Adding a Compiled Library to CMake

This page describes how to add a new, compiled library to the CMake-based build system. If your library is a "header-only" library, and does not require separate compilation of object files into a library binary, you can safely skip this step. Before adding compiled libraries to CMake, make sure you have already followed the directions for adding a library to CMake, so that the CMake system recognizes your Boost library.

In this page, we will assume that your library resides in the subdirectory libs/libname, and that we want to create the compiled library boost_libname. We will also assume that the sources for this library reside in the subdirectory libs/libname/src. The source directory should be listed via SRCDIRS in the use of the boost_library_project macro, as described in the previous section, Adding a Library to CMake. Follow these steps to add this new library into Boost's build system. If your library has multiple source directories listed after SRCDIRS, follow these steps for each one.

1. Create a new file libs/libname/src/CMakeLists.txtwith your favorite text editor. This file will contain build rules for your compiled library. In this file, we will create one or more invocations of the boost_add_library macro, which adds a compiled Boost library to the CMake system. The boost_add_library macro provides the name of the library, the source files from which the library will be built, and any specific compiler and linker options needed to help build the library. Let's start by adding a simple library with a few source files:

   boost_add_library(boost_libname
     mysrc1.cpp mysrc2.cpp
     )
   

This invocation will build several variants of the boost_libname library from the source files mysrc1.cpp and mysrc2.cpp. For example, it will build both static and shared library, single- and multi-threaded, debug and release, etc. This invocation also handles the installation of these libraries.

2. For simple libraries, that's it! Rebuilding via CMake (e.g., running make or reloading and rebuilding the Boost project in your IDE) will build the new library, including several different variants for different compilation options. Your Boost library will also be included when the user installs Boost or builds a binary package of Boost. Within the CMake configuration, you will also see an option BUILD_LIBNAME, which allows the user to decide whether or not to build this Boost library.

3. Many libraries will need specific compilation options when building, need to link against other libraries (Boost or otherwise), or rely on certain features of the compilation process to proceed. Follow the instructions in the remaining part of this page to address these library-specific needs.

Compilation Flags

Many libraries require certain compilation flags when we are building the library binaries themselves (rather than when the library headers are included by the user). For example, we want to define the macro "BUILDING_BOOST_LIBNAME" when building the library. We can do so by passing the COMPILE_FLAGS option to boost_add_library:

boost_add_library(boost_libname
  mysrc1.cpp mysrc2.cpp
  COMPILE_FLAGS "-DBUILDING_BOOST_LIBNAME=1"
  )

Now when CMake builds the library, it will pass the flag -DBUILDING_BOOST_LIBNAME=1 to the compiler.

On Windows, shared libraries are built very differently from static libraries. In particular, when building a shared library, one needs to be sure to export the right symbols from the DLL using dllexport. When users use the shared library, these symbols will be imported (via dllimport). The typical way to handle this is to define a macro (say, BOOST_LIBNAME_DYN_LINK) when building the shared library. This macro instructs the library headers to dllexport everything that needs to be exported. We can do this with variant-specific compile flags, e.g.,

boost_add_library(boost_libname
  mysrc1.cpp mysrc2.cpp
  COMPILE_FLAGS "-DBUILDING_BOOST_LIBNAME=1"
  SHARED_COMPILE_FLAGS "-DBOOST_LIBNAME_DYN_LINK=1"
  )

When building a shared library, the SHARED_COMPILE_FLAGS options will be combined with the COMPILE_FLAGS options. When building a static library, the SHARED_COMPILE_FLAGS options will be ignored. There are other options that can be specified per-feature, such as LINK_FLAGS and LINK_LIBS; refer to the boost_add_library macro reference for more information.

Linking to Other Libraries

Some Boost libraries depends on other Boost libraries. For example, perhaps our library uses the Boost.Filesystem library under the hood. We can use the DEPENDS feature of the boost_add_library macro to state which libraries our library depends on. In this example, we'll link against boost_filesystem:

boost_add_library(boost_libname
  mysrc1.cpp mysrc2.cpp
  COMPILE_FLAGS "-DBUILDING_BOOST_LIBNAME=1"
  SHARED_COMPILE_FLAGS "-DBOOST_LIBNAME_DYN_LINK=1"
  DEPENDS boost_filesystem
  )

Now, each variant of the boost_libname library will link against the appropriate boost_filesystem library variant. Whenever boost_filesystem changes, our library will be relinked appropriately.

Linking External Libraries/Optional Sources

Sometimes, Boost libraries need to link against other libraries supplied by the system. The primary challenge in linking against these libraries is finding those libraries, and their associated headers, on the system. If the library is found, we usually want to pass some extra compilation flags to our library and add in additional sources. Otherwise, we just skip these extra sources.

Often, CMake already contains a module that will search for a common system library or tool; search the ​CMake Documentation for existing modules that do what you need. For example, say we want to link against the system's PNG library for portable network graphics. We can use the supplied FindPNG module by adding the following early in our CMakeLists.txt file:

include(FindPNG)

Documentation for CMake modules is typically found in the module file itself. Look into the Modules subdirectory of your CMake installation, either in Program Files\CMake (on Windows) or /usr/share/cmake-version (on Unix variants) to find the module of the same name. The module will typically provide a variable that indicates whether the library was found. For the FindPNG module, this variable is called PNG_FOUND. We can use this variable to optionally add sources to a variable EXTRA_SOURCES:

include(FindPNG)
set(EXTRA_SOURCES)
if (PNG_FOUND)
  list(APPEND EXTRA_SOURCES png.cpp)
endif (PNG_FOUND)

CMake modules also typically define macros specifying the include directories needed for the library and linking information for the library binary. For the FindPNG module, these variables are called PNG_PNG_INCLUDE_DIR and PNG_LIBRARY, respectively. The include directory should be added via the CMake include_directories macro, e.g.,

include_directories(${PNG_PNG_INCLUDE_DIR})

while the PNG_LIBRARY value should be added to the LINK_LIBS option to the boost_add_library macro. Using these features together, we can search for the PNG library on the system and optionally include PNG support into our library:

include(FindPNG)
set(EXTRA_SOURCES)
if (PNG_FOUND)
  include_directories(${PNG_PNG_INCLUDE_DIR})
  list(APPEND EXTRA_SOURCES png.cpp)
endif (PNG_FOUND)

boost_add_library(boost_libname
  mysrc1.cpp mysrc2.cpp
  ${EXTRA_SOURCES}
  COMPILE_FLAGS "-DBUILDING_BOOST_LIBNAME=1"
  LINK_LIBS "${PNG_LIBRARY}"
  SHARED_COMPILE_FLAGS "-DBOOST_LIBNAME_DYN_LINK=1"
  DEPENDS boost_filesystem
  )

If CMake does not provide a module to search for the library you need, don't worry! You can write your own module relatively easily, following the examples from the CMake Modules directory or some of the Boost-specific examples, such as ​tools/build/CMake/FindICU.cmake. For a real-life example of finding system libraries and using that information to add optional, extra capabilities to a Boost library, check out the build rules for the Boost.IOStreams library at ​libs/iostreams/src/CMakeLists.txt.

Build Variants

The Boost build system defines many different build features, which describe specific properties of certain builds. For example, the SHARED feature indicates that we are building a shared library, while the MULTI_THREADED feature indicates that we are building a multi-threaded library. A specific set of features is called a variant, e.g., RELEASE and MULTI_THREADED and SHARED. By default, the CMake-based build system builds several different variants of each Boost library.

Since some features conflict with certain libraries (a threading library cannot be SINGLE_THREADED!), one can pass additional flags to boost_add_library stating which features should the library cannot be built with. For example, say that our library cannot be built as a multi-threaded library, because it uses thread-unsafe routines from the underlying C library. To disable multi-threaded variants of the library, pass the option NOT_MULTI_THREADED:

boost_add_library(boost_libname
  mysrc1.cpp mysrc2.cpp
  COMPILE_FLAGS "-DBUILDING_BOOST_LIBNAME=1"
  SHARED_COMPILE_FLAGS "-DBOOST_LIBNAME_DYN_LINK=1"
  DEPENDS boost_filesystem
  NOT_MULTI_THREADED
  )