As seen in Section 1, “File Format of *.pc Files”, pkg-config
allows the definition of arbitrary variables, in addition to the fixed-meaning keywords.
The use of arbitrary variables is often used to provide information decided at build-time but not necessarily consumed by the compiler and linked, for example the path in which to install plug-in objects for some software.
Defining a custom variable is the same as defining pre-defined and temporary variables, such as
libdir. To query these variables within autoconf
you should use the PKG_CHECK_VAR macro, provided by
pkg-config.
The syntax of this macro follows part of the conventions of Section 3, “The PKG_CHECK_MODULES Macro”, and is effectively a wrapper around AC_ARG_VAR.
PKG_CHECK_VAR(var-name, module, pkgconfig-variable, action-if-found, action-if-not-found)
The var-name parameter defines the name of the variable, to be used in
configure.ac, Makefile.am and other similar
files. Since this is declared through AC_ARG_VAR, defining this variable
in the environment or command-line will skip the execution of
pkg-config.
The module and pkgconfig-variable parameters are
used to identify the file and the variable to fetch through
pkg-config. You can request explicitly a minimum or maximum version
number for the module, as described in Section 3.3, “Modules specification”, but unlike
PKG_CHECK_MODULES you can only provide a single module to query.
While the action-if-found and
action-if-not-found follow the usual semantics of
autoconf macros, the latter is a catch-all for failures, and is
invoked in either of the following cases: module not found, module version constraints not
fulfilled, missing variable in module file.
Additionally, unlike PKG_CHECK_MODULES, this macro does not, by default,
error out if the variable cannot be defined. Together with the above-noted caveat of being a
no-op if the variable is already defined, it is possible to call the macro multiple times with
the same var-name to find the variable in one of multiple possible
modules.
Example 4.3. Reading target path for plugins via PKG_CHECK_VAR
The following example assumes that there is an application called Foo that provides a plug-in interface. Details of building flags can be seen in Section 2, “Building plugins”.
dnl In configure.ac PKG_CHECK_VAR([FOO_PLUGINDIR], [foo], [plugindir]) AC_MSG_CHECKING([foo plugins path]) AS_IF([test "x$FOO_PLUGINDIR" = "x"], [ AC_MSG_FAILURE([Unable to identify foo plugin path.]) ]) # In Makefile.am fooplugindir = $(FOO_PLUGINDIR) fooplugin_LTLIBRARIES = myplugin.la