Forcing C99 in CMake (to use ‘for’ loop initial declaration)

After creating a target such as a library or executable, put a line like this in your CMakeLists.txt file:

set_property(TARGET tgt PROPERTY C_STANDARD 99)

where tgt is the name of your target.

I think this was added in CMake 3.1, and the documentation is here:

http://www.cmake.org/cmake/help/latest/prop_tgt/C_STANDARD.html

If you need to support versions of CMake older than 3.1, you can use this macro:

macro(use_c99)
  if (CMAKE_VERSION VERSION_LESS "3.1")
    if (CMAKE_C_COMPILER_ID STREQUAL "GNU")
      set (CMAKE_C_FLAGS "-std=gnu99 ${CMAKE_C_FLAGS}")
    endif ()
  else ()
    set (CMAKE_C_STANDARD 99)
  endif ()
endmacro(use_c99)

After putting that macro in your top-level file so it is visible everywhere, you can just write use_c99() at the top of any CMakeLists file that defines a target with C99 code in it.

CMake issue #15943 for clang users targeting macOS

If you are using CMake and clang to target MacOS there is a bug that can cause the CMAKE_C_STANDARD feature to simply not work (not add any compiler flags). Make sure that you do one of the following things:

• Use cmake_minimum_required to require CMake 3.0 or later, or

• Set policy CMP0025 to NEW with the following code at the top of your CMakeLists.txt file before the project command:

    # Fix behavior of CMAKE_C_STANDARD when targeting macOS.
    if (POLICY CMP0025)
      cmake_policy(SET CMP0025 NEW)
    endif ()