A checklist for writing pkg-config files

Posted on 09/12/2014 by Philip Withnall

tl;dr: Use AX_PKG_CHECK_MODULES to split public/private dependencies; use AC_CONFIG_FILES to magically include the API version in the .pc file name.

A few tips for creating a pkg-config file which you will never need to think about maintaining again — because one of the most common problems with pkg-config files is that their dependency lists are years out of date compared to the dependencies checked for inconfigure.ac. See lower down for some example automake snippets.

• Include the project’s major API version1 in the pkg-config file name. e.g. libfoo-1.pcrather than libfoo.pc. This will allow parallel installation of two API-incompatible versions of the library if it becomes necessary in future.
• Split private and public dependencies between Requires and Requires.private. Thiseliminates over-linking when dynamically linking against the project, since in that case the private dependencies are not needed. This is easily done using theAX_PKG_CHECK_MODULES macro (and perhaps using an upstream macro in future — seepkg-config bug #87154). A dependency is public when its symbols are exposed in public headers installed by your project; it is private otherwise.
• Include useful ancillary variables, such as the paths to any utilities, directories or daemons which ship with the project. For example, glib-2.0.pc has variables giving the paths for its utilities: glib-genmarshal, gobject-query and glib-mkenums. libosinfo-1.0.pchas variables for its database directories.
• Substitute in the Name and Version using @PACKAGE_NAME@ and @PACKAGE_VERSION@ so they don’t fall out of sync.
• Place the .pc.in template in the source code subdirectory for the library it’s for — so if your project produces multiple libraries (or might do in future), the .pc.in files don’t get mixed up at the top level.

Given all those suggestions, here’s a template libmy-project/my-project.pc.in file:

prefix=@prefix@
exec_prefix=@exec_prefix@
libdir=@libdir@
includedir=@includedir@
 
my_project_utility=my-project-utility-binary-name
my_project_db_dir=@sysconfdir@/my-project/db
 
Name: @PACKAGE_NAME@
Description: Some brief but informative description
Version: @PACKAGE_VERSION@
Libs: -L${libdir} -lmy-project-@API_VERSION@
Cflags: -I${includedir}/my-project-@API_VERSION@
Requires: @AX_PACKAGE_REQUIRES@
Requires.private: @AX_PACKAGE_REQUIRES_PRIVATE@

And here’s a a few snippets from a template configure.ac:

# Release version
m4_define([package_version_major],[1])
m4_define([package_version_minor],[2])
m4_define([package_version_micro],[3])
 
# API version
m4_define([api_version],[1])
 
AC_INIT([my-project],
        [package_version_major.package_version_minor.package_version_micro],
        …)
 
# Dependencies
PKG_PROG_PKG_CONFIG
 
glib_reqs=2.40
gio_reqs=2.42
gthread_reqs=2.40
nice_reqs=0.1.6
 
# The first list on each line is public; the second is private.
# The AX_PKG_CHECK_MODULES macro substitutes AX_PACKAGE_REQUIRES and
# AX_PACKAGE_REQUIRES_PRIVATE.
AX_PKG_CHECK_MODULES([GLIB],
                     [glib-2.0 >= $glib_reqs gio-2.0 >= $gio_reqs],
                     [gthread-2.0 >= $gthread_reqs])
AX_PKG_CHECK_MODULES([NICE],
                     [nice >= $nice_reqs],
                     [])
 
AC_SUBST([PACKAGE_VERSION_MAJOR],package_version_major)
AC_SUBST([PACKAGE_VERSION_MINOR],package_version_minor)
AC_SUBST([PACKAGE_VERSION_MICRO],package_version_micro)
AC_SUBST([API_VERSION],api_version)
 
# Output files
# Rename the template .pc file to include the API version on configure
AC_CONFIG_FILES([
libmy-project/my-project-$API_VERSION.pc:libmy-project/my-project.pc.in
…
],[],
[API_VERSION='$API_VERSION'])
AC_OUTPUT

And finally, the top-level Makefile.am:

# Install the pkg-config file.
pkgconfigdir = $(libdir)/pkgconfig
pkgconfig_DATA = libmy-project/my-project-$(API_VERSION).pc
 
DISTCLEANFILES += $(pkgconfig_DATA)
EXTRA_DIST += libmy-project/my-project.pc.in

Once that’s all built, you’ll end up with an installed my-project-1.pc file containing the following (assuming a prefix of /usr):

prefix=/usr
exec_prefix=/usr
libdir=/usr/lib
includedir=/usr/include
 
my_project_utility=my-project-utility-binary-name
my_project_db_dir=/etc/my-project/db
 
Name: my-project
Description: Some brief but informative description
Version: 1.2.3
Libs: -L${libdir} -lmy-project-1
Cflags: -I${includedir}/my-project-1
Requires: glib-2.0 >= 2.40 gio-2.0 >= 2.42 nice >= 0.1.6
Requires.private: gthread-2.0 >= 2.40

All code samples in this post are released into the public domain.

1. Assuming this is the number which will change if backwards-incompatible API/ABI changes are made.

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