case 'G':		/* Start Copy In */
+				case PqMsg_CopyInResponse:
 					if (getCopyStart(conn, PGRES_COPY_IN))
 						return;
 					conn->asyncStatus = PGASYNC_COPY_IN;
 					break;
-				case 'H':		/* Start Copy Out */
+				case PqMsg_CopyOutResponse:
 					if (getCopyStart(conn, PGRES_COPY_OUT))
 						return;
 					conn->asyncStatus = PGASYNC_COPY_OUT;
 					conn->copy_already_done = 0;
 					break;
-				case 'W':		/* Start Copy Both */
+				case PqMsg_CopyBothResponse:
 					if (getCopyStart(conn, PGRES_COPY_BOTH))
 						return;
 					conn->asyncStatus = PGASYNC_COPY_BOTH;
 					conn->copy_already_done = 0;
 					break;
-				case 'd':		/* Copy Data */
+				case PqMsg_CopyData:
 
 					/*
 					 * If we see Copy Data, just silently drop it.  This would
@@ -431,7 +435,7 @@ pqParseInput3(PGconn *conn)
 					 */
 					conn->inCursor += msgLength;
 					break;
-				case 'c':		/* Copy Done */
+				case PqMsg_CopyDone:
 
 					/*
 					 * If we see Copy Done, just silently drop it.  This is
@@ -1692,21 +1696,21 @@ getCopyDataMessage(PGconn *conn)
 		 */
 		switch (id)
 		{
-			case 'A':			/* NOTIFY */
+			case PqMsg_NotificationResponse:
 				if (getNotify(conn))
 					return 0;
 				break;
-			case 'N':			/* NOTICE */
+			case PqMsg_NoticeResponse:
 				if (pqGetErrorNotice3(conn, false))
 					return 0;
 				break;
-			case 'S':			/* ParameterStatus */
+			case PqMsg_ParameterStatus:
 				if (getParameterStatus(conn))
 					return 0;
 				break;
-			case 'd':			/* Copy Data, pass it back to caller */
+			case PqMsg_CopyData:
 				return msgLength;
-			case 'c':
+			case PqMsg_CopyDone:
 
 				/*
 				 * If this is a CopyDone message, exit COPY_OUT mode and let
@@ -1929,7 +1933,7 @@ pqEndcopy3(PGconn *conn)
 	if (conn->asyncStatus == PGASYNC_COPY_IN ||
 		conn->asyncStatus == PGASYNC_COPY_BOTH)
 	{
-		if (pqPutMsgStart('c', conn) < 0 ||
+		if (pqPutMsgStart(PqMsg_CopyDone, conn) < 0 ||
 			pqPutMsgEnd(conn) < 0)
 			return 1;
 
@@ -1940,7 +1944,7 @@ pqEndcopy3(PGconn *conn)
 		if (conn->cmd_queue_head &&
 			conn->cmd_queue_head->queryclass != PGQUERY_SIMPLE)
 		{
-			if (pqPutMsgStart('S', conn) < 0 ||
+			if (pqPutMsgStart(PqMsg_Sync, conn) < 0 ||
 				pqPutMsgEnd(conn) < 0)
 				return 1;
 		}
@@ -2023,7 +2027,7 @@ pqFunctionCall3(PGconn *conn, Oid fnid,
 
 	/* PQfn already validated connection state */
 
-	if (pqPutMsgStart('F', conn) < 0 || /* function call msg */
+	if (pqPutMsgStart(PqMsg_FunctionCall, conn) < 0 ||
 		pqPutInt(fnid, 4, conn) < 0 ||	/* function id */
 		pqPutInt(1, 2, conn) < 0 || /* # of format codes */
 		pqPutInt(1, 2, conn) < 0 || /* format code: BINARY */
diff --git a/src/interfaces/libpq/fe-trace.c b/src/interfaces/libpq/fe-trace.c
index 402784f40e..b18e3deab6 100644
--- a/src/interfaces/libpq/fe-trace.c
+++ b/src/interfaces/libpq/fe-trace.c
@@ -562,110 +562,120 @@ pqTraceOutputMessage(PGconn *conn, const char *message, bool toServer)
 
 	switch (id)
 	{
-		case '1':
+		case PqMsg_ParseComplete:
 			fprintf(conn->Pfdebug, "ParseComplete");
 			/* No message content */
 			break;
-		case '2':
+		case PqMsg_BindComplete:
 			fprintf(conn->Pfdebug, "BindComplete");
 			/* No message content */
 			break;
-		case '3':
+		case PqMsg_CloseComplete:
 			fprintf(conn->Pfdebug, "CloseComplete");
 			/* No message content */
 			break;
-		case 'A':				/* Notification Response */
+		case PqMsg_NotificationResponse:
 			pqTraceOutputA(conn->Pfdebug, message, &logCursor, regress);
 			break;
-		case 'B':				/* Bind */
+		case PqMsg_Bind:
 			pqTraceOutputB(conn->Pfdebug, message, &logCursor);
 			break;
-		case 'c':
+		case PqMsg_CopyDone:
 			fprintf(conn->Pfdebug, "CopyDone");
 			/* No message content */
 			break;
-		case 'C':				/* Close(F) or Command Complete(B) */
+		case PqMsg_CommandComplete:
+			/* Close(F) and CommandComplete(B) use the same identifier. */
+			Assert(PqMsg_Close == PqMsg_CommandComplete);
 			pqTraceOutputC(conn->Pfdebug, toServer, message, &logCursor);
 			break;
-		case 'd':				/* Copy Data */
+		case PqMsg_CopyData:
 			/* Drop COPY data to reduce the overhead of logging. */
 			break;
-		case 'D':				/* Describe(F) or Data Row(B) */
+		case PqMsg_Describe:
+			/* Describe(F) and DataRow(B) use the same identifier. */
+			Assert(PqMsg_Describe == PqMsg_DataRow);
 			pqTraceOutputD(conn->Pfdebug, toServer, message, &logCursor);
 			break;
-		case 'E':				/* Execute(F) or Error Response(B) */
+		case PqMsg_Execute:
+			/* Execute(F) and ErrorResponse(B) use the same identifier. */
+			Assert(PqMsg_Execute == PqMsg_ErrorResponse);
 			pqTraceOutputE(conn->Pfdebug, toServer, message, &logCursor,
 						   regress);
 			break;
-		case 'f':				/* Copy Fail */
+		case PqMsg_CopyFail:
 			pqTraceOutputf(conn->Pfdebug, message, &logCursor);
 			break;
-		case 'F':				/* Function Call */
+		case PqMsg_FunctionCall:
 			pqTraceOutputF(conn->Pfdebug, message, &logCursor, regress);
 			break;
-		case 'G':				/* Start Copy In */
+		case PqMsg_CopyInResponse:
 			pqTraceOutputG(conn->Pfdebug, message, &logCursor);
 			break;
-		case 'H':				/* Flush(F) or Start Copy Out(B) */
+		case PqMsg_Flush:
+			/* Flush(F) and CopyOutResponse(B) use the same identifier */
+			Assert(PqMsg_CopyOutResponse == PqMsg_Flush);
 			if (!toServer)
 				pqTraceOutputH(conn->Pfdebug, message, &logCursor);
 			else
 				fprintf(conn->Pfdebug, "Flush");	/* no message content */
 			break;
-		case 'I':
+		case PqMsg_EmptyQueryResponse:
 			fprintf(conn->Pfdebug, "EmptyQueryResponse");
 			/* No message content */
 			break;
-		case 'K':				/* secret key data from the backend */
+		case PqMsg_BackendKeyData:
 			pqTraceOutputK(conn->Pfdebug, message, &logCursor, regress);
 			break;
-		case 'n':
+		case PqMsg_NoData:
 			fprintf(conn->Pfdebug, "NoData");
 			/* No message content */
 			break;
-		case 'N':
+		case PqMsg_NoticeResponse:
 			pqTraceOutputNR(conn->Pfdebug, "NoticeResponse", message,
 							&logCursor, regress);
 			break;
-		case 'P':				/* Parse */
+		case PqMsg_Parse:
 			pqTraceOutputP(conn->Pfdebug, message, &logCursor, regress);
 			break;
-		case 'Q':				/* Query */
+		case PqMsg_Query:
 			pqTraceOutputQ(conn->Pfdebug, message, &logCursor);
 			break;
-		case 'R':				/* Authentication */
+		case PqMsg_AuthenticationRequest:
 			pqTraceOutputR(conn->Pfdebug, message, &logCursor);
 			break;
-		case 's':
+		case PqMsg_PortalSuspended:
 			fprintf(conn->Pfdebug, "PortalSuspended");
 			/* No message content */
 			break;
-		case 'S':				/* Parameter Status(B) or Sync(F) */
+		case PqMsg_Sync:
+			/* Parameter Status(B) and Sync(F) use the same identifier */
+			Assert(PqMsg_ParameterStatus == PqMsg_Sync);
 			if (!toServer)
 				pqTraceOutputS(conn->Pfdebug, message, &logCursor);
 			else
 				fprintf(conn->Pfdebug, "Sync"); /* no message content */
 			break;
-		case 't':				/* Parameter Description */
+		case PqMsg_ParameterDescription:
 			pqTraceOutputt(conn->Pfdebug, message, &logCursor, regress);
 			break;
-		case 'T':				/* Row Description */
+		case PqMsg_RowDescription:
 			pqTraceOutputT(conn->Pfdebug, message, &logCursor, regress);
 			break;
-		case 'v':				/* Negotiate Protocol Version */
+		case PqMsg_NegotiateProtocolVersion:
 			pqTraceOutputv(conn->Pfdebug, message, &logCursor);
 			break;
-		case 'V':				/* Function Call response */
+		case PqMsg_FunctionCallResponse:
 			pqTraceOutputV(conn->Pfdebug, message, &logCursor);
 			break;
-		case 'W':				/* Start Copy Both */
+		case PqMsg_CopyBothResponse:
 			pqTraceOutputW(conn->Pfdebug, message, &logCursor, length);
 			break;
-		case 'X':
+		case PqMsg_Terminate:
 			fprintf(conn->Pfdebug, "Terminate");
 			/* No message content */
 			break;
-		case 'Z':				/* Ready For Query */
+		case PqMsg_ReadyForQuery:
 			pqTraceOutputZ(conn->Pfdebug, message, &logCursor);
 			break;
 		default:
-- 
2.25.1

From 69f94689857a469333fecbfd2057868140796516 Mon Sep 17 00:00:00 2001
From: Nathan Bossart <nathan@postgresql.org>
Date: Thu, 17 Aug 2023 09:00:46 -0700
Subject: [PATCH v7 1/1] Introduce macros for protocol characters.

Author: Dave Cramer
Reviewed-by: Alvaro Herrera, Tatsuo Ishii, Peter Smith, Robert Haas, Tom Lane, Peter Eisentraut, Michael Paquier
Discussion: https://postgr.es/m/CADK3HHKbBmK-PKf1bPNFoMC%2BoBt%2BpD9PH8h5nvmBQskEHm-Ehw%40mail.gmail.com
---
 src/backend/access/common/printsimple.c |  5 +-
 src/backend/access/transam/parallel.c   | 14 ++--
 src/backend/backup/basebackup_copy.c    | 16 ++---
 src/backend/commands/async.c            |  2 +-
 src/backend/commands/copyfromparse.c    | 22 +++----
 src/backend/commands/copyto.c           |  6 +-
 src/backend/libpq/auth-sasl.c           |  2 +-
 src/backend/libpq/auth.c                |  8 +--
 src/backend/postmaster/postmaster.c     |  2 +-
 src/backend/replication/walsender.c     | 18 +++---
 src/backend/tcop/dest.c                 |  8 +--
 src/backend/tcop/fastpath.c             |  2 +-
 src/backend/tcop/postgres.c             | 68 ++++++++++----------
 src/backend/utils/error/elog.c          |  5 +-
 src/backend/utils/misc/guc.c            |  2 +-
 src/include/Makefile                    |  3 +-
 src/include/libpq/pqcomm.h              | 23 ++-----
 src/include/libpq/protocol.h            | 85 +++++++++++++++++++++++++
 src/include/meson.build                 |  1 +
 src/interfaces/libpq/fe-auth.c          |  2 +-
 src/interfaces/libpq/fe-connect.c       | 19 ++++--
 src/interfaces/libpq/fe-exec.c          | 54 ++++++++--------
 src/interfaces/libpq/fe-protocol3.c     | 70 ++++++++++----------
 src/interfaces/libpq/fe-trace.c         | 70 +++++++++++---------
 src/tools/msvc/Install.pm               |  2 +
 25 files changed, 305 insertions(+), 204 deletions(-)
 create mode 100644 src/include/libpq/protocol.h

diff --git a/src/backend/access/common/printsimple.c b/src/backend/access/common/printsimple.c
index ef818228ac..675b744db2 100644
--- a/src/backend/access/common/printsimple.c
+++ b/src/backend/access/common/printsimple.c
@@ -20,6 +20,7 @@
 
 #include "access/printsimple.h"
 #include "catalog/pg_type.h"
+#include "libpq/protocol.h"
 #include "libpq/pqformat.h"
 #include "utils/builtins.h"
 
@@ -32,7 +33,7 @@ printsimple_startup(DestReceiver *self, int operation, TupleDesc tupdesc)
 	StringInfoData buf;
 	int			i;
 
-	pq_beginmessage(&buf, 'T'); /* RowDescription */
+	pq_beginmessage(&buf, PqMsg_RowDescription);
 	pq_sendint16(&buf, tupdesc->natts);
 
 	for (i = 0; i < tupdesc->natts; ++i)
@@ -65,7 +66,7 @@ printsimple(TupleTableSlot *slot, DestReceiver *self)
 	slot_getallattrs(slot);
 
 	/* Prepare and send message */
-	pq_beginmessage(&buf, 'D');
+	pq_beginmessage(&buf, PqMsg_DataRow);
 	pq_sendint16(&buf, tupdesc->natts);
 
 	for (i = 0; i < tupdesc->natts; ++i)
diff --git a/src/backend/access/transam/parallel.c b/src/backend/access/transam/parallel.c
index 1738aecf1f..194a1207be 100644
--- a/src/backend/access/transam/parallel.c
+++ b/src/backend/access/transam/parallel.c
@@ -1127,7 +1127,7 @@ HandleParallelMessage(ParallelContext *pcxt, int i, StringInfo msg)
 
 	switch (msgtype)
 	{
-		case 'K':				/* BackendKeyData */
+		case PqMsg_BackendKeyData:
 			{
 				int32		pid = pq_getmsgint(msg, 4);
 
@@ -1137,8 +1137,8 @@ HandleParallelMessage(ParallelContext *pcxt, int i, StringInfo msg)
 				break;
 			}
 
-		case 'E':				/* ErrorResponse */
-		case 'N':				/* NoticeResponse */
+		case PqMsg_ErrorResponse:
+		case PqMsg_NoticeResponse:
 			{
 				ErrorData	edata;
 				ErrorContextCallback *save_error_context_stack;
@@ -1183,7 +1183,7 @@ HandleParallelMessage(ParallelContext *pcxt, int i, StringInfo msg)
 				break;
 			}
 
-		case 'A':				/* NotifyResponse */
+		case PqMsg_NotificationResponse:
 			{
 				/* Propagate NotifyResponse. */
 				int32		pid;
@@ -1217,7 +1217,7 @@ HandleParallelMessage(ParallelContext *pcxt, int i, StringInfo msg)
 				break;
 			}
 
-		case 'X':				/* Terminate, indicating clean exit */
+		case PqMsg_Terminate:
 			{
 				shm_mq_detach(pcxt->worker[i].error_mqh);
 				pcxt->worker[i].error_mqh = NULL;
@@ -1372,7 +1372,7 @@ ParallelWorkerMain(Datum main_arg)
 	 * protocol message is defined, but it won't actually be used for anything
 	 * in this case.
 	 */
-	pq_beginmessage(&msgbuf, 'K');
+	pq_beginmessage(&msgbuf, PqMsg_BackendKeyData);
 	pq_sendint32(&msgbuf, (int32) MyProcPid);
 	pq_sendint32(&msgbuf, (int32) MyCancelKey);
 	pq_endmessage(&msgbuf);
@@ -1550,7 +1550,7 @@ ParallelWorkerMain(Datum main_arg)
 	DetachSession();
 
 	/* Report success. */
-	pq_putmessage('X', NULL, 0);
+	pq_putmessage(PqMsg_Terminate, NULL, 0);
 }
 
 /*
diff --git a/src/backend/backup/basebackup_copy.c b/src/backend/backup/basebackup_copy.c
index 1db80cde1b..fee30c21e1 100644
--- a/src/backend/backup/basebackup_copy.c
+++ b/src/backend/backup/basebackup_copy.c
@@ -152,7 +152,7 @@ bbsink_copystream_begin_backup(bbsink *sink)
 	SendTablespaceList(state->tablespaces);
 
 	/* Send a CommandComplete message */
-	pq_puttextmessage('C', "SELECT");
+	pq_puttextmessage(PqMsg_CommandComplete, "SELECT");
 
 	/* Begin COPY stream. This will be used for all archives + manifest. */
 	SendCopyOutResponse();
@@ -169,7 +169,7 @@ bbsink_copystream_begin_archive(bbsink *sink, const char *archive_name)
 	StringInfoData buf;
 
 	ti = list_nth(state->tablespaces, state->tablespace_num);
-	pq_beginmessage(&buf, 'd'); /* CopyData */
+	pq_beginmessage(&buf, PqMsg_CopyData);
 	pq_sendbyte(&buf, 'n');		/* New archive */
 	pq_sendstring(&buf, archive_name);
 	pq_sendstring(&buf, ti->path == NULL ? "" : ti->path);
@@ -220,7 +220,7 @@ bbsink_copystream_archive_contents(bbsink *sink, size_t len)
 		{
 			mysink->last_progress_report_time = now;
 
-			pq_beginmessage(&buf, 'd'); /* CopyData */
+			pq_beginmessage(&buf, PqMsg_CopyData);
 			pq_sendbyte(&buf, 'p'); /* Progress report */
 			pq_sendint64(&buf, state->bytes_done);
 			pq_endmessage(&buf);
@@ -246,7 +246,7 @@ bbsink_copystream_end_archive(bbsink *sink)
 
 	mysink->bytes_done_at_last_time_check = state->bytes_done;
 	mysink->last_progress_report_time = GetCurrentTimestamp();
-	pq_beginmessage(&buf, 'd'); /* CopyData */
+	pq_beginmessage(&buf, PqMsg_CopyData);
 	pq_sendbyte(&buf, 'p');		/* Progress report */
 	pq_sendint64(&buf, state->bytes_done);
 	pq_endmessage(&buf);
@@ -261,7 +261,7 @@ bbsink_copystream_begin_manifest(bbsink *sink)
 {
 	StringInfoData buf;
 
-	pq_beginmessage(&buf, 'd'); /* CopyData */
+	pq_beginmessage(&buf, PqMsg_CopyData);
 	pq_sendbyte(&buf, 'm');		/* Manifest */
 	pq_endmessage(&buf);
 }
@@ -318,7 +318,7 @@ SendCopyOutResponse(void)
 {
 	StringInfoData buf;
 
-	pq_beginmessage(&buf, 'H');
+	pq_beginmessage(&buf, PqMsg_CopyOutResponse);
 	pq_sendbyte(&buf, 0);		/* overall format */
 	pq_sendint16(&buf, 0);		/* natts */
 	pq_endmessage(&buf);
@@ -330,7 +330,7 @@ SendCopyOutResponse(void)
 static void
 SendCopyDone(void)
 {
-	pq_putemptymessage('c');
+	pq_putemptymessage(PqMsg_CopyDone);
 }
 
 /*
@@ -368,7 +368,7 @@ SendXlogRecPtrResult(XLogRecPtr ptr, TimeLineID tli)
 	end_tup_output(tstate);
 
 	/* Send a CommandComplete message */
-	pq_puttextmessage('C', "SELECT");
+	pq_puttextmessage(PqMsg_CommandComplete, "SELECT");
 }
 
 /*
diff --git a/src/backend/commands/async.c b/src/backend/commands/async.c
index ef909cf4e0..d148d10850 100644
--- a/src/backend/commands/async.c
+++ b/src/backend/commands/async.c
@@ -2281,7 +2281,7 @@ NotifyMyFrontEnd(const char *channel, const char *payload, int32 srcPid)
 	{
 		StringInfoData buf;
 
-		pq_beginmessage(&buf, 'A');
+		pq_beginmessage(&buf, PqMsg_NotificationResponse);
 		pq_sendint32(&buf, srcPid);
 		pq_sendstring(&buf, channel);
 		pq_sendstring(&buf, payload);
diff --git a/src/backend/commands/copyfromparse.c b/src/backend/commands/copyfromparse.c
index 232768a6e1..f553734582 100644
--- a/src/backend/commands/copyfromparse.c
+++ b/src/backend/commands/copyfromparse.c
@@ -174,7 +174,7 @@ ReceiveCopyBegin(CopyFromState cstate)
 	int16		format = (cstate->opts.binary ? 1 : 0);
 	int			i;
 
-	pq_beginmessage(&buf, 'G');
+	pq_beginmessage(&buf, PqMsg_CopyInResponse);
 	pq_sendbyte(&buf, format);	/* overall format */
 	pq_sendint16(&buf, natts);
 	for (i = 0; i < natts; i++)
@@ -279,13 +279,13 @@ CopyGetData(CopyFromState cstate, void *databuf, int minread, int maxread)
 					/* Validate message type and set packet size limit */
 					switch (mtype)
 					{
-						case 'd':	/* CopyData */
+						case PqMsg_CopyData:
 							maxmsglen = PQ_LARGE_MESSAGE_LIMIT;
 							break;
-						case 'c':	/* CopyDone */
-						case 'f':	/* CopyFail */
-						case 'H':	/* Flush */
-						case 'S':	/* Sync */
+						case PqMsg_CopyDone:
+						case PqMsg_CopyFail:
+						case PqMsg_Flush:
+						case PqMsg_Sync:
 							maxmsglen = PQ_SMALL_MESSAGE_LIMIT;
 							break;
 						default:
@@ -305,20 +305,20 @@ CopyGetData(CopyFromState cstate, void *databuf, int minread, int maxread)
 					/* ... and process it */
 					switch (mtype)
 					{
-						case 'd':	/* CopyData */
+						case PqMsg_CopyData:
 							break;
-						case 'c':	/* CopyDone */
+						case PqMsg_CopyDone:
 							/* COPY IN correctly terminated by frontend */
 							cstate->raw_reached_eof = true;
 							return bytesread;
-						case 'f':	/* CopyFail */
+						case PqMsg_CopyFail:
 							ereport(ERROR,
 									(errcode(ERRCODE_QUERY_CANCELED),
 									 errmsg("COPY from stdin failed: %s",
 											pq_getmsgstring(cstate->fe_msgbuf))));
 							break;
-						case 'H':	/* Flush */
-						case 'S':	/* Sync */
+						case PqMsg_Flush:
+						case PqMsg_Sync:
 
 							/*
 							 * Ignore Flush/Sync for the convenience of client
diff --git a/src/backend/commands/copyto.c b/src/backend/commands/copyto.c
index 9e4b2437a5..eaa3172793 100644
--- a/src/backend/commands/copyto.c
+++ b/src/backend/commands/copyto.c
@@ -144,7 +144,7 @@ SendCopyBegin(CopyToState cstate)
 	int16		format = (cstate->opts.binary ? 1 : 0);
 	int			i;
 
-	pq_beginmessage(&buf, 'H');
+	pq_beginmessage(&buf, PqMsg_CopyOutResponse);
 	pq_sendbyte(&buf, format);	/* overall format */
 	pq_sendint16(&buf, natts);
 	for (i = 0; i < natts; i++)
@@ -159,7 +159,7 @@ SendCopyEnd(CopyToState cstate)
 	/* Shouldn't have any unsent data */
 	Assert(cstate->fe_msgbuf->len == 0);
 	/* Send Copy Done message */
-	pq_putemptymessage('c');
+	pq_putemptymessage(PqMsg_CopyDone);
 }
 
 /*----------
@@ -247,7 +247,7 @@ CopySendEndOfRow(CopyToState cstate)
 				CopySendChar(cstate, '\n');
 
 			/* Dump the accumulated row as one CopyData message */
-			(void) pq_putmessage('d', fe_msgbuf->data, fe_msgbuf->len);
+			(void) pq_putmessage(PqMsg_CopyData, fe_msgbuf->data, fe_msgbuf->len);
 			break;
 		case COPY_CALLBACK:
 			cstate->data_dest_cb(fe_msgbuf->data, fe_msgbuf->len);
diff --git a/src/backend/libpq/auth-sasl.c b/src/backend/libpq/auth-sasl.c
index 684680897b..c535bc5383 100644
--- a/src/backend/libpq/auth-sasl.c
+++ b/src/backend/libpq/auth-sasl.c
@@ -87,7 +87,7 @@ CheckSASLAuth(const pg_be_sasl_mech *mech, Port *port, char *shadow_pass,
 	{
 		pq_startmsgread();
 		mtype = pq_getbyte();
-		if (mtype != 'p')
+		if (mtype != PqMsg_SASLResponse)
 		{
 			/* Only log error if client didn't disconnect. */
 			if (mtype != EOF)
diff --git a/src/backend/libpq/auth.c b/src/backend/libpq/auth.c
index 315a24bb3f..0356fe3e45 100644
--- a/src/backend/libpq/auth.c
+++ b/src/backend/libpq/auth.c
@@ -665,7 +665,7 @@ sendAuthRequest(Port *port, AuthRequest areq, const char *extradata, int extrale
 
 	CHECK_FOR_INTERRUPTS();
 
-	pq_beginmessage(&buf, 'R');
+	pq_beginmessage(&buf, PqMsg_AuthenticationRequest);
 	pq_sendint32(&buf, (int32) areq);
 	if (extralen > 0)
 		pq_sendbytes(&buf, extradata, extralen);
@@ -698,7 +698,7 @@ recv_password_packet(Port *port)
 
 	/* Expect 'p' message type */
 	mtype = pq_getbyte();
-	if (mtype != 'p')
+	if (mtype != PqMsg_PasswordMessage)
 	{
 		/*
 		 * If the client just disconnects without offering a password, don't
@@ -961,7 +961,7 @@ pg_GSS_recvauth(Port *port)
 		CHECK_FOR_INTERRUPTS();
 
 		mtype = pq_getbyte();
-		if (mtype != 'p')
+		if (mtype != PqMsg_GSSResponse)
 		{
 			/* Only log error if client didn't disconnect. */
 			if (mtype != EOF)
@@ -1232,7 +1232,7 @@ pg_SSPI_recvauth(Port *port)
 	{
 		pq_startmsgread();
 		mtype = pq_getbyte();
-		if (mtype != 'p')
+		if (mtype != PqMsg_GSSResponse)
 		{
 			if (sspictx != NULL)
 			{
diff --git a/src/backend/postmaster/postmaster.c b/src/backend/postmaster/postmaster.c
index 9c8ec779f9..07d376d77e 100644
--- a/src/backend/postmaster/postmaster.c
+++ b/src/backend/postmaster/postmaster.c
@@ -2357,7 +2357,7 @@ SendNegotiateProtocolVersion(List *unrecognized_protocol_options)
 	StringInfoData buf;
 	ListCell   *lc;
 
-	pq_beginmessage(&buf, 'v'); /* NegotiateProtocolVersion */
+	pq_beginmessage(&buf, PqMsg_NegotiateProtocolVersion);
 	pq_sendint32(&buf, PG_PROTOCOL_LATEST);
 	pq_sendint32(&buf, list_length(unrecognized_protocol_options));
 	foreach(lc, unrecognized_protocol_options)
diff --git a/src/backend/replication/walsender.c b/src/backend/replication/walsender.c
index d27ef2985d..80374c55be 100644
--- a/src/backend/replication/walsender.c
+++ b/src/backend/replication/walsender.c
@@ -603,7 +603,7 @@ SendTimeLineHistory(TimeLineHistoryCmd *cmd)
 	dest->rStartup(dest, CMD_SELECT, tupdesc);
 
 	/* Send a DataRow message */
-	pq_beginmessage(&buf, 'D');
+	pq_beginmessage(&buf, PqMsg_DataRow);
 	pq_sendint16(&buf, 2);		/* # of columns */
 	len = strlen(histfname);
 	pq_sendint32(&buf, len);	/* col1 len */
@@ -801,7 +801,7 @@ StartReplication(StartReplicationCmd *cmd)
 		WalSndSetState(WALSNDSTATE_CATCHUP);
 
 		/* Send a CopyBothResponse message, and start streaming */
-		pq_beginmessage(&buf, 'W');
+		pq_beginmessage(&buf, PqMsg_CopyBothResponse);
 		pq_sendbyte(&buf, 0);
 		pq_sendint16(&buf, 0);
 		pq_endmessage(&buf);
@@ -1294,7 +1294,7 @@ StartLogicalReplication(StartReplicationCmd *cmd)
 	WalSndSetState(WALSNDSTATE_CATCHUP);
 
 	/* Send a CopyBothResponse message, and start streaming */
-	pq_beginmessage(&buf, 'W');
+	pq_beginmessage(&buf, PqMsg_CopyBothResponse);
 	pq_sendbyte(&buf, 0);
 	pq_sendint16(&buf, 0);
 	pq_endmessage(&buf);
@@ -1923,11 +1923,11 @@ ProcessRepliesIfAny(void)
 		/* Validate message type and set packet size limit */
 		switch (firstchar)
 		{
-			case 'd':
+			case PqMsg_CopyData:
 				maxmsglen = PQ_LARGE_MESSAGE_LIMIT;
 				break;
-			case 'c':
-			case 'X':
+			case PqMsg_CopyDone:
+			case PqMsg_Terminate:
 				maxmsglen = PQ_SMALL_MESSAGE_LIMIT;
 				break;
 			default:
@@ -1955,7 +1955,7 @@ ProcessRepliesIfAny(void)
 				/*
 				 * 'd' means a standby reply wrapped in a CopyData packet.
 				 */
-			case 'd':
+			case PqMsg_CopyData:
 				ProcessStandbyMessage();
 				received = true;
 				break;
@@ -1964,7 +1964,7 @@ ProcessRepliesIfAny(void)
 				 * CopyDone means the standby requested to finish streaming.
 				 * Reply with CopyDone, if we had not sent that already.
 				 */
-			case 'c':
+			case PqMsg_CopyDone:
 				if (!streamingDoneSending)
 				{
 					pq_putmessage_noblock('c', NULL, 0);
@@ -1978,7 +1978,7 @@ ProcessRepliesIfAny(void)
 				/*
 				 * 'X' means that the standby is closing down the socket.
 				 */
-			case 'X':
+			case PqMsg_Terminate:
 				proc_exit(0);
 
 			default:
diff --git a/src/backend/tcop/dest.c b/src/backend/tcop/dest.c
index c0406e2ee5..06d1872b9a 100644
--- a/src/backend/tcop/dest.c
+++ b/src/backend/tcop/dest.c
@@ -176,7 +176,7 @@ EndCommand(const QueryCompletion *qc, CommandDest dest, bool force_undecorated_o
 
 			len = BuildQueryCompletionString(completionTag, qc,
 											 force_undecorated_output);
-			pq_putmessage('C', completionTag, len + 1);
+			pq_putmessage(PqMsg_Close, completionTag, len + 1);
 
 		case DestNone:
 		case DestDebug:
@@ -200,7 +200,7 @@ EndCommand(const QueryCompletion *qc, CommandDest dest, bool force_undecorated_o
 void
 EndReplicationCommand(const char *commandTag)
 {
-	pq_putmessage('C', commandTag, strlen(commandTag) + 1);
+	pq_putmessage(PqMsg_Close, commandTag, strlen(commandTag) + 1);
 }
 
 /* ----------------
@@ -220,7 +220,7 @@ NullCommand(CommandDest dest)
 		case DestRemoteSimple:
 
 			/* Tell the FE that we saw an empty query string */
-			pq_putemptymessage('I');
+			pq_putemptymessage(PqMsg_EmptyQueryResponse);
 			break;
 
 		case DestNone:
@@ -258,7 +258,7 @@ ReadyForQuery(CommandDest dest)
 			{
 				StringInfoData buf;
 
-				pq_beginmessage(&buf, 'Z');
+				pq_beginmessage(&buf, PqMsg_ReadyForQuery);
 				pq_sendbyte(&buf, TransactionBlockStatusCode());
 				pq_endmessage(&buf);
 			}
diff --git a/src/backend/tcop/fastpath.c b/src/backend/tcop/fastpath.c
index 2f70ebd5fa..71f161dbe2 100644
--- a/src/backend/tcop/fastpath.c
+++ b/src/backend/tcop/fastpath.c
@@ -69,7 +69,7 @@ SendFunctionResult(Datum retval, bool isnull, Oid rettype, int16 format)
 {
 	StringInfoData buf;
 
-	pq_beginmessage(&buf, 'V');
+	pq_beginmessage(&buf, PqMsg_FunctionCallResponse);
 
 	if (isnull)
 	{
diff --git a/src/backend/tcop/postgres.c b/src/backend/tcop/postgres.c
index 36cc99ec9c..e4756f8be2 100644
--- a/src/backend/tcop/postgres.c
+++ b/src/backend/tcop/postgres.c
@@ -402,37 +402,37 @@ SocketBackend(StringInfo inBuf)
 	 */
 	switch (qtype)
 	{
-		case 'Q':				/* simple query */
+		case PqMsg_Query:
 			maxmsglen = PQ_LARGE_MESSAGE_LIMIT;
 			doing_extended_query_message = false;
 			break;
 
-		case 'F':				/* fastpath function call */
+		case PqMsg_FunctionCall:
 			maxmsglen = PQ_LARGE_MESSAGE_LIMIT;
 			doing_extended_query_message = false;
 			break;
 
-		case 'X':				/* terminate */
+		case PqMsg_Terminate:
 			maxmsglen = PQ_SMALL_MESSAGE_LIMIT;
 			doing_extended_query_message = false;
 			ignore_till_sync = false;
 			break;
 
-		case 'B':				/* bind */
-		case 'P':				/* parse */
+		case PqMsg_Bind:
+		case PqMsg_Parse:
 			maxmsglen = PQ_LARGE_MESSAGE_LIMIT;
 			doing_extended_query_message = true;
 			break;
 
-		case 'C':				/* close */
-		case 'D':				/* describe */
-		case 'E':				/* execute */
-		case 'H':				/* flush */
+		case PqMsg_Close:
+		case PqMsg_Describe:
+		case PqMsg_Execute:
+		case PqMsg_Flush:
 			maxmsglen = PQ_SMALL_MESSAGE_LIMIT;
 			doing_extended_query_message = true;
 			break;
 
-		case 'S':				/* sync */
+		case PqMsg_Sync:
 			maxmsglen = PQ_SMALL_MESSAGE_LIMIT;
 			/* stop any active skip-till-Sync */
 			ignore_till_sync = false;
@@ -440,13 +440,13 @@ SocketBackend(StringInfo inBuf)
 			doing_extended_query_message = false;
 			break;
 
-		case 'd':				/* copy data */
+		case PqMsg_CopyData:
 			maxmsglen = PQ_LARGE_MESSAGE_LIMIT;
 			doing_extended_query_message = false;
 			break;
 
-		case 'c':				/* copy done */
-		case 'f':				/* copy fail */
+		case PqMsg_CopyDone:
+		case PqMsg_CopyFail:
 			maxmsglen = PQ_SMALL_MESSAGE_LIMIT;
 			doing_extended_query_message = false;
 			break;
@@ -1589,7 +1589,7 @@ exec_parse_message(const char *query_string,	/* string to execute */
 	 * Send ParseComplete.
 	 */
 	if (whereToSendOutput == DestRemote)
-		pq_putemptymessage('1');
+		pq_putemptymessage(PqMsg_ParseComplete);
 
 	/*
 	 * Emit duration logging if appropriate.
@@ -2047,7 +2047,7 @@ exec_bind_message(StringInfo input_message)
 	 * Send BindComplete.
 	 */
 	if (whereToSendOutput == DestRemote)
-		pq_putemptymessage('2');
+		pq_putemptymessage(PqMsg_BindComplete);
 
 	/*
 	 * Emit duration logging if appropriate.
@@ -2290,7 +2290,7 @@ exec_execute_message(const char *portal_name, long max_rows)
 	{
 		/* Portal run not complete, so send PortalSuspended */
 		if (whereToSendOutput == DestRemote)
-			pq_putemptymessage('s');
+			pq_putemptymessage(PqMsg_PortalSuspended);
 
 		/*
 		 * Set XACT_FLAGS_PIPELINING whenever we suspend an Execute message,
@@ -2683,7 +2683,7 @@ exec_describe_statement_message(const char *stmt_name)
 								  NULL);
 	}
 	else
-		pq_putemptymessage('n');	/* NoData */
+		pq_putemptymessage(PqMsg_NoData);
 }
 
 /*
@@ -2736,7 +2736,7 @@ exec_describe_portal_message(const char *portal_name)
 								  FetchPortalTargetList(portal),
 								  portal->formats);
 	else
-		pq_putemptymessage('n');	/* NoData */
+		pq_putemptymessage(PqMsg_NoData);
 }
 
 
@@ -4239,7 +4239,7 @@ PostgresMain(const char *dbname, const char *username)
 	{
 		StringInfoData buf;
 
-		pq_beginmessage(&buf, 'K');
+		pq_beginmessage(&buf, PqMsg_BackendKeyData);
 		pq_sendint32(&buf, (int32) MyProcPid);
 		pq_sendint32(&buf, (int32) MyCancelKey);
 		pq_endmessage(&buf);
@@ -4618,7 +4618,7 @@ PostgresMain(const char *dbname, const char *username)
 
 		switch (firstchar)
 		{
-			case 'Q':			/* simple query */
+			case PqMsg_Query:
 				{
 					const char *query_string;
 
@@ -4642,7 +4642,7 @@ PostgresMain(const char *dbname, const char *username)
 				}
 				break;
 
-			case 'P':			/* parse */
+			case PqMsg_Parse:
 				{
 					const char *stmt_name;
 					const char *query_string;
@@ -4672,7 +4672,7 @@ PostgresMain(const char *dbname, const char *username)
 				}
 				break;
 
-			case 'B':			/* bind */
+			case PqMsg_Bind:
 				forbidden_in_wal_sender(firstchar);
 
 				/* Set statement_timestamp() */
@@ -4687,7 +4687,7 @@ PostgresMain(const char *dbname, const char *username)
 				/* exec_bind_message does valgrind_report_error_query */
 				break;
 
-			case 'E':			/* execute */
+			case PqMsg_Execute:
 				{
 					const char *portal_name;
 					int			max_rows;
@@ -4707,7 +4707,7 @@ PostgresMain(const char *dbname, const char *username)
 				}
 				break;
 
-			case 'F':			/* fastpath function call */
+			case PqMsg_FunctionCall:
 				forbidden_in_wal_sender(firstchar);
 
 				/* Set statement_timestamp() */
@@ -4742,7 +4742,7 @@ PostgresMain(const char *dbname, const char *username)
 				send_ready_for_query = true;
 				break;
 
-			case 'C':			/* close */
+			case PqMsg_Close:
 				{
 					int			close_type;
 					const char *close_target;
@@ -4782,13 +4782,13 @@ PostgresMain(const char *dbname, const char *username)
 					}
 
 					if (whereToSendOutput == DestRemote)
-						pq_putemptymessage('3');	/* CloseComplete */
+						pq_putemptymessage(PqMsg_CloseComplete);
 
 					valgrind_report_error_query("CLOSE message");
 				}
 				break;
 
-			case 'D':			/* describe */
+			case PqMsg_Describe:
 				{
 					int			describe_type;
 					const char *describe_target;
@@ -4822,13 +4822,13 @@ PostgresMain(const char *dbname, const char *username)
 				}
 				break;
 
-			case 'H':			/* flush */
+			case PqMsg_Flush:
 				pq_getmsgend(&input_message);
 				if (whereToSendOutput == DestRemote)
 					pq_flush();
 				break;
 
-			case 'S':			/* sync */
+			case PqMsg_Sync:
 				pq_getmsgend(&input_message);
 				finish_xact_command();
 				valgrind_report_error_query("SYNC message");
@@ -4847,7 +4847,7 @@ PostgresMain(const char *dbname, const char *username)
 
 				/* FALLTHROUGH */
 
-			case 'X':
+			case PqMsg_Terminate:
 
 				/*
 				 * Reset whereToSendOutput to prevent ereport from attempting
@@ -4865,9 +4865,9 @@ PostgresMain(const char *dbname, const char *username)
 				 */
 				proc_exit(0);
 
-			case 'd':			/* copy data */
-			case 'c':			/* copy done */
-			case 'f':			/* copy fail */
+			case PqMsg_CopyData:
+			case PqMsg_CopyDone:
+			case PqMsg_CopyFail:
 
 				/*
 				 * Accept but ignore these messages, per protocol spec; we
@@ -4897,7 +4897,7 @@ forbidden_in_wal_sender(char firstchar)
 {
 	if (am_walsender)
 	{
-		if (firstchar == 'F')
+		if (firstchar == PqMsg_FunctionCall)
 			ereport(ERROR,
 					(errcode(ERRCODE_PROTOCOL_VIOLATION),
 					 errmsg("fastpath function calls not supported in a replication connection")));
diff --git a/src/backend/utils/error/elog.c b/src/backend/utils/error/elog.c
index 5898100acb..8e1f3e8521 100644
--- a/src/backend/utils/error/elog.c
+++ b/src/backend/utils/error/elog.c
@@ -3465,7 +3465,10 @@ send_message_to_frontend(ErrorData *edata)
 		char		tbuf[12];
 
 		/* 'N' (Notice) is for nonfatal conditions, 'E' is for errors */
-		pq_beginmessage(&msgbuf, (edata->elevel < ERROR) ? 'N' : 'E');
+		if (edata->elevel < ERROR)
+			pq_beginmessage(&msgbuf, PqMsg_NoticeResponse);
+		else
+			pq_beginmessage(&msgbuf, PqMsg_ErrorResponse);
 
 		sev = error_severity(edata->elevel);
 		pq_sendbyte(&msgbuf, PG_DIAG_SEVERITY);
diff --git a/src/backend/utils/misc/guc.c b/src/backend/utils/misc/guc.c
index 99bb2fdd19..84e7ad4d90 100644
--- a/src/backend/utils/misc/guc.c
+++ b/src/backend/utils/misc/guc.c
@@ -2593,7 +2593,7 @@ ReportGUCOption(struct config_generic *record)
 	{
 		StringInfoData msgbuf;
 
-		pq_beginmessage(&msgbuf, 'S');
+		pq_beginmessage(&msgbuf, PqMsg_ParameterStatus);
 		pq_sendstring(&msgbuf, record->name);
 		pq_sendstring(&msgbuf, val);
 		pq_endmessage(&msgbuf);
diff --git a/src/include/Makefile b/src/include/Makefile
index 5d213187e2..2d5242561c 100644
--- a/src/include/Makefile
+++ b/src/include/Makefile
@@ -40,6 +40,7 @@ install: all installdirs
 	$(INSTALL_DATA) $(srcdir)/port.h         '$(DESTDIR)$(includedir_internal)'
 	$(INSTALL_DATA) $(srcdir)/postgres_fe.h  '$(DESTDIR)$(includedir_internal)'
 	$(INSTALL_DATA) $(srcdir)/libpq/pqcomm.h '$(DESTDIR)$(includedir_internal)/libpq'
+	$(INSTALL_DATA) $(srcdir)/libpq/protocol.h '$(DESTDIR)$(includedir_internal)/libpq'
 # These headers are needed for server-side development
 	$(INSTALL_DATA) pg_config.h     '$(DESTDIR)$(includedir_server)'
 	$(INSTALL_DATA) pg_config_ext.h '$(DESTDIR)$(includedir_server)'
@@ -65,7 +66,7 @@ installdirs:
 
 uninstall:
 	rm -f $(addprefix '$(DESTDIR)$(includedir)'/, pg_config.h pg_config_ext.h pg_config_os.h pg_config_manual.h postgres_ext.h libpq/libpq-fs.h)
-	rm -f $(addprefix '$(DESTDIR)$(includedir_internal)'/, c.h port.h postgres_fe.h libpq/pqcomm.h)
+	rm -f $(addprefix '$(DESTDIR)$(includedir_internal)'/, c.h port.h postgres_fe.h libpq/pqcomm.h libpq/protocol.h)
 # heuristic...
 	rm -rf $(addprefix '$(DESTDIR)$(includedir_server)'/, $(SUBDIRS) *.h)
 
diff --git a/src/include/libpq/pqcomm.h b/src/include/libpq/pqcomm.h
index 3da00f7983..46a0946b8b 100644
--- a/src/include/libpq/pqcomm.h
+++ b/src/include/libpq/pqcomm.h
@@ -21,6 +21,12 @@
 #include <netdb.h>
 #include <netinet/in.h>
 
+/*
+ * The definitions for the request/response codes are kept in a separate file
+ * for ease of use in third party programs.
+ */
+#include "libpq/protocol.h"
+
 typedef struct
 {
 	struct sockaddr_storage addr;
@@ -112,23 +118,6 @@ typedef uint32 PacketLen;
 #define MAX_STARTUP_PACKET_LENGTH 10000
 
 
-/* These are the authentication request codes sent by the backend. */
-
-#define AUTH_REQ_OK			0	/* User is authenticated  */
-#define AUTH_REQ_KRB4		1	/* Kerberos V4. Not supported any more. */
-#define AUTH_REQ_KRB5		2	/* Kerberos V5. Not supported any more. */
-#define AUTH_REQ_PASSWORD	3	/* Password */
-#define AUTH_REQ_CRYPT		4	/* crypt password. Not supported any more. */
-#define AUTH_REQ_MD5		5	/* md5 password */
-/* 6 is available.  It was used for SCM creds, not supported any more. */
-#define AUTH_REQ_GSS		7	/* GSSAPI without wrap() */
-#define AUTH_REQ_GSS_CONT	8	/* Continue GSS exchanges */
-#define AUTH_REQ_SSPI		9	/* SSPI negotiate without wrap() */
-#define AUTH_REQ_SASL	   10	/* Begin SASL authentication */
-#define AUTH_REQ_SASL_CONT 11	/* Continue SASL authentication */
-#define AUTH_REQ_SASL_FIN  12	/* Final SASL message */
-#define AUTH_REQ_MAX	   AUTH_REQ_SASL_FIN	/* maximum AUTH_REQ_* value */
-
 typedef uint32 AuthRequest;
 
 
diff --git a/src/include/libpq/protocol.h b/src/include/libpq/protocol.h
new file mode 100644
index 0000000000..cc46f4b586
--- /dev/null
+++ b/src/include/libpq/protocol.h
@@ -0,0 +1,85 @@
+/*-------------------------------------------------------------------------
+ *
+ * protocol.h
+ *		Definitions of the request/response codes for the wire protocol.
+ *
+ *
+ * Portions Copyright (c) 1996-2023, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ *
+ * src/include/libpq/protocol.h
+ *
+ *-------------------------------------------------------------------------
+ */
+#ifndef PROTOCOL_H
+#define PROTOCOL_H
+
+/* These are the request codes sent by the frontend. */
+
+#define PqMsg_Bind					'B'
+#define PqMsg_Close					'C'
+#define PqMsg_Describe				'D'
+#define PqMsg_Execute				'E'
+#define PqMsg_FunctionCall			'F'
+#define PqMsg_Flush					'H'
+#define PqMsg_Parse					'P'
+#define PqMsg_Query					'Q'
+#define PqMsg_Sync					'S'
+#define PqMsg_Terminate				'X'
+#define PqMsg_CopyFail				'f'
+#define PqMsg_GSSResponse			'p'
+#define PqMsg_PasswordMessage		'p'
+#define PqMsg_SASLInitialResponse	'p'
+#define PqMsg_SASLResponse			'p'
+
+
+/* These are the response codes sent by the backend. */
+
+#define PqMsg_ParseComplete			'1'
+#define PqMsg_BindComplete			'2'
+#define PqMsg_CloseComplete			'3'
+#define PqMsg_NotificationResponse	'A'
+#define PqMsg_CommandComplete		'C'
+#define PqMsg_DataRow				'D'
+#define PqMsg_ErrorResponse			'E'
+#define PqMsg_CopyInResponse		'G'
+#define PqMsg_CopyOutResponse		'H'
+#define PqMsg_EmptyQueryResponse	'I'
+#define PqMsg_BackendKeyData		'K'
+#define PqMsg_NoticeResponse		'N'
+#define PqMsg_AuthenticationRequest 'R'
+#define PqMsg_ParameterStatus		'S'
+#define PqMsg_RowDescription		'T'
+#define PqMsg_FunctionCallResponse	'V'
+#define PqMsg_CopyBothResponse		'W'
+#define PqMsg_ReadyForQuery			'Z'
+#define PqMsg_NoData				'n'
+#define PqMsg_PortalSuspended		's'
+#define PqMsg_ParameterDescription	't'
+#define PqMsg_NegotiateProtocolVersion 'v'
+
+
+/* These are the codes sent by both the frontend and backend. */
+
+#define PqMsg_CopyDone				'c'
+#define PqMsg_CopyData				'd'
+
+
+/* These are the authentication request codes sent by the backend. */
+
+#define AUTH_REQ_OK			0	/* User is authenticated  */
+#define AUTH_REQ_KRB4		1	/* Kerberos V4. Not supported any more. */
+#define AUTH_REQ_KRB5		2	/* Kerberos V5. Not supported any more. */
+#define AUTH_REQ_PASSWORD	3	/* Password */
+#define AUTH_REQ_CRYPT		4	/* crypt password. Not supported any more. */
+#define AUTH_REQ_MD5		5	/* md5 password */
+/* 6 is available.  It was used for SCM creds, not supported any more. */
+#define AUTH_REQ_GSS		7	/* GSSAPI without wrap() */
+#define AUTH_REQ_GSS_CONT	8	/* Continue GSS exchanges */
+#define AUTH_REQ_SSPI		9	/* SSPI negotiate without wrap() */
+#define AUTH_REQ_SASL	   10	/* Begin SASL authentication */
+#define AUTH_REQ_SASL_CONT 11	/* Continue SASL authentication */
+#define AUTH_REQ_SASL_FIN  12	/* Final SASL message */
+#define AUTH_REQ_MAX	   AUTH_REQ_SASL_FIN	/* maximum AUTH_REQ_* value */
+
+#endif							/* PROTOCOL_H */
diff --git a/src/include/meson.build b/src/include/meson.build
index d7e1ecd4c9..d50897c9fd 100644
--- a/src/include/meson.build
+++ b/src/include/meson.build
@@ -94,6 +94,7 @@ install_headers(
 
 install_headers(
   'libpq/pqcomm.h',
+  'libpq/protocol.h',
   install_dir: dir_include_internal / 'libpq',
 )
 
diff --git a/src/interfaces/libpq/fe-auth.c b/src/interfaces/libpq/fe-auth.c
index 887ca5e9e1..912aa14821 100644
--- a/src/interfaces/libpq/fe-auth.c
+++ b/src/interfaces/libpq/fe-auth.c
@@ -586,7 +586,7 @@ pg_SASL_init(PGconn *conn, int payloadlen)
 	/*
 	 * Build a SASLInitialResponse message, and send it.
 	 */
-	if (pqPutMsgStart('p', conn))
+	if (pqPutMsgStart(PqMsg_SASLInitialResponse, conn))
 		goto error;
 	if (pqPuts(selected_mechanism, conn))
 		goto error;
diff --git a/src/interfaces/libpq/fe-connect.c b/src/interfaces/libpq/fe-connect.c
index 837c5321aa..bf83a9b569 100644
--- a/src/interfaces/libpq/fe-connect.c
+++ b/src/interfaces/libpq/fe-connect.c
@@ -3591,7 +3591,9 @@ keep_going:						/* We will come back to here until there is
 				 * Anything else probably means it's not Postgres on the other
 				 * end at all.
 				 */
-				if (!(beresp == 'R' || beresp == 'v' || beresp == 'E'))
+				if (beresp != PqMsg_AuthenticationRequest &&
+					beresp != PqMsg_ErrorResponse &&
+					beresp != PqMsg_NegotiateProtocolVersion)
 				{
 					libpq_append_conn_error(conn, "expected authentication request from server, but received %c",
 											beresp);
@@ -3618,19 +3620,22 @@ keep_going:						/* We will come back to here until there is
 				 * version 14, the server also used the old protocol for
 				 * errors that happened before processing the startup packet.)
 				 */
-				if (beresp == 'R' && (msgLength < 8 || msgLength > 2000))
+				if (beresp == PqMsg_AuthenticationRequest &&
+					(msgLength < 8 || msgLength > 2000))
 				{
 					libpq_append_conn_error(conn, "received invalid authentication request");
 					goto error_return;
 				}
-				if (beresp == 'v' && (msgLength < 8 || msgLength > 2000))
+				if (beresp == PqMsg_NegotiateProtocolVersion &&
+					(msgLength < 8 || msgLength > 2000))
 				{
 					libpq_append_conn_error(conn, "received invalid protocol negotiation message");
 					goto error_return;
 				}
 
 #define MAX_ERRLEN 30000
-				if (beresp == 'E' && (msgLength < 8 || msgLength > MAX_ERRLEN))
+				if (beresp == PqMsg_ErrorResponse &&
+					(msgLength < 8 || msgLength > MAX_ERRLEN))
 				{
 					/* Handle error from a pre-3.0 server */
 					conn->inCursor = conn->inStart + 1; /* reread data */
@@ -3693,7 +3698,7 @@ keep_going:						/* We will come back to here until there is
 				}
 
 				/* Handle errors. */
-				if (beresp == 'E')
+				if (beresp == PqMsg_ErrorResponse)
 				{
 					if (pqGetErrorNotice3(conn, true))
 					{
@@ -3770,7 +3775,7 @@ keep_going:						/* We will come back to here until there is
 
 					goto error_return;
 				}
-				else if (beresp == 'v')
+				else if (beresp == PqMsg_NegotiateProtocolVersion)
 				{
 					if (pqGetNegotiateProtocolVersion3(conn))
 					{
@@ -4540,7 +4545,7 @@ sendTerminateConn(PGconn *conn)
 		 * Try to send "close connection" message to backend. Ignore any
 		 * error.
 		 */
-		pqPutMsgStart('X', conn);
+		pqPutMsgStart(PqMsg_Terminate, conn);
 		pqPutMsgEnd(conn);
 		(void) pqFlush(conn);
 	}
diff --git a/src/interfaces/libpq/fe-exec.c b/src/interfaces/libpq/fe-exec.c
index a868284ff8..974d462d4b 100644
--- a/src/interfaces/libpq/fe-exec.c
+++ b/src/interfaces/libpq/fe-exec.c
@@ -1458,7 +1458,7 @@ PQsendQueryInternal(PGconn *conn, const char *query, bool newQuery)
 
 	/* Send the query message(s) */
 	/* construct the outgoing Query message */
-	if (pqPutMsgStart('Q', conn) < 0 ||
+	if (pqPutMsgStart(PqMsg_Query, conn) < 0 ||
 		pqPuts(query, conn) < 0 ||
 		pqPutMsgEnd(conn) < 0)
 	{
@@ -1571,7 +1571,7 @@ PQsendPrepare(PGconn *conn,
 		return 0;				/* error msg already set */
 
 	/* construct the Parse message */
-	if (pqPutMsgStart('P', conn) < 0 ||
+	if (pqPutMsgStart(PqMsg_Parse, conn) < 0 ||
 		pqPuts(stmtName, conn) < 0 ||
 		pqPuts(query, conn) < 0)
 		goto sendFailed;
@@ -1599,7 +1599,7 @@ PQsendPrepare(PGconn *conn,
 	/* Add a Sync, unless in pipeline mode. */
 	if (conn->pipelineStatus == PQ_PIPELINE_OFF)
 	{
-		if (pqPutMsgStart('S', conn) < 0 ||
+		if (pqPutMsgStart(PqMsg_Sync, conn) < 0 ||
 			pqPutMsgEnd(conn) < 0)
 			goto sendFailed;
 	}
@@ -1784,7 +1784,7 @@ PQsendQueryGuts(PGconn *conn,
 	if (command)
 	{
 		/* construct the Parse message */
-		if (pqPutMsgStart('P', conn) < 0 ||
+		if (pqPutMsgStart(PqMsg_Parse, conn) < 0 ||
 			pqPuts(stmtName, conn) < 0 ||
 			pqPuts(command, conn) < 0)
 			goto sendFailed;
@@ -1808,7 +1808,7 @@ PQsendQueryGuts(PGconn *conn,
 	}
 
 	/* Construct the Bind message */
-	if (pqPutMsgStart('B', conn) < 0 ||
+	if (pqPutMsgStart(PqMsg_Bind, conn) < 0 ||
 		pqPuts("", conn) < 0 ||
 		pqPuts(stmtName, conn) < 0)
 		goto sendFailed;
@@ -1874,14 +1874,14 @@ PQsendQueryGuts(PGconn *conn,
 		goto sendFailed;
 
 	/* construct the Describe Portal message */
-	if (pqPutMsgStart('D', conn) < 0 ||
+	if (pqPutMsgStart(PqMsg_Describe, conn) < 0 ||
 		pqPutc('P', conn) < 0 ||
 		pqPuts("", conn) < 0 ||
 		pqPutMsgEnd(conn) < 0)
 		goto sendFailed;
 
 	/* construct the Execute message */
-	if (pqPutMsgStart('E', conn) < 0 ||
+	if (pqPutMsgStart(PqMsg_Execute, conn) < 0 ||
 		pqPuts("", conn) < 0 ||
 		pqPutInt(0, 4, conn) < 0 ||
 		pqPutMsgEnd(conn) < 0)
@@ -1890,7 +1890,7 @@ PQsendQueryGuts(PGconn *conn,
 	/* construct the Sync message if not in pipeline mode */
 	if (conn->pipelineStatus == PQ_PIPELINE_OFF)
 	{
-		if (pqPutMsgStart('S', conn) < 0 ||
+		if (pqPutMsgStart(PqMsg_Sync, conn) < 0 ||
 			pqPutMsgEnd(conn) < 0)
 			goto sendFailed;
 	}
@@ -2422,7 +2422,7 @@ PQdescribePrepared(PGconn *conn, const char *stmt)
 {
 	if (!PQexecStart(conn))
 		return NULL;
-	if (!PQsendTypedCommand(conn, 'D', 'S', stmt))
+	if (!PQsendTypedCommand(conn, PqMsg_Describe, 'S', stmt))
 		return NULL;
 	return PQexecFinish(conn);
 }
@@ -2441,7 +2441,7 @@ PQdescribePortal(PGconn *conn, const char *portal)
 {
 	if (!PQexecStart(conn))
 		return NULL;
-	if (!PQsendTypedCommand(conn, 'D', 'P', portal))
+	if (!PQsendTypedCommand(conn, PqMsg_Describe, 'P', portal))
 		return NULL;
 	return PQexecFinish(conn);
 }
@@ -2456,7 +2456,7 @@ PQdescribePortal(PGconn *conn, const char *portal)
 int
 PQsendDescribePrepared(PGconn *conn, const char *stmt)
 {
-	return PQsendTypedCommand(conn, 'D', 'S', stmt);
+	return PQsendTypedCommand(conn, PqMsg_Describe, 'S', stmt);
 }
 
 /*
@@ -2469,7 +2469,7 @@ PQsendDescribePrepared(PGconn *conn, const char *stmt)
 int
 PQsendDescribePortal(PGconn *conn, const char *portal)
 {
-	return PQsendTypedCommand(conn, 'D', 'P', portal);
+	return PQsendTypedCommand(conn, PqMsg_Describe, 'P', portal);
 }
 
 /*
@@ -2488,7 +2488,7 @@ PQclosePrepared(PGconn *conn, const char *stmt)
 {
 	if (!PQexecStart(conn))
 		return NULL;
-	if (!PQsendTypedCommand(conn, 'C', 'S', stmt))
+	if (!PQsendTypedCommand(conn, PqMsg_Close, 'S', stmt))
 		return NULL;
 	return PQexecFinish(conn);
 }
@@ -2506,7 +2506,7 @@ PQclosePortal(PGconn *conn, const char *portal)
 {
 	if (!PQexecStart(conn))
 		return NULL;
-	if (!PQsendTypedCommand(conn, 'C', 'P', portal))
+	if (!PQsendTypedCommand(conn, PqMsg_Close, 'P', portal))
 		return NULL;
 	return PQexecFinish(conn);
 }
@@ -2521,7 +2521,7 @@ PQclosePortal(PGconn *conn, const char *portal)
 int
 PQsendClosePrepared(PGconn *conn, const char *stmt)
 {
-	return PQsendTypedCommand(conn, 'C', 'S', stmt);
+	return PQsendTypedCommand(conn, PqMsg_Close, 'S', stmt);
 }
 
 /*
@@ -2534,7 +2534,7 @@ PQsendClosePrepared(PGconn *conn, const char *stmt)
 int
 PQsendClosePortal(PGconn *conn, const char *portal)
 {
-	return PQsendTypedCommand(conn, 'C', 'P', portal);
+	return PQsendTypedCommand(conn, PqMsg_Close, 'P', portal);
 }
 
 /*
@@ -2542,8 +2542,8 @@ PQsendClosePortal(PGconn *conn, const char *portal)
  *	 Common code to send a Describe or Close command
  *
  * Available options for "command" are
- *	 'C' for Close; or
- *	 'D' for Describe.
+ *	 PqMsg_Close for Close; or
+ *	 PqMsg_Describe for Describe.
  *
  * Available options for "type" are
  *	 'S' to run a command on a prepared statement; or
@@ -2577,17 +2577,17 @@ PQsendTypedCommand(PGconn *conn, char command, char type, const char *target)
 	/* construct the Sync message */
 	if (conn->pipelineStatus == PQ_PIPELINE_OFF)
 	{
-		if (pqPutMsgStart('S', conn) < 0 ||
+		if (pqPutMsgStart(PqMsg_Sync, conn) < 0 ||
 			pqPutMsgEnd(conn) < 0)
 			goto sendFailed;
 	}
 
 	/* remember if we are doing a Close or a Describe */
-	if (command == 'C')
+	if (command == PqMsg_Close)
 	{
 		entry->queryclass = PGQUERY_CLOSE;
 	}
-	else if (command == 'D')
+	else if (command == PqMsg_Describe)
 	{
 		entry->queryclass = PGQUERY_DESCRIBE;
 	}
@@ -2696,7 +2696,7 @@ PQputCopyData(PGconn *conn, const char *buffer, int nbytes)
 				return pqIsnonblocking(conn) ? 0 : -1;
 		}
 		/* Send the data (too simple to delegate to fe-protocol files) */
-		if (pqPutMsgStart('d', conn) < 0 ||
+		if (pqPutMsgStart(PqMsg_CopyData, conn) < 0 ||
 			pqPutnchar(buffer, nbytes, conn) < 0 ||
 			pqPutMsgEnd(conn) < 0)
 			return -1;
@@ -2731,7 +2731,7 @@ PQputCopyEnd(PGconn *conn, const char *errormsg)
 	if (errormsg)
 	{
 		/* Send COPY FAIL */
-		if (pqPutMsgStart('f', conn) < 0 ||
+		if (pqPutMsgStart(PqMsg_CopyFail, conn) < 0 ||
 			pqPuts(errormsg, conn) < 0 ||
 			pqPutMsgEnd(conn) < 0)
 			return -1;
@@ -2739,7 +2739,7 @@ PQputCopyEnd(PGconn *conn, const char *errormsg)
 	else
 	{
 		/* Send COPY DONE */
-		if (pqPutMsgStart('c', conn) < 0 ||
+		if (pqPutMsgStart(PqMsg_CopyDone, conn) < 0 ||
 			pqPutMsgEnd(conn) < 0)
 			return -1;
 	}
@@ -2751,7 +2751,7 @@ PQputCopyEnd(PGconn *conn, const char *errormsg)
 	if (conn->cmd_queue_head &&
 		conn->cmd_queue_head->queryclass != PGQUERY_SIMPLE)
 	{
-		if (pqPutMsgStart('S', conn) < 0 ||
+		if (pqPutMsgStart(PqMsg_Sync, conn) < 0 ||
 			pqPutMsgEnd(conn) < 0)
 			return -1;
 	}
@@ -3263,7 +3263,7 @@ PQpipelineSync(PGconn *conn)
 	entry->query = NULL;
 
 	/* construct the Sync message */
-	if (pqPutMsgStart('S', conn) < 0 ||
+	if (pqPutMsgStart(PqMsg_Sync, conn) < 0 ||
 		pqPutMsgEnd(conn) < 0)
 		goto sendFailed;
 
@@ -3311,7 +3311,7 @@ PQsendFlushRequest(PGconn *conn)
 		return 0;
 	}
 
-	if (pqPutMsgStart('H', conn) < 0 ||
+	if (pqPutMsgStart(PqMsg_Flush, conn) < 0 ||
 		pqPutMsgEnd(conn) < 0)
 	{
 		return 0;
diff --git a/src/interfaces/libpq/fe-protocol3.c b/src/interfaces/libpq/fe-protocol3.c
index 7bc6355d17..5613c56b14 100644
--- a/src/interfaces/libpq/fe-protocol3.c
+++ b/src/interfaces/libpq/fe-protocol3.c
@@ -34,8 +34,13 @@
  * than a couple of kilobytes).
  */
 #define VALID_LONG_MESSAGE_TYPE(id) \
-	((id) == 'T' || (id) == 'D' || (id) == 'd' || (id) == 'V' || \
-	 (id) == 'E' || (id) == 'N' || (id) == 'A')
+	((id) == PqMsg_CopyData || \
+	 (id) == PqMsg_DataRow || \
+	 (id) == PqMsg_ErrorResponse || \
+	 (id) == PqMsg_FunctionCallResponse || \
+	 (id) == PqMsg_NoticeResponse || \
+	 (id) == PqMsg_NotificationResponse || \
+	 (id) == PqMsg_RowDescription)
 
 
 static void handleSyncLoss(PGconn *conn, char id, int msgLength);
@@ -140,12 +145,12 @@ pqParseInput3(PGconn *conn)
 		 * from config file due to SIGHUP), but otherwise we hold off until
 		 * BUSY state.
 		 */
-		if (id == 'A')
+		if (id == PqMsg_NotificationResponse)
 		{
 			if (getNotify(conn))
 				return;
 		}
-		else if (id == 'N')
+		else if (id == PqMsg_NoticeResponse)
 		{
 			if (pqGetErrorNotice3(conn, false))
 				return;
@@ -165,12 +170,12 @@ pqParseInput3(PGconn *conn)
 			 * it is about to close the connection, so we don't want to just
 			 * discard it...)
 			 */
-			if (id == 'E')
+			if (id == PqMsg_ErrorResponse)
 			{
 				if (pqGetErrorNotice3(conn, false /* treat as notice */ ))
 					return;
 			}
-			else if (id == 'S')
+			else if (id == PqMsg_ParameterStatus)
 			{
 				if (getParameterStatus(conn))
 					return;
@@ -192,7 +197,7 @@ pqParseInput3(PGconn *conn)
 			 */
 			switch (id)
 			{
-				case 'C':		/* command complete */
+				case PqMsg_CommandComplete:
 					if (pqGets(&conn->workBuffer, conn))
 						return;
 					if (!pgHavePendingResult(conn))
@@ -210,13 +215,12 @@ pqParseInput3(PGconn *conn)
 								CMDSTATUS_LEN);
 					conn->asyncStatus = PGASYNC_READY;
 					break;
-				case 'E':		/* error return */
+				case PqMsg_ErrorResponse:
 					if (pqGetErrorNotice3(conn, true))
 						return;
 					conn->asyncStatus = PGASYNC_READY;
 					break;
-				case 'Z':		/* sync response, backend is ready for new
-								 * query */
+				case PqMsg_ReadyForQuery:
 					if (getReadyForQuery(conn))
 						return;
 					if (conn->pipelineStatus != PQ_PIPELINE_OFF)
@@ -246,7 +250,7 @@ pqParseInput3(PGconn *conn)
 						conn->asyncStatus = PGASYNC_IDLE;
 					}
 					break;
-				case 'I':		/* empty query */
+				case PqMsg_EmptyQueryResponse:
 					if (!pgHavePendingResult(conn))
 					{
 						conn->result = PQmakeEmptyPGresult(conn,
@@ -259,7 +263,7 @@ pqParseInput3(PGconn *conn)
 					}
 					conn->asyncStatus = PGASYNC_READY;
 					break;
-				case '1':		/* Parse Complete */
+				case PqMsg_ParseComplete:
 					/* If we're doing PQprepare, we're done; else ignore */
 					if (conn->cmd_queue_head &&
 						conn->cmd_queue_head->queryclass == PGQUERY_PREPARE)
@@ -277,10 +281,10 @@ pqParseInput3(PGconn *conn)
 						conn->asyncStatus = PGASYNC_READY;
 					}
 					break;
-				case '2':		/* Bind Complete */
+				case PqMsg_BindComplete:
 					/* Nothing to do for this message type */
 					break;
-				case '3':		/* Close Complete */
+				case PqMsg_CloseComplete:
 					/* If we're doing PQsendClose, we're done; else ignore */
 					if (conn->cmd_queue_head &&
 						conn->cmd_queue_head->queryclass == PGQUERY_CLOSE)
@@ -298,11 +302,11 @@ pqParseInput3(PGconn *conn)
 						conn->asyncStatus = PGASYNC_READY;
 					}
 					break;
-				case 'S':		/* parameter status */
+				case PqMsg_ParameterStatus:
 					if (getParameterStatus(conn))
 						return;
 					break;
-				case 'K':		/* secret key data from the backend */
+				case PqMsg_BackendKeyData:
 
 					/*
 					 * This is expected only during backend startup, but it's
@@ -314,7 +318,7 @@ pqParseInput3(PGconn *conn)
 					if (pqGetInt(&(conn->be_key), 4, conn))
 						return;
 					break;
-				case 'T':		/* Row Description */
+				case PqMsg_RowDescription:
 					if (conn->error_result ||
 						(conn->result != NULL &&
 						 conn->result->resultStatus == PGRES_FATAL_ERROR))
@@ -346,7 +350,7 @@ pqParseInput3(PGconn *conn)
 						return;
 					}
 					break;
-				case 'n':		/* No Data */
+				case PqMsg_NoData:
 
 					/*
 					 * NoData indicates that we will not be seeing a
@@ -374,11 +378,11 @@ pqParseInput3(PGconn *conn)
 						conn->asyncStatus = PGASYNC_READY;
 					}
 					break;
-				case 't':		/* Parameter Description */
+				case PqMsg_ParameterDescription:
 					if (getParamDescriptions(conn, msgLength))
 						return;
 					break;
-				case 'D':		/* Data Row */
+				case PqMsg_DataRow:
 					if (conn->result != NULL &&
 						conn->result->resultStatus == PGRES_TUPLES_OK)
 					{
@@ -405,24 +409,24 @@ pqParseInput3(PGconn *conn)
 						conn->inCursor += msgLength;
 					}
 					break;
-				case 'G':		/* Start Copy In */
+				case PqMsg_CopyInResponse:
 					if (getCopyStart(conn, PGRES_COPY_IN))
 						return;
 					conn->asyncStatus = PGASYNC_COPY_IN;
 					break;
-				case 'H':		/* Start Copy Out */
+				case PqMsg_CopyOutResponse:
 					if (getCopyStart(conn, PGRES_COPY_OUT))
 						return;
 					conn->asyncStatus = PGASYNC_COPY_OUT;
 					conn->copy_already_done = 0;
 					break;
-				case 'W':		/* Start Copy Both */
+				case PqMsg_CopyBothResponse:
 					if (getCopyStart(conn, PGRES_COPY_BOTH))
 						return;
 					conn->asyncStatus = PGASYNC_COPY_BOTH;
 					conn->copy_already_done = 0;
 					break;
-				case 'd':		/* Copy Data */
+				case PqMsg_CopyData:
 
 					/*
 					 * If we see Copy Data, just silently drop it.  This would
@@ -431,7 +435,7 @@ pqParseInput3(PGconn *conn)
 					 */
 					conn->inCursor += msgLength;
 					break;
-				case 'c':		/* Copy Done */
+				case PqMsg_CopyDone:
 
 					/*
 					 * If we see Copy Done, just silently drop it.  This is
@@ -1692,21 +1696,21 @@ getCopyDataMessage(PGconn *conn)
 		 */
 		switch (id)
 		{
-			case 'A':			/* NOTIFY */
+			case PqMsg_NotificationResponse:
 				if (getNotify(conn))
 					return 0;
 				break;
-			case 'N':			/* NOTICE */
+			case PqMsg_NoticeResponse:
 				if (pqGetErrorNotice3(conn, false))
 					return 0;
 				break;
-			case 'S':			/* ParameterStatus */
+			case PqMsg_ParameterStatus:
 				if (getParameterStatus(conn))
 					return 0;
 				break;
-			case 'd':			/* Copy Data, pass it back to caller */
+			case PqMsg_CopyData:
 				return msgLength;
-			case 'c':
+			case PqMsg_CopyDone:
 
 				/*
 				 * If this is a CopyDone message, exit COPY_OUT mode and let
@@ -1929,7 +1933,7 @@ pqEndcopy3(PGconn *conn)
 	if (conn->asyncStatus == PGASYNC_COPY_IN ||
 		conn->asyncStatus == PGASYNC_COPY_BOTH)
 	{
-		if (pqPutMsgStart('c', conn) < 0 ||
+		if (pqPutMsgStart(PqMsg_CopyDone, conn) < 0 ||
 			pqPutMsgEnd(conn) < 0)
 			return 1;
 
@@ -1940,7 +1944,7 @@ pqEndcopy3(PGconn *conn)
 		if (conn->cmd_queue_head &&
 			conn->cmd_queue_head->queryclass != PGQUERY_SIMPLE)
 		{
-			if (pqPutMsgStart('S', conn) < 0 ||
+			if (pqPutMsgStart(PqMsg_Sync, conn) < 0 ||
 				pqPutMsgEnd(conn) < 0)
 				return 1;
 		}
@@ -2023,7 +2027,7 @@ pqFunctionCall3(PGconn *conn, Oid fnid,
 
 	/* PQfn already validated connection state */
 
-	if (pqPutMsgStart('F', conn) < 0 || /* function call msg */
+	if (pqPutMsgStart(PqMsg_FunctionCall, conn) < 0 ||
 		pqPutInt(fnid, 4, conn) < 0 ||	/* function id */
 		pqPutInt(1, 2, conn) < 0 || /* # of format codes */
 		pqPutInt(1, 2, conn) < 0 || /* format code: BINARY */
diff --git a/src/interfaces/libpq/fe-trace.c b/src/interfaces/libpq/fe-trace.c
index 402784f40e..b18e3deab6 100644
--- a/src/interfaces/libpq/fe-trace.c
+++ b/src/interfaces/libpq/fe-trace.c
@@ -562,110 +562,120 @@ pqTraceOutputMessage(PGconn *conn, const char *message, bool toServer)
 
 	switch (id)
 	{
-		case '1':
+		case PqMsg_ParseComplete:
 			fprintf(conn->Pfdebug, "ParseComplete");
 			/* No message content */
 			break;
-		case '2':
+		case PqMsg_BindComplete:
 			fprintf(conn->Pfdebug, "BindComplete");
 			/* No message content */
 			break;
-		case '3':
+		case PqMsg_CloseComplete:
 			fprintf(conn->Pfdebug, "CloseComplete");
 			/* No message content */
 			break;
-		case 'A':				/* Notification Response */
+		case PqMsg_NotificationResponse:
 			pqTraceOutputA(conn->Pfdebug, message, &logCursor, regress);
 			break;
-		case 'B':				/* Bind */
+		case PqMsg_Bind:
 			pqTraceOutputB(conn->Pfdebug, message, &logCursor);
 			break;
-		case 'c':
+		case PqMsg_CopyDone:
 			fprintf(conn->Pfdebug, "CopyDone");
 			/* No message content */
 			break;
-		case 'C':				/* Close(F) or Command Complete(B) */
+		case PqMsg_CommandComplete:
+			/* Close(F) and CommandComplete(B) use the same identifier. */
+			Assert(PqMsg_Close == PqMsg_CommandComplete);
 			pqTraceOutputC(conn->Pfdebug, toServer, message, &logCursor);
 			break;
-		case 'd':				/* Copy Data */
+		case PqMsg_CopyData:
 			/* Drop COPY data to reduce the overhead of logging. */
 			break;
-		case 'D':				/* Describe(F) or Data Row(B) */
+		case PqMsg_Describe:
+			/* Describe(F) and DataRow(B) use the same identifier. */
+			Assert(PqMsg_Describe == PqMsg_DataRow);
 			pqTraceOutputD(conn->Pfdebug, toServer, message, &logCursor);
 			break;
-		case 'E':				/* Execute(F) or Error Response(B) */
+		case PqMsg_Execute:
+			/* Execute(F) and ErrorResponse(B) use the same identifier. */
+			Assert(PqMsg_Execute == PqMsg_ErrorResponse);
 			pqTraceOutputE(conn->Pfdebug, toServer, message, &logCursor,
 						   regress);
 			break;
-		case 'f':				/* Copy Fail */
+		case PqMsg_CopyFail:
 			pqTraceOutputf(conn->Pfdebug, message, &logCursor);
 			break;
-		case 'F':				/* Function Call */
+		case PqMsg_FunctionCall:
 			pqTraceOutputF(conn->Pfdebug, message, &logCursor, regress);
 			break;
-		case 'G':				/* Start Copy In */
+		case PqMsg_CopyInResponse:
 			pqTraceOutputG(conn->Pfdebug, message, &logCursor);
 			break;
-		case 'H':				/* Flush(F) or Start Copy Out(B) */
+		case PqMsg_Flush:
+			/* Flush(F) and CopyOutResponse(B) use the same identifier */
+			Assert(PqMsg_CopyOutResponse == PqMsg_Flush);
 			if (!toServer)
 				pqTraceOutputH(conn->Pfdebug, message, &logCursor);
 			else
 				fprintf(conn->Pfdebug, "Flush");	/* no message content */
 			break;
-		case 'I':
+		case PqMsg_EmptyQueryResponse:
 			fprintf(conn->Pfdebug, "EmptyQueryResponse");
 			/* No message content */
 			break;
-		case 'K':				/* secret key data from the backend */
+		case PqMsg_BackendKeyData:
 			pqTraceOutputK(conn->Pfdebug, message, &logCursor, regress);
 			break;
-		case 'n':
+		case PqMsg_NoData:
 			fprintf(conn->Pfdebug, "NoData");
 			/* No message content */
 			break;
-		case 'N':
+		case PqMsg_NoticeResponse:
 			pqTraceOutputNR(conn->Pfdebug, "NoticeResponse", message,
 							&logCursor, regress);
 			break;
-		case 'P':				/* Parse */
+		case PqMsg_Parse:
 			pqTraceOutputP(conn->Pfdebug, message, &logCursor, regress);
 			break;
-		case 'Q':				/* Query */
+		case PqMsg_Query:
 			pqTraceOutputQ(conn->Pfdebug, message, &logCursor);
 			break;
-		case 'R':				/* Authentication */
+		case PqMsg_AuthenticationRequest:
 			pqTraceOutputR(conn->Pfdebug, message, &logCursor);
 			break;
-		case 's':
+		case PqMsg_PortalSuspended:
 			fprintf(conn->Pfdebug, "PortalSuspended");
 			/* No message content */
 			break;
-		case 'S':				/* Parameter Status(B) or Sync(F) */
+		case PqMsg_Sync:
+			/* Parameter Status(B) and Sync(F) use the same identifier */
+			Assert(PqMsg_ParameterStatus == PqMsg_Sync);
 			if (!toServer)
 				pqTraceOutputS(conn->Pfdebug, message, &logCursor);
 			else
 				fprintf(conn->Pfdebug, "Sync"); /* no message content */
 			break;
-		case 't':				/* Parameter Description */
+		case PqMsg_ParameterDescription:
 			pqTraceOutputt(conn->Pfdebug, message, &logCursor, regress);
 			break;
-		case 'T':				/* Row Description */
+		case PqMsg_RowDescription:
 			pqTraceOutputT(conn->Pfdebug, message, &logCursor, regress);
 			break;
-		case 'v':				/* Negotiate Protocol Version */
+		case PqMsg_NegotiateProtocolVersion:
 			pqTraceOutputv(conn->Pfdebug, message, &logCursor);
 			break;
-		case 'V':				/* Function Call response */
+		case PqMsg_FunctionCallResponse:
 			pqTraceOutputV(conn->Pfdebug, message, &logCursor);
 			break;
-		case 'W':				/* Start Copy Both */
+		case PqMsg_CopyBothResponse:
 			pqTraceOutputW(conn->Pfdebug, message, &logCursor, length);
 			break;
-		case 'X':
+		case PqMsg_Terminate:
 			fprintf(conn->Pfdebug, "Terminate");
 			/* No message content */
 			break;
-		case 'Z':				/* Ready For Query */
+		case PqMsg_ReadyForQuery:
 			pqTraceOutputZ(conn->Pfdebug, message, &logCursor);
 			break;
 		default:
diff --git a/src/tools/msvc/Install.pm b/src/tools/msvc/Install.pm
index 05548d7c0a..b6dd2c3bba 100644
--- a/src/tools/msvc/Install.pm
+++ b/src/tools/msvc/Install.pm
@@ -614,6 +614,8 @@ sub CopyIncludeFiles
 		'src/include/', 'c.h', 'port.h', 'postgres_fe.h');
 	lcopy('src/include/libpq/pqcomm.h', $target . '/include/internal/libpq/')
 	  || croak 'Could not copy pqcomm.h';
+	lcopy('src/include/libpq/protocol.h', $target . '/include/internal/libpq/')
+	  || croak 'Could not copy protocol.h';
 
 	CopyFiles(
 		'Server headers',
-- 
2.25.1