Is Cmake set variable recursive?

Question

I am trying to modify compiler flags for all the directories below a certain directory (i.e. for all the current directories subdirectories and all their subdirectories recursively). So I found here there is two ways:

add_directory(dir1)
# ...
add_directory(dirN)

add_compile_options(flag1 flag2 ...)
# or for CMake versions < 3.0 to do something more like:
set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} flag1 flag2 ...")

The man page for add_compile_options is very clear in stating that the effect will be "current directory and below" (which is what I want) but for set(CMAKE_CXX_FLAGS ...) I am not so sure.

Is Cmake set variable recursive?

Answer 1

The short answer is, that each sub-directory has it's own variable scope initialized with a copy of the current variable values at the time of the add_subdirectory() call.

For the long answer please see What's the CMake syntax to set and use variables?

Directory & Target Properties vs (Global) Variables

There is a difference between how add_compile_options() and CMAKE_CXX_FLAGS are processed by CMake:

• Everything that you specify with add_compile_options() is appended to the COMPILE_OPTIONS directory property. Then "this property is used to initialize the COMPILE_OPTIONS target property when a target is created" with add_library() or add_executable().

  And the current state of directory properties are used to initialize sub-directory properties when the parser gets to a add_subdirectory() call.

• The CMAKE_CXX_FLAGS is a global cached variable. You can extend/overwrite it by defining a local directory scoped variable (hiding the globally cached one).

  Those variable's context is copied into a sub-directories scope on add_subdirectory() (propagating to sub-directories).

  And CMake looks into its value at the end of the each CMakeLists.txt file and applies this to all targets in the same CMakeLists.txt (allowing late declarations, see also Complete Formula and Test Code below).

• So for CMake versions < 3.0 the equivalent to add_compile_options() was add_definitions(). The functionality is still there, but it was strange to mix definitions with compile options. So add_compile_options() was invented.

The complete Generator-Formula for Compiler Flags

It's in CMake's code (see cmCommonTargetGenerator::GetFlags(), cmLocalGenerator::AddCompileOptions() and cmLocalGenerator::AddLanguageFlags()).

This example shows a DEBUG build configuration library without exports, not taking into account the feature-based flags or something like CMAKE_CXX_USE_RESPONSE_FILE_FOR_INCLUDES or CMAKE_QUOTE_INCLUDE_PATHS:

  CMAKE_CXX_FLAGS       // as set at the end of target's CMakeLists.txt
+ CMAKE_CXX_FLAGS_DEBUG

+ Include Directories   // pefixed with CMAKE_INCLUDE_FLAG_CXX/CMAKE_INCLUDE_SYSTEM_FLAG_CXX

    (CMAKE_INCLUDE_CURRENT_DIR) ? 
        + CMAKE_CURRENT_SOURCE_DIR + CMAKE_CURRENT_BINARY_DIR
    + CMAKE_CXX_IMPLICIT_INCLUDE_DIRECTORIES
    + Target[INCLUDE_DIRECTORIES]
    + DependingTargets[INTERFACE_INCLUDE_DIRECTORIES] 

+ Define Flags         // compiler flags given with add_definitions()
+ Target[COMPILE_FLAGS] // deprecated
    - Filtered by CMAKE_CXX_FLAG_REGEX
+ Target[COMPILE_OPTIONS]
+ DependingTargets[INTERFACE_COMPILE_OPTIONS]

Test Code

For a better understanding here is my code for testing the compiler options and the results I get:

Note: Normally I would use add_definitions() and target_compile_definitions() instead of add_compile_options() and target_compile_options() to set compiler definitions, but to demonstrate the propagating of compiler options I (mis-)used -D flags.

CMakeLists.txt

cmake_minimum_required(VERSION 3.0)

project(CxxFlagsTest)

set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -DCXX_FLAG")
add_compile_options("-DCOMPILE_OPTION")

add_subdirectory(lib)

file(WRITE main.cpp "int main() { return 0; }")
add_executable(main main.cpp)
target_link_libraries(main lib)

target_compile_options(main PRIVATE "-DMAIN_COMPILE_OPTION")
set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -DLATE_CXX_FLAG")

get_target_property(_main_compile_options main COMPILE_OPTIONS)
message(STATUS "main COMPILE_OPTIONS: ${_main_compile_options}")
get_directory_property(_root_compile_options COMPILE_OPTIONS)
message(STATUS "root COMPILE_OPTIONS: ${_root_compile_options}")
message(STATUS "root CMAKE_CXX_FLAGS: ${CMAKE_CXX_FLAGS}")

lib/CMakeLists.txt

set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -DSUB_CXX_FLAG")
add_compile_options("-DSUB_COMPILE_OPTION")

file(WRITE lib.cpp "")
add_library(lib lib.cpp)

target_compile_options(lib PUBLIC "-DLIB_COMPILE_OPTION")
set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -DLATE_SUB_CXX_FLAG")

get_target_property(_lib_compile_options lib COMPILE_OPTIONS)
message(STATUS "lib COMPILE_OPTIONS: ${_lib_compile_options}")
get_directory_property(_sub_compile_options COMPILE_OPTIONS)
message(STATUS "sub COMPILE_OPTIONS: ${_sub_compile_options}")
message(STATUS "sub CMAKE_CXX_FLAGS: ${CMAKE_CXX_FLAGS}")

Would result in the following messages:

-- lib COMPILE_OPTIONS: -DCOMPILE_OPTION;-DSUB_COMPILE_OPTION;-DLIB_COMPILE_OPTION
-- sub COMPILE_OPTIONS: -DCOMPILE_OPTION;-DSUB_COMPILE_OPTION
-- sub CMAKE_CXX_FLAGS:  ... -DCXX_FLAG -DSUB_CXX_FLAG -DLATE_SUB_CXX_FLAG
-- main COMPILE_OPTIONS: -DCOMPILE_OPTION;-DMAIN_COMPILE_OPTION
-- root COMPILE_OPTIONS: -DCOMPILE_OPTION
-- root CMAKE_CXX_FLAGS:  ... -DCXX_FLAG -DLATE_CXX_FLAG

And the following pre-processor definitions being set:

lib

CXX_FLAG
SUB_CXX_FLAG
LATE_SUB_CXX_FLAG
COMPILE_OPTION
SUB_COMPILE_OPTION
LIB_COMPILE_OPTION

main

CXX_FLAG
LATE_CXX_FLAG
COMPILE_OPTION
MAIN_COMPILE_OPTION
LIB_COMPILE_OPTION

The interesting parts here are the LATE CXX flags and the LIB compile option propagated the the linked library.

References

• cmake - Global linker flag setting (for all targets in directory)
• What's the CMake syntax to set and use variables?