How do I configure portable parallel builds in CMake?

Question

Is it somehow possible to be able to have a parallel build no matter which build tool is used?

Under Unix we can add make -jN where N are the number of threads, and under Windows I added to the CXX_FLAG "/MP" which is then used in Visual Studio to parallel build...(?) How can I make my version such that CMAKE_MAKE_PROGRAM is not always extended when I run CMake?

What is a general solution?

I came up with this:

# Add some multithreaded build support MARK_AS_ADVANCED(MULTITHREADED_BUILD) set(MULTITHREADED_BUILD 12 CACHE STRING "How many threads are used to build the project") if(MULTITHREADED_BUILD)     if(${CMAKE_GENERATOR} MATCHES "Unix Makefiles")             message(STATUS ${CMAKE_BUILD_TOOL})             set(CMAKE_MAKE_PROGRAM "${CMAKE_MAKE_PROGRAM} -j${MULTITHREADED_BUILD}")             message(STATUS "Added arguments to CMAKE_BUILD_TOOL: ${CMAKE_MAKE_PROGRAM}")     elseif(MSVC)       set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} /MP")       message(STATUS "Added parallel build arguments to CMAKE_CXX_FLAGS: ${CMAKE_CXX_FLAGS}")     endif() endif() 

Answer 1

With CMake 3.12 this is possible. From the release notes:

The cmake(1) Build a Project (cmake --build) gained --parallel [<jobs>] and -j [<jobs>] options to specify a parallel build level. They map to corresponding options of the native build tool.

As mentioned by dkg, you can also set the environment variable CMAKE_BUILD_PARALLEL_LEVEL.

Links to CMake's documentation:

• Build a Project
• CMAKE_BUILD_PARALLEL_LEVEL

Answer 2

If you have CMake v2.8.8 or higher, you may use Ninja as an alternative of GNU make:

mkdir build cd    build cmake -G Ninja .. ninja              # Parallel build (no need -j12) 

or

mkdir build cd    build cmake -G Ninja .. cmake --build .    # Parallel build using Ninja 

As you can see, no need to use CMAKE_MAKE_PROGRAM, the build is run in parallel by default, optimizing the number of jobs depending on available CPU cores.

Ninja is based on a low-level JSON configuration to speed up the startup phase. Therefore its JSON configuration is not easy to write by hand, and I always generate it using a high-level tool/IDE:

• CMake v2.8.8 (2012)
• Qt Creator v2.6 (2012)
• KDevelop v4.6 (2013)
• Meson on Linux (2013)
• ... see the generators of Ninja configuration at https://github.com/ninja-build/ninja/wiki/List-of-generators-producing-ninja-build-files

As a C++ build often requires lots of memory, your computer must provide as much memory as the number of CPU cores.

As pointed out by Ruslan, CMake 3.12 (2018) has a new option cmake --build -j <N> to limit build to <N> cores (jobs) thus limiting the memory consumption (see also the documentation). If you use an older CMake version, you can still use cmake --build -- -j <N>. The option -- tells to CMake to pass the rest directly to the underlying builder tool, here it is Ninja.