Adding Features

There are lots of compiler and linker settings. When you need to add something special, you could check first to see if CMake supports it; if it does, you can avoid explicitly tying yourself to a compiler version. And, better yet, you explain what you mean in your CMakeLists, rather than spewing flags.

The first and most common feature was C++ standards support, which got it’s own chapter.

Position independent code

This is best known as the -fPIC flag. Much of the time, you don’t need to do anything. CMake will include the flag for SHARED or MODULE libraries. If you do explicitly need it:

  1. set(CMAKE_POSITION_INDEPENDENT_CODE ON)

will do it globally, or:

  1. set_target_properties(lib1 PROPERTIES POSITION_INDEPENDENT_CODE ON)

to explicitly turn it ON (or OFF) for a target.

Little libraries

If you need to link to the dl library, with -ldl on Linux, just use the built-in CMake variable ${CMAKE_DL_LIBS} in a target_link_libraries command. No module or find_package needed. (This adds whatever is needed to get dlopen and dlclose)

Unfortunately, the math library is not so lucky. If you need to explicitly link to it, you can always do target_link_libraries(MyTarget PUBLIC m), but it might be better to use CMake’s generic find_library:

  1. find_library(MATH_LIBRARY m)
  2. if(MATH_LIBRARY)
  3.     target_link_libraries(MyTarget PUBLIC ${MATH_LIBRARY})
  4. endif()

You can pretty easily find Find*.cmake’s for this and other libraries that you need with a quick search; most major packages have a helper library of CMake modules. See the chapter on existing package inclusion for more.

Interprocedural optimization

INTERPROCEDURAL_OPTIMIZATION, best known as link time optimization and the -flto flag, is available on very recent versions of CMake. You can turn this on with CMAKE_INTERPROCEDURAL_OPTIMIZATION (CMake 3.9+ only) or the INTERPROCEDURAL_OPTIMIZATION property on targets. Support for GCC and Clang was added in CMake 3.8. If you set cmake_minimum_required(VERSION 3.9) or better (see CMP0069, setting this to ON on a target is an error if the compiler doesn’t support it. You can use check_ipo_supported(), from the built-in CheckIPOSupported module, to see if support is available before hand. An example of 3.9 style usage:

  1. include(CheckIPOSupported)
  2. check_ipo_supported(RESULT result)
  3. if(result)
  4.   set_target_properties(foo PROPERTIES INTERPROCEDURAL_OPTIMIZATION TRUE)
  5. endif()