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:
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
-Ioption 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 += -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_configto find out where the header files are on the local system:
$pkg-config --cflags libpq
Note that this will already include the
-Iin 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
-lpqso that the libpq library gets pulled in, as well as the option
-Lto 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
-Loption before the
-lpqoption. For example:
cc -o testprog testprog1.o testprog2.o -L/usr/local/pgsql/lib -lpq
You can find out the library directory using
Or again use
$pkg-config --libs libpq
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
/usr/bin/ld: cannot find -lpq
This means you forgot the
-Loption or did not specify the right directory.