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If you want to compile a source file including the ‘gpgme.h’ header file, you must make sure that the compiler can find it in the directory hierarchy. This is accomplished by adding the path to the directory in which the header file is located to the compilers include file search path (via the -I option).
However, the path to the include file is determined at the time the
source is configured. To solve this problem, gpgme ships with
gpgme.pc file, that knows about the path to the include file
and other configuration options. The command, pkg-config,
can be used to handle information with gpgme.pc file. In an
environment which doesn’t have pkg-config (like the one in
early stage of OS bootstrap), for Automake, you can use
gpgme.m4 which invokes gpgrt-config with
gpgme.pc. (In the past, gpgme used to ship with a small helper
program gpgme-config. This functionality of
gpgme-config is replaced by pkg-config with
gpgme.pc file.)
The options that need to be added to the compiler invocation at
compile time are output by the --cflags option to
pkg-config gpgme. The following example shows how it can be
used at the command line:
gcc -c foo.c `pkg-config --cflags gpgme`
Adding the output of ‘pkg-config --cflags gpgme’ to the compiler command line will ensure that the compiler can find the GPGME header file.
A similar problem occurs when linking the program with the library.
Again, the compiler has to find the library files. For this to work,
the path to the library files has to be added to the library search
path (via the -L option). For this, the option
--libs to pkg-config gpgme can be used. For
convenience, this option also outputs all other options that are
required to link the program with GPGME (in particular, the
‘-lgpgme’ option). The example shows how to link foo.o
with the GPGME library to a program foo.
gcc -o foo foo.o `pkg-config --libs gpgme`
Of course you can also combine both examples to a single command by
specifying both options to pkg-config gpgme:
gcc -o foo foo.c `pkg-config --cflags --libs gpgme`
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