Example 31-2. libpq Example Program 2
I need notifications to a C application on inserts to a table. The
notification funcion is listed below. My program and "Example 31-2. libpq
Example Program 2" receive the notification, but the payload message in
"PGnotify *notify->extra" is invalid.
The example code is located here:
https://www.postgresql.org/docs/current/libpq-example.html
I modified the code to include the "extra" member in the following
statement:
fprintf(stderr,
"ASYNC NOTIFY of '%s' received from backend PID %d:
%s\n",
notify->relname, notify->be_pid, *notify->extra*);
Both the example and my application core on an invalid address for "extra".
Using psql with LISTEN works correctly. My application works as expected
except for the payload message. I was unable to locate any example of how
to retrieve the payload in C. The "extra" member is a char*.
================================================
CREATE OR REPLACE FUNCTION OS_FB_UPDATE_FCNFY()
RETURNS trigger
LANGUAGE 'plpgsql'
AS $BODY$
DECLARE
notification TEXT;
BEGIN
notification :=
NEW.node_id || ':' ||
NEW.block_id || ':' ||
NEW.operation || ':' ||
NEW.update_class || ':' ||
NEW.update_data || ':' ||
to_char(COALESCE(NEW.time_stamp, current_timestamp), 'MM-DD-YYY
HH24:MI:SS');
PERFORM pg_notify('notifyondatachange', row_to_json(NEW)::text);
RETURN NEW;
EXCEPTION
WHEN others THEN
RAISE WARNING '[ONSITE.OS_FB_UPDATE_FCNFY] - UDF ERROR [OTHER] -
SQLSTATE: %, SQLERRM: %', SQLSTATE, SQLERRM;
END
$BODY$;
On Wednesday, March 10, 2021, Niko Ware <nikowareinc@gmail.com> wrote:
I modified the code to include the "extra" member in the following
statement:fprintf(stderr,
"ASYNC NOTIFY of '%s' received from backend PID %d:
%s\n",
notify->relname, notify->be_pid, *notify->extra*);I was unable to locate any example of how to retrieve the payload in C.
Since psql does it correctly that is a good place to find code.
Yes, your modification should work. Looking at the history that extra
field is 18 years old. I’m not in a position to confirm whether or not this
is somehow broken on head but has gone undiscovered, but it seems
unlikely. Only advice I can think of besides waiting for a more
experienced hacker is to check the relevant header files and
breakpoint/trace the code and see what “notify” actually looks like. Or
start from scratch on a clean setup and try again confirming you have
checked out master/head from the official repository.
David J.
Niko Ware <nikowareinc@gmail.com> writes:
I modified the code to include the "extra" member in the following
statement:
fprintf(stderr,
"ASYNC NOTIFY of '%s' received from backend PID %d: %s\n",
notify->relname, notify->be_pid, *notify->extra*);
Both the example and my application core on an invalid address for "extra".
Modifying testlibpq2.c like that works for me, FWIW.
I speculate maybe you have a macro definition of "extra" that is bollixing
things somehow? Kind of a weak theory, but nothing else comes to mind.
regards, tom lane