pkg-config

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Recently I have been learning how to use google-test and reading its source code. When I ran make install, I noticed that besides installing header files into /usr/local/ and a shared library into /usr/lib/, it also installed two .pc files into /usr/local/lib/pkgconfig/. So what exactly is pkg-config?

From this guide, we can find all the useful basic information about pkg-config.

Overview

Modern computer systems use many layers of components to provide APIs to users. A major challenge is how to integrate these different layers properly. The pkg-config tool collects metadata for libraries installed on the system, and users can conveniently inspect that metadata. For example, one of the pc files installed by google-test, gtest.pc, contains:

It includes all the information needed to use the gtest library, such as the header installation path, the shared library location, and the compiler options needed at compile time. In other words, to use gtest, reading this metadata is enough.

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prefix=${pcfiledir}/../..
libdir=${prefix}/lib
includedir=${prefix}/include

Name: gtest
Description: GoogleTest (without main() function)
Version: 1.9.0
URL: https://github.com/google/googletest
Libs: -L${libdir} -lgtest 
Cflags: -I${includedir} -DGTEST_HAS_PTHREAD=1

Without a metadata system like pkg-config on a computer system, locating and obtaining the details of services provided by the system would be difficult.
For a developer, installing pkg-config metadata along with your package can greatly simplify how users consume your API.

The main use of pkg-config is to provide the necessary details when a program is compiled and linked against a library. This metadata is stored in pkg-config files. These files use the .pc suffix and are placed in specific paths known by the pkg-config tool.
A .pc file contains two kinds of information: metadata keywords and freeform variables.
The former starts with a keyword followed by a colon and value, such as “Name: gtest”.
The latter connects a variable name and value with =, such as “prefix=…”.
Keywords are defined and exported by pkg-config.
Variables are optional, but they can be used to represent information not covered by pkg-config, or be referenced by keywords to make keyword definitions more flexible.

Ideally, one pc file should correspond to one library file. The filename, except for the suffix, should also be the same.

The most important metadata fields are Requires, Requires.private, Cflags, Libs, and Libs.private. External projects can use them to compile and link against this library. Prefer using the private fields to avoid exposing unnecessary libraries to users. If the user does not directly use symbols from the required library, they should not link against that library directly.