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 -Idirectory 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 -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.