## 31.20. Building libpq Programs

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 Postgres Pro header files were installed, by supplying the `-I`directory`` 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:

      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`directory`` 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.