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  <h1><a name="top">Guide to pkg-config</a></h1>

<h3>Dan Nicholson</h3>

<ul>
    <li><a href="#overview">Overview</a></li>
    <li><a href="#why">Why?</a></li>
    <li><a href="#concepts">Concepts</a></li>
    <li><a href="#writing">Writing pkg-config files</a></li>
    <li><a href="#using">Using pkg-config files</a></li>
    
    <li><a href="#faq">Frequently asked questions</a></li>
  </ul>

<h2><a name="overview">Overview</a></h2>

<p>This document aims to give an overview to using the <tt>pkg-config</tt>
  tool from the perspective of both a user and a developer. It reviews the
  concepts behind <tt>pkg-config</tt>, how to write <tt>pkg-config</tt> files
  to support your project, and how to use <tt>pkg-config</tt> to integrate
  with 3rd party projects.</p>

<p>More information on <tt>pkg-config</tt> can be found at the
  <a href="http://pkg-config.freedesktop.org/">website</a> and in the
  <tt>pkg-config(1)</tt> manual page.</p>

<p>This document assumes usage of <tt>pkg-config</tt> on a Unix-like
  operating system such as Linux. Some of the details may be different on
  other platforms.</p>

<p>Modern computer systems use many layered components to provide
  applications to the user. One of the difficulties in assembling these parts
  is properly integrating them. <tt>pkg-config</tt> collects metadata about
  the installed libraries on the system and easily provides it to the user.
  </p>

<p>Without a metadata system such as <tt>pkg-config</tt>, it can be very
  difficult to locate and obtain details about the services provided on a
  given computer. For a developer, installing <tt>pkg-config</tt> files with
  your package greatly eases adoption of your API.</p>

<h2><a name="concepts">Concepts</a></h2>

<p>The primary use of <tt>pkg-config</tt> is to provide the necessary
  details for compiling and linking a program to a library. This metadata is
  stored in <tt>pkg-config</tt> files. These files have the suffix
  <tt>.pc</tt> and reside in specific locations known to the
  <tt>pkg-config</tt> tool. This will be described in more detail later.</p>

<p>The file format contains predefined metadata keywords and freeform
  variables. An example may be illustrative:<p>

<pre>prefix=/usr/local
exec_prefix=${prefix}
includedir=${prefix}/include
libdir=${exec_prefix}/lib

Name: foo
Description: The foo library
Version: 1.0.0
Cflags: -I${includedir}/foo
Libs: -L${libdir} -lfoo</pre>

<p>The keyword definitions such as <tt>Name:</tt> begin with a keyword
  followed by a colon and the value. The variables such as <tt>prefix=</tt>
  are a string and value separated by an equals sign. The keywords are defined
  and exported by <tt>pkg-config</tt>. The variables are not necessary, but
  can be used by the keyword definitions for flexibility or to store data not
  covered by <tt>pkg-config</tt>.</p>

<p>Here is a short description of the keyword fields. A more in depth
  description of these fields and how to use them effectively will be given in
  the <a href="#writing">Writing pkg-config files</a> section.</p>

<ul>
    <li><b>Name</b>: A human-readable name for the library or package. This
    does not affect usage of the <tt>pkg-config</tt> tool, which uses the name
    of the <tt>.pc</tt> file.</li>

<li><b>Description</b>: A brief description of the package.</li>

<li><b>URL</b>: An URL where people can get more information about and
    download the package.</li>

<li><b>Version</b>: A string specifically defining the version of the
    package.</li>

<li><b>Requires</b>: A list of packages required by this package. The
    versions of these packages may be specified using the comparison operators
    =, &lt;, &gt;, &lt;= or &gt;=.</li>

<li><b>Requires.private</b>: A list of private packages required by this
    package but not exposed to applications. The version specific rules from
    the <tt>Requires</tt> field also apply here.</li>

<li><b>Conflicts</b>: An optional field describing packages that this one
    conflicts with. The version specific rules from the <tt>Requires</tt>
    field also apply here. This field also takes multiple instances of the
    same package. E.g., <tt>Conflicts: bar < 1.2.3, bar >= 1.3.0</tt>.</li>

<li><b>Cflags</b>: The compiler flags specific to this package and any
    required libraries that don't support <tt>pkg-config</tt>. If the required
    libraries support <tt>pkg-config</tt>, they should be added to
    <tt>Requires</tt> or <tt>Requires.private</tt>.</li>

<li><b>Libs</b>: The link flags specific to this package and any required
    libraries that don't support <tt>pkg-config</tt>. The same rule as
    <tt>Cflags</tt> applies here.</li>

<li><b>Libs.private</b>: The link flags for private libraries required by
    this package but not exposed to applications. The same rule as
    <tt>Cflags</tt> applies here.</li>
  </ul>

<h2><a name="writing">Writing pkg-config files</a></h2>

<p>When creating <tt>pkg-config</tt> files for a package, it is first
  necessary to decide how they will be distributed. Each file is best used to
  describe a single library, so each package should have at least as many
  <tt>pkg-config</tt> files as they do installed libraries.</p>

<p>The package name is determined through the filename of the
  <tt>pkg-config</tt> metadata file. This is the portion of the filename prior
  to the <tt>.pc</tt> suffix. A common choice is to match the library name to
  the <tt>.pc</tt> name. For instance, a package installing <tt>libfoo.so</tt>
  would have a corresponding <tt>libfoo.pc</tt> file containing the
  <tt>pkg-config</tt> metadata. This choice is not necessary; the <tt>.pc</tt>
  file should simply be a unique identifier for your library. Following the
  above example, <tt>foo.pc</tt> or <tt>foolib.pc</tt> would probably work
  just as well.</p>

<p>The <tt>Name</tt>, <tt>Description</tt> and <tt>URL</tt> fields are
  purely informational and should be easy to fill in. The <tt>Version</tt>
  field is a bit trickier to ensure that it is usable by consumers of the
  data. <tt>pkg-config</tt> uses the algorithm from
  <a href="http://rpm.org/">RPM</a> for version comparisons. This works best
  with a dotted decimal number such as <tt>1.2.3</tt> since letters can cause
  unexpected results. The number should be monotonically increasing and be
  as specific as possible in describing the library. Usually it's sufficient
  to use the package's version number here since it's easy for consumers to
  track.</p>

<p>Before describing the more useful fields, it will be helpful to
  demonstrate variable definitions. The most common usage is to define the
  installation paths so that they don't clutter the metadata fields. Since
  the variables are expanded recursively, this is very helpful when used in
  conjunction with autoconf derived paths.</p>

<pre>prefix=/usr/local
includedir=${prefix}/include

Cflags: -I${includedir}/foo</pre>

<p>The most important <tt>pkg-config</tt> metadata fields are
  <tt>Requires</tt>, <tt>Requires.private</tt>, <tt>Cflags</tt>, <tt>Libs</tt>
  and <tt>Libs.private</tt>. They will define the metadata used by external
  projects to compile and link with the library.</p>

<p><tt>Requires</tt> and <tt>Requires.private</tt> define other modules
  needed by the library. It is usually preferred to use the private variant of
  <tt>Requires</tt> to avoid exposing unnecessary libraries to the program
  that is linking with your library. If the program will not be using the
  symbols of the required library, it should not be linking directly to that
  library. See the discussion of
  <a href="http://wiki.mandriva.com/en/Overlinking">overlinking</a> for a more
  thorough explanation.</p>

<p>Since <tt>pkg-config</tt> always exposes the link flags of the
  <tt>Requires</tt> libraries, these modules will become direct dependencies
  of the program. On the other hand, libraries from <tt>Requires.private</tt>
  will only be included when static linking. For this reason, it is usually
  only appropriate to add modules from the same package in <tt>Requires</tt>.
  </p>

<p>The <tt>Libs</tt> field contains the link flags necessary to use that
  library. In addition, <tt>Libs</tt> and <tt>Libs.private</tt> contain link
  flags for other libraries not supported by <tt>pkg-config</tt>. Similar to
  the <tt>Requires</tt> field, it is preferred to add link flags for external
  libraries to the <tt>Libs.private</tt> field so programs do not acquire an
  additional direct dependency.</p>

<p>Finally, the <tt>Cflags</tt> contains the compiler flags for using the
  library. Unlike the <tt>Libs</tt> field, there is not a private variant of
  <tt>Cflags</tt>. This is because the data types and macro definitions are
  needed regardless of the linking scenario.</p>

<h2><a name="using">Using pkg-config files</a></h2>

<p>Assuming that there are <tt>.pc</tt> files installed on the system, the
  <tt>pkg-config</tt> tool is used to extract the metadata for usage. A short
  description of the options can be seen by executing
  <tt>pkg-config --help</tt>. A more in depth discussion can be found in the
  <tt>pkg-config(1)</tt> manual page. This section will provide a brief
  explanation of common usages.</tt>

<p>Consider a system with two modules, <tt>foo</tt> and <tt>bar</tt>.
  Their <tt>.pc</tt> files might look like this:</p>

<pre>foo.pc:
prefix=/usr
exec_prefix=${prefix}
includedir=${prefix}/include
libdir=${exec_prefix}/lib

Name: foo
Description: The foo library
Version: 1.0.0
Cflags: -I${includedir}/foo
Libs: -L${libdir} -lfoo

bar.pc:
prefix=/usr
exec_prefix=${prefix}
includedir=${prefix}/include
libdir=${exec_prefix}/lib

Name: bar
Description: The bar library
Version: 2.1.2
Requires.private: foo >= 0.7
Cflags: -I${includedir}
Libs: -L${libdir} -lbar</pre>

<p>The version of the modules can be obtained with the <tt>--modversion</tt>
  option.</p>

<pre>$ pkg-config --modversion foo
1.0.0
$ pkg-config --modversion bar
2.1.2</pre>

<p>To print the link flags needed for each module, use the <tt>--libs</tt>
  option.</p>

<pre>$ pkg-config --libs foo
-lfoo
$ pkg-config --libs bar
-lbar</pre>

<p>Notice that <tt>pkg-config</tt> has suppressed part of the <tt>Libs</tt>
  field for both modules. This is because it treats the <tt>-L</tt> flag
  specially and knows that the <tt>${libdir}</tt> directory <tt>/usr/lib</tt>
  is part of the system linker search path. This keeps <tt>pkg-config</tt>
  from interfering with the linker operation.</p>

<p>Also, although <tt>foo</tt> is required by <tt>bar</tt>, the link flags
  for <tt>foo</tt> are not output. This is because <tt>foo</tt> is not
  directly needed by an application that only wants to use the <tt>bar</tt>
  library. For statically linking a <tt>bar</tt> application, we need both
  sets of linker flags:</p>

<pre>$ pkg-config --libs --static bar
-lbar -lfoo</pre>

<p><tt>pkg-config</tt> needs to output both sets of link flags in this case
  to ensure that the statically linked application will find all the necessary
  symbols. On the other hand, it will always output all the <tt>Cflags</tt>.
  </p>

<pre>$ pkg-config --cflags bar
-I/usr/include/foo
$ pkg-config --cflags --static bar
-I/usr/include/foo</pre>

<p>Another useful option, <tt>--exists</tt>, can be used to test for a
  module's availability.</p>

<pre>$ pkg-config --exists foo
$ echo $?
0</pre>

<p>One of the nicest features of <tt>pkg-config</tt> is providing version
  checking. It can be used to determine if a sufficient version is available.
  </p>

<pre>$ pkg-config --libs "bar >= 2.7"
Requested 'bar >= 2.7' but version of bar is 2.1.2</pre>

<p>Some commands will provide more verbose output when combined with the
  <tt>--print-errors</tt> option.</p>

<pre>$ pkg-config --exists --print-errors xoxo
Package xoxo was not found in the pkg-config search path.
Perhaps you should add the directory containing `xoxo.pc'
to the PKG_CONFIG_PATH environment variable
No package 'xoxo' found</pre>

<p>The message above references the <tt>PKG_CONFIG_PATH</tt> environment
  variable. This variable is used to augment <tt>pkg-config</tt>'s search
  path. On a typical Unix system, it will search in the directories
  <tt>/usr/lib/pkgconfig</tt> and <tt>/usr/share/pkgconfig</tt>. This will
  usually cover system installed modules. However, some local modules may be
  installed in a different prefix such as <tt>/usr/local</tt>. In that case,
  it's necessary to prepend the search path so that <tt>pkg-config</tt> can
  locate the <tt>.pc</tt> files.</p>

<pre>$ pkg-config --modversion hello
Package hello was not found in the pkg-config search path.
Perhaps you should add the directory containing `hello.pc'
to the PKG_CONFIG_PATH environment variable
No package 'hello' found
$ export PKG_CONFIG_PATH=/usr/local/lib/pkgconfig
$ pkg-config --modversion hello
1.0.0</pre>

<p>A few <a href="http://www.gnu.org/software/autoconf/">autoconf</a> macros
  are also provided to ease integration of <tt>pkg-config</tt> modules into
  projects.</p>

<ul>
    <li><b>PKG_PROG_PKG_CONFIG([MIN-VERSION])</b>: Locates the
    <tt>pkg-config</tt> tool on the system and checks the version for
    compatibility.</li>

<li><b>PKG_CHECK_EXISTS(MODULES, [ACTION-IF-FOUND], [ACTION-IF-NOT-FOUND])</b>:
    Checks to see whether a particular set of modules exists.</li>

<li><b>PKG_CHECK_MODULES(VARIABLE-PREFIX, MODULES, [ACTION-IF-FOUND], [ACTION-IF-NOT-FOUND])</b>:
    Checks to see whether a particular set of modules exists. If so, it sets
    <tt>&lt;VARIABLE-PREFIX&gt;_CFLAGS</tt> and
    <tt>&lt;VARIABLE-PREFIX&gt;_LIBS</tt> according to the output from
    <tt>pkg-config --cflags</tt> and <tt>pkg-config --libs</tt>.</li>
  </ul>

<h2><a name="faq">Frequently asked questions</a></h2>

<ol>
    <li>My program uses library <tt>x</tt>. What do I do?</li>

<p>The <tt>pkg-config</tt> output can easily be used on the compiler
    command line. Assuming the <tt>x</tt> library has a <tt>x.pc</tt>
    <tt>pkg-config</tt> file:</p>

<pre>cc `pkg-config --cflags --libs x` -o myapp myapp.c</pre>

<p>The integration can be more robust when used with
    <a href="http://www.gnu.org/software/autoconf/">autoconf</a> and
    <a href="http://www.gnu.org/software/automake/">automake</a>. By using the
    supplied <tt>PKG_CHECK_MODULES</tt> macro, the metadata is easily accessed
    in the build process.</p>

<pre>configure.ac:
PKG_CHECK_MODULES([X], [x])

Makefile.am:
myapp_CFLAGS = $(X_CFLAGS)
myapp_LDADD = $(X_LIBS)</pre>

<p>If the <tt>x</tt> module is found, the macro will fill and substitute
    the <tt>X_CFLAGS</tt> and <tt>X_LIBS</tt> variables. If the module is not
    found, an error will be produced. Optional 3rd and 4th arguments can be
    supplied to <tt>PKG_CHECK_MODULES</tt> to control actions when the module
    is found or not.</p>

<li>My library <tt>z</tt> installs header files which include <tt>libx</tt>
    headers. What do I put in my <tt>z.pc</tt> file?</li>

<p>If the <tt>x</tt> library has <tt>pkg-config</tt> support, add it to
    the <tt>Requires.private</tt> field. If it does not, augment the
    <tt>Cflags</tt> field with the necessary compiler flags for using the
    <tt>libx</tt> headers. In either case, <tt>pkg-config</tt> will output
    the compiler flags when <tt>--static</tt> is used or not.</p>

<li>My library <tt>z</tt> uses <tt>libx</tt> internally, but does not
    expose <tt>libx</tt> data types in its public API. What do I put in my
    <tt>z.pc</tt> file?</li>

<p>Again, add the module to <tt>Requires.private</tt> if it supports
    <tt>pkg-config</tt>. In this case, the compiler flags will be emitted
    unnecessarily, but it ensures that the linker flags will be present when
    linking statically. If <tt>libx</tt> does not support <tt>pkg-config</tt>,
    add the necessary linker flags to <tt>Libs.private</tt>.</p>
  </ol>

<hr/>

<address>Dan Nicholson &lt;dbn.lists (at) gmail (dot) com&gt;</address>

<p>Copyright (C) 2010  Dan Nicholson.<br/>
  This document is licensed under the
  <a href="http://www.gnu.org/licenses/old-licenses/gpl-2.0.txt">GNU General Public License, Version 2</a>
  or any later version.</p>

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