Fail during the compilation building libpq program

To build (i.e., compile and link) a program using libpq you need to do all of the following things:

* Include the libpq-fe.h header file:
#include "libpq-fe.h"

If you failed to do that then you will normally get error messages from your compiler similar to:

foo.c: In function `main':
foo.c:34: `PGconn' undeclared (first use in this function)
foo.c:35: `PGresult' undeclared (first use in this function)
foo.c:54: `CONNECTION_BAD' undeclared (first use in this function)
foo.c:68: `PGRES_COMMAND_OK' undeclared (first use in this function)
foo.c:95: `PGRES_TUPLES_OK' undeclared (first use in this function)

When linking the final program, specify the option -lpq so that the libpq library gets pulled in, as well as the option -Ldirectory to point the compiler to the directory where the libpq library resides. (Again, the compiler will search some directories by default.) For maximum portability, put the -L option before the -lpq option. For example:

cc -o testprog testprog1.o testprog2.o -L/usr/local/pgsql/lib -lpq

You can find out the library directory using pg_config as well:

$ pg_config --libdir
/usr/local/pgsql/lib

Or again use pkg-config:

$ pkg-config --libs libpq
-L/usr/local/pgsql/lib -lpq

Note again that this prints the full options, not only the path.

Error messages that point to problems in this area could look like the following:

testlibpq.o: In function `main':
testlibpq.o(.text+0x60): undefined reference to `PQsetdbLogin'
testlibpq.o(.text+0x71): undefined reference to `PQstatus'
testlibpq.o(.text+0xa4): undefined reference to `PQerrorMessage'

This means you forgot -lpq.

/usr/bin/ld: cannot find -lpq

This means you forgot the -L option or did not specify the right directory.

#libpq #gcc #postgresql