# Guide to pkg-config

### Dan Nicholson

- Overview
- Why?
- Concepts
- Writing pkg-config files
- Using pkg-config files
- Frequently asked questions

## Overview

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

More information on `pkg-config` can be found at the website and in the `pkg-config(1)` manual page.

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

## Why?

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. `pkg-config` collects metadata about the installed libraries on the system and easily provides it to the user.

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

## Concepts

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

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

    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

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

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 Writing pkg-config files section.

- Name: A human-readable name for the library or package. This does not affect usage of the `pkg-config` tool, which uses the name of the `.pc` file.
- Description: A brief description of the package.
- URL: An URL where people can get more information about and download the package.
- Version: A string specifically defining the version of the package.
- Requires: A list of packages required by this package. The versions of these packages may be specified using the comparison operators =, <, >, <= or >=.
- Requires.private: A list of private packages required by this package but not exposed to applications. The version specific rules from the `Requires` field also apply here.
- Conflicts: An optional field describing packages that this one conflicts with. The version specific rules from the `Requires` field also apply here. This field also takes multiple instances of the same package. E.g., `Conflicts: bar < 1.2.3, bar >= 1.3.0`.
- Cflags: The compiler flags specific to this package and any required libraries that don't support `pkg-config`. If the required libraries support `pkg-config`, they should be added to `Requires` or `Requires.private`.
- Libs: The link flags specific to this package and any required libraries that don't support `pkg-config`. The same rule as `Cflags` applies here.
- Libs.private: The link flags for private libraries required by this package but not exposed to applications. The same rule as `Cflags` applies here.

## Writing pkg-config files

When creating `pkg-config` 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 `pkg-config` files as they do installed libraries.

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

The `Name`, `Description` and `URL` fields are purely informational and should be easy to fill in. The `Version` field is a bit trickier to ensure that it is usable by consumers of the data. `pkg-config` uses the algorithm from RPM for version comparisons. This works best with a dotted decimal number such as `1.2.3` 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.

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.

    prefix=/usr/local
    includedir=${prefix}/include

    Cflags: -I${includedir}/foo

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

`Requires` and `Requires.private` define other modules needed by the library. It is usually preferred to use the private variant of `Requires` 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 overlinking for a more thorough explanation.

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

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

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

## Using pkg-config files

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

Consider a system with two modules, `foo` and `bar`. Their `.pc` files might look like this:

    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

The version of the modules can be obtained with the `--modversion` option.

    $ pkg-config --modversion foo
    1.0.0
    $ pkg-config --modversion bar
    2.1.2

To print the link flags needed for each module, use the `--libs` option.

    $ pkg-config --libs foo
    -lfoo
    $ pkg-config --libs bar
    -lbar

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

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

    $ pkg-config --libs --static bar
    -lbar -lfoo

`pkg-config` 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 `Cflags`.

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

Another useful option, `--exists`, can be used to test for a module's availability.

    $ pkg-config --exists foo
    $ echo $?
    0

One of the nicest features of `pkg-config` is providing version checking. It can be used to determine if a sufficient version is available.

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

Some commands will provide more verbose output when combined with the `--print-errors` option.

    $ 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

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

    $ 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

A few autoconf macros are also provided to ease integration of `pkg-config` modules into projects.

- PKG_PREREQ(MIN-VERSION): Ensures that the version of the `pkg-config` `autoconf` macros in use is at least `MIN-VERSION`.
- PKG_PROG_PKG_CONFIG([MIN-VERSION]): Locates the `pkg-config` tool on the system and checks the version for compatibility.
- PKG_CHECK_EXISTS(MODULES, [ACTION-IF-FOUND], [ACTION-IF-NOT-FOUND]): Checks to see whether a particular set of modules exists.
- PKG_CHECK_MODULES(VARIABLE-PREFIX, MODULES, [ACTION-IF-FOUND], [ACTION-IF-NOT-FOUND]): Checks to see whether a particular set of modules exists. If so, it sets `<VARIABLE-PREFIX>_CFLAGS` and `<VARIABLE-PREFIX>_LIBS` according to the output from `pkg-config --cflags` and `pkg-config --libs`.

## Frequently asked questions

1. My program uses library `x`. What do I do?

The `pkg-config` output can easily be used on the compiler command line. Assuming the `x` library has a `x.pc` `pkg-config` file:

    cc `pkg-config --cflags --libs x` -o myapp myapp.c

The integration can be more robust when used with autoconf and automake. By using the supplied `PKG_CHECK_MODULES` macro, the metadata is easily accessed in the build process.

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

    Makefile.am:
    myapp_CFLAGS = $(X_CFLAGS)
    myapp_LDADD = $(X_LIBS)

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

2. My library `z` installs header files which include `libx` headers. What do I put in my `z.pc` file?

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

3. My library `z` uses `libx` internally, but does not expose `libx` data types in its public API. What do I put in my `z.pc` file?

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

Copyright (C) 2010 Dan Nicholson.
This document is licensed under the GNU General Public License, Version 2 or any later version.