PostgreSQL C function: libpq doesn't link? - postgresql

Question

I want to write a C function for PostgreSQL. For this function I will need to query some data using libpq, so I started by writing a dummy function to test this part:
#define _XOPEN_SOURCE
#include <libpq-fe.h>
#include <postgres.h>
#include <fmgr.h>
#include <funcapi.h>
#include <executor/executor.h>
#include "test.h"
PG_FUNCTION_INFO_V1(getAnnotation);
Datum getAnnotation(PG_FUNCTION_ARGS) {
// Connection to the database
PGconn *conn = PQsetdbLogin("localhost",
"5432",
"",
"",
"postgres",
"postgres",
"password");
// Databases names
PGresult *res = PQexec (conn, "SELECT user FROM activity LIMIT 1;");
VarChar* i = PQgetvalue(res, 0, 0);
PG_RETURN_VARCHAR_P(i);
}
It is just supposed to return the first column of the first line of one of my tables. Pretty simple right ? Well it doesn't work.
When I try to use it within psql, it says :
ERROR: could not load library "/usr/local/lib/postgresql/test.so": Error relocating /usr/local/lib/postgresql/test.so: PQexec: symbol not found
The weird part is that both PQsetdbLogin and PQexec are in the libpq-fe.h file, but only the second one causes an error. If I comment the PQexec line, then PQsetdbLogin raises an error as well.
Here is the Makefile I use to build the code:
PG_CPPFLAGS = -I$(libpq_srcdir)
LDFLAGS_INTERNAL = -L$(libdir)
SHLIB_LINK_INTERNAL = $(libpq)
SHLIB_PREREQS = submake-libpq
EXTENSION = test
DATA = test--0.1.sql
MODULES = test
# REGRESS = ... # Script for tests
PG_CONFIG = pg_config
PGXS := $(shell $(PG_CONFIG) --pgxs)
include $(PGXS)
As you can see, I linked libpq therem so everything should work... But it doesn't and I don't know why.
I am using PostgreSQL 9.6 in a docker container.

Answer 1

If you want to link with external libraries, you need SHLIB_LINK, but that only works if you use MODULE_big instead of MODULES.
A working Makefile would be
PG_CPPFLAGS = -I$(libpq_srcdir)
SHLIB_LINK = $(libpq)
EXTENSION = test
DATA = test--0.1.sql
MODULE_big = test
OBJS = test.o
PG_CONFIG = pg_config
PGXS := $(shell $(PG_CONFIG) --pgxs)
include $(PGXS)
But there are other problems with your function:
VarChar is a varlena, but PQgetvalue returns a char *.
You'd have to convert the value to a varlena with code similar to this:
VarChar *result;
char *c = PQgetvalue(res, 0, 0);
result = (VarChar *) palloc(strlen(c) + VARHDRSZ);
strncpy(VARDATA(result), c, strlen(c));
SET_VARSIZE(result, strlen(c) + VARHDRSZ);
It is usually the wrong idea to write client code in a server function.
If all you need is to run a query inside the current session, use the Server Programming Interface.
I guess your function is only sample code, but you need to close the connection before the backend terminates, else you have a connection leak.