PostgreSQL C Language Extension with C++ Code

TalGloz <glozmantal@gmail.com> writes:

I've searched information about my problem in the archives and on the
internet, but it didn't help. I have this small myfunc.cpp
[ that doesn't work ]

16: #ifdef PG_MODULE_MAGIC
17: PG_MODULE_MAGIC;
18: #endif

Hmm ... don't use an #ifdef there. If you don't have the macro defined,
you want to fail, not silently build an extension without it.

It's possible you need extern "C" { ... } around the macro, too.

regards, tom lane

Hi,

On Sun, Aug 12, 2018 at 12:05 PM, TalGloz <glozmantal@gmail.com> wrote:

Hi,

I've searched information about my problem in the archives and on the
internet, but it didn't help. I have this small myfunc.cpp

1: #include <postgres.h>
2: #include <utils/rel.h>
3: #include <postgres.h>
4: #include <fmgr.h>
5: #include <utils/array.h>
6: #include <utils/builtins.h>
7: #include <catalog/pg_type.h>
8: #include <string>
9: #include <vector>
10: #include <stdlib.h>
11: #include <stdint.h>
12: #include <iostream>
13: #include <fstream>
14: #include <seal/seal.h> // external compiled c++ library linked on
running 'make'
15:
16: #ifdef PG_MODULE_MAGIC

I think you're missing "n" here.
Should be:

#ifndef PG_MODULE_MAGIC

.

Thank you.

OK now I have this code:

1: extern "C" { // The C header should go here
2: #include <postgres.h>
3: #include <utils/rel.h>
4: #include <fmgr.h>
5: #include <utils/array.h>
6: #include <utils/builtins.h>
7: #include <catalog/pg_type.h>
8: #include <stdlib.h>
9: #include <stdint.h>
10:
11: PG_MODULE_MAGIC;
12: }
13:
14: // CPP header without extern "C"
15: #include <string>
16: #include <vector>
17: #include <iostream>
18: #include <fstream>
19: #include <seal/seal.h> // external compiled c++ library linked on
running 'make'
20:
21: Datum sum_of_numbers(PG_FUNCTION_ARGS);
22: PG_FUNCTION_INFO_V1(sum_of_numbers);
33:
34: extern "C" { // CPP function
35: int64_t sum_of_numbers(){
36: std::vector<int64_t> numbers {23, 445, 64};
37: auto sum = 0;
38: for (auto &item : numbers){
39: sum += item;
40: }
41: return sum;
42: }
43: }

The error this time for PostgreSQL is:
*ERROR: could not find function information for function "sum_of_numbers"
HINT: SQL-callable functions need an accompanying
PG_FUNCTION_INFO_V1(funcname).
SQL state: 42883*

Thanks,
Tal

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TalGloz <glozmantal@gmail.com> writes:

The error this time for PostgreSQL is:
*ERROR: could not find function information for function "sum_of_numbers"
HINT: SQL-callable functions need an accompanying
PG_FUNCTION_INFO_V1(funcname).
SQL state: 42883*

Probably need extern "C" around the PG_FUNCTION_INFO_V1 macro, too.
Both those macros produce C function definitions under the hood,
and said functions need to not be name-mangled by C++.

regards, tom lane

I did it with the macros

extern "C" {
Datum sum_of_numbers(PG_FUNCTION_ARGS);
PG_FUNCTION_INFO_V1(sum_of_numbers);
}

But now my compiler throws some other errors when running make:

g++ --std=c++17 -fPIC -Wall -Werror -g3 -O0 -o myfunc.o -c myfunc.cpp
-I/usr/pgsql-10/include/server -L"/usr/local/lib/libseal.a"
-I"/usr/local/include/seal" -I"/usr/local/include/cppcodec"
myfunc.cpp:29:10: error: conflicting declaration of C function ‘int64_t
sum_of_numbers()’
int64_t sum_of_numbers(){
^~~~~~~~~~~~~~
In file included from /usr/pgsql-10/include/server/utils/array.h:64,
from /usr/pgsql-10/include/server/utils/acl.h:38,
from
/usr/pgsql-10/include/server/catalog/objectaddress.h:18,
from
/usr/pgsql-10/include/server/catalog/pg_publication.h:21,
from /usr/pgsql-10/include/server/utils/rel.h:21,
from myfunc.cpp:3:
myfunc.cpp:25:21: note: previous declaration ‘Datum
sum_of_numbers(FunctionCallInfo)’
PG_FUNCTION_INFO_V1(sum_of_numbers);
^~~~~~~~~~~~~~
/usr/pgsql-10/include/server/fmgr.h:374:14: note: in definition of macro
‘PG_FUNCTION_INFO_V1’
extern Datum funcname(PG_FUNCTION_ARGS); \
^~~~~~~~
make: *** [Makefile:16: myfunc.o] Error 1

Why is it so hard to use C++ with PostgerSQL for a C extension? :)

Regards,
Tal

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вс, 12 авг. 2018 г. в 21:40, TalGloz <glozmantal@gmail.com>:

I did it with the macros

extern "C" {
Datum sum_of_numbers(PG_FUNCTION_ARGS);
PG_FUNCTION_INFO_V1(sum_of_numbers);
}

But now my compiler throws some other errors when running make:

g++ --std=c++17 -fPIC -Wall -Werror -g3 -O0 -o myfunc.o -c myfunc.cpp
-I/usr/pgsql-10/include/server -L"/usr/local/lib/libseal.a"
-I"/usr/local/include/seal" -I"/usr/local/include/cppcodec"
myfunc.cpp:29:10: error: conflicting declaration of C function ‘int64_t
sum_of_numbers()’
int64_t sum_of_numbers(){
^~~~~~~~~~~~~~
In file included from /usr/pgsql-10/include/server/utils/array.h:64,
from /usr/pgsql-10/include/server/utils/acl.h:38,
from
/usr/pgsql-10/include/server/catalog/objectaddress.h:18,
from
/usr/pgsql-10/include/server/catalog/pg_publication.h:21,
from /usr/pgsql-10/include/server/utils/rel.h:21,
from myfunc.cpp:3:
myfunc.cpp:25:21: note: previous declaration ‘Datum
sum_of_numbers(FunctionCallInfo)’
PG_FUNCTION_INFO_V1(sum_of_numbers);
^~~~~~~~~~~~~~
/usr/pgsql-10/include/server/fmgr.h:374:14: note: in definition of macro
‘PG_FUNCTION_INFO_V1’
extern Datum funcname(PG_FUNCTION_ARGS); \
^~~~~~~~
make: *** [Makefile:16: myfunc.o] Error 1

Why is it so hard to use C++ with PostgerSQL for a C extension? :)

It is not.

TalGloz <glozmantal@gmail.com> writes:

But now my compiler throws some other errors when running make:

myfunc.cpp:29:10: error: conflicting declaration of C function ‘int64_t
sum_of_numbers()’
int64_t sum_of_numbers(){
^~~~~~~~~~~~~~

Well, yeah, you forgot to repeat the argument list here. For that
matter, spelling Datum as int64_t is a recipe for trouble, even if
it works on some particular platform. Should be

Datum sum_of_numbers(PG_FUNCTION_ARGS) { ...

Why is it so hard to use C++ with PostgerSQL for a C extension? :)

Probably because you've failed to break your bad C++ habits.

regards, tom lane

Let me see if I understood you correctly. I cant have a code like this:

extern "C" {
int sum_of_numbers(){
std::vector<int> numbers {23, 445, 64};
int sum = 0;
for (auto &item : numbers){
sum += item;
}
return sum;
}
}

And I have to declare my function with inside the
Datum sum_of_numbers(PG_FUNCTION_ARGS){
// function code here
}

Sorry for the log mails. I'm having problems to understand, how to change
the construction of myfunc.cpp to run the code.

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OK It worked. This is how I did it, hopefully it is right

extern "C" {
#include <postgres.h>
#include <utils/rel.h>
#include <fmgr.h>
#include <utils/array.h>
#include <utils/builtins.h>
#include <catalog/pg_type.h>
#include <stdlib.h>
#include <stdint.h>

PG_MODULE_MAGIC;
}

#include <string>
#include <vector>
#include <iostream>
#include <fstream>
#include <seal/seal.h> // external compiled c++ library linked on running
'make'

extern "C" {
Datum sum_of_numbers(PG_FUNCTION_ARGS){
std::vector<int> numbers {23, 445, 64};
int sum = 0;
for (auto &item : numbers){
sum += item;
}
return sum;
};
PG_FUNCTION_INFO_V1(sum_of_numbers);
}

I've managed to create and execute the function in my PostgreSQL database.

So basically I can execute any CPP code as long as I declare my functions
like this:

extern "C" {
Datum function_name(PG_FUNCTION_ARGS){
// CPP code here
};

PG_FUNCTION_INFO_V1(function_name);
}

In addition tho thath, all the C headers should be * inside a extern "C" {
}* block and all the CPP headers *outside the extern "C" { }* block, did I
get it right?

Thanks,
Tal

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