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)
      Point your compiler to the directory where the PostgreSQL header
      files were installed, by supplying the
      -I option
      to your compiler.  (In some cases the compiler will look into
      the directory in question by default, so you can omit this
      option.)  For instance, your compile command line could look
      like:
directory
cc -c -I/usr/local/pgsql/include testprog.c
      If you are using makefiles then add the option to the
      CPPFLAGS variable:
CPPFLAGS += -I/usr/local/pgsql/include
      If there is any chance that your program might be compiled by
      other users then you should not hardcode the directory location
      like that.  Instead, you can run the utility
      pg_config to find out where the header
      files are on the local system:
$pg_config --includedir/usr/local/include
      If you
      have pkg-config installed, you can run instead:
$pkg-config --cflags libpq-I/usr/local/include
      Note that this will already include the -I in front of
      the path.
     
Failure to specify the correct option to the compiler will result in an error message such as:
testlibpq.c:8:22: libpq-fe.h: No such file or directory
      When linking the final program, specify the option
      -lpq so that the libpq
      library gets pulled in, as well as the option
      -L 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 directory-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.