= Compile Flags :toc: :toc-placement!: toc::[] # Introduction CMake supports setting compile flags in a number of different ways: * using +target_compile_definitions()+ function * using the +CMAKE_C_FLAGS+ and +CMAKE_CXX_FLAGS+ variables. The files in this tutorial are below: ``` $ tree . ├── CMakeLists.txt ├── main.cpp ``` * link:CMakeLists.txt[] - Contains the CMake commands you wish to run * link:main.cpp[] - The source file with main # Concepts # Set Per-Target C++ Flags The recommended way to set C++ flags in modern CMake is to use per-target flags which can be populated to other targets through the +target_compile_definitions()+ link:https://cmake.org/cmake/help/v3.0/command/target_compile_definitions.html?highlight=target_compile_definitions[function]. This will populate the link:https://cmake.org/cmake/help/v3.0/prop_tgt/INTERFACE_COMPILE_DEFINITIONS.html#prop_tgt:INTERFACE_COMPILE_DEFINITIONS[INTERFACE_COMPILE_DEFINITIONS] for the library and push the definition to the linked target depending on the scope. [source,cmake] ---- target_compile_definitions(cmake_examples_compile_flags PRIVATE EX3 ) ---- This will cause the compiler to add the definition +-DEX3+ when compiling the target. In the target was a library, and the scope +PUBLIC+ or +INTERFACE+ has been chosen the definition would also be included in any executables that link this target. For compiler options you can also use the +target_compile_options()+ link:https://cmake.org/cmake/help/v3.0/command/target_compile_options.html[function]. ## Set Default C++ Flags The default `CMAKE_CXX_FLAGS` is either empty or contains the appropriate flags for the build type. To set additional default compile flags you can add the following to your top level CMakeLists.txt [source,cmake] ---- set (CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -DEX2" CACHE STRING "Set C++ Compiler Flags" FORCE) ---- Similarly to +CMAKE_CXX_FLAGS+ other options include: * Setting C compiler flags using +CMAKE_C_FLAGS+ * Setting linker flags using +CMAKE_LINKER_FLAGS+. [NOTE] ==== The values `CACHE STRING "Set C++ Compiler Flags" FORCE` from the above command are used to force this variable to be set in the CMakeCache.txt file. For more details, see https://cmake.org/cmake/help/v3.0/command/set.html[here] ==== Once set the +CMAKE_C_FLAGS+ and +CMAKE_CXX_FLAGS+ will set a compler flag / definition globally for all targets in this directory or any included sub-directories. This method is not recommended for general usage now and the +target_compile_definitions+ function is preferred. ### Set CMake Flags Similar to the build type a global C++ compiler flag can be set using the following methods. - Using a gui tool such as ccmake / cmake-gui image::cmake-gui-set-cxx-flag.png[cmake-gui set cxx flag] - Passing into cmake [source,cmake] ---- cmake .. -DCMAKE_CXX_FLAGS="-DEX3" ---- # Building the Example [source,bash] ---- $ mkdir build $ cd build/ $ cmake .. -- The C compiler identification is GNU 4.8.4 -- The CXX compiler identification is GNU 4.8.4 -- Check for working C compiler: /usr/bin/cc -- Check for working C compiler: /usr/bin/cc -- works -- Detecting C compiler ABI info -- Detecting C compiler ABI info - done -- Check for working CXX compiler: /usr/bin/c++ -- Check for working CXX compiler: /usr/bin/c++ -- works -- Detecting CXX compiler ABI info -- Detecting CXX compiler ABI info - done -- Configuring done -- Generating done -- Build files have been written to: /home/matrim/workspace/cmake-examples/01-basic/G-compile-flags/build $ make VERBOSE=1 /usr/bin/cmake -H/home/matrim/workspace/cmake-examples/01-basic/G-compile-flags -B/home/matrim/workspace/cmake-examples/01-basic/G-compile-flags/build --check-build-system CMakeFiles/Makefile.cmake 0 /usr/bin/cmake -E cmake_progress_start /home/matrim/workspace/cmake-examples/01-basic/G-compile-flags/build/CMakeFiles /home/matrim/workspace/cmake-examples/01-basic/G-compile-flags/build/CMakeFiles/progress.marks make -f CMakeFiles/Makefile2 all make[1]: Entering directory `/home/matrim/workspace/cmake-examples/01-basic/G-compile-flags/build' make -f CMakeFiles/cmake_examples_compile_flags.dir/build.make CMakeFiles/cmake_examples_compile_flags.dir/depend make[2]: Entering directory `/home/matrim/workspace/cmake-examples/01-basic/G-compile-flags/build' cd /home/matrim/workspace/cmake-examples/01-basic/G-compile-flags/build && /usr/bin/cmake -E cmake_depends "Unix Makefiles" /home/matrim/workspace/cmake-examples/01-basic/G-compile-flags /home/matrim/workspace/cmake-examples/01-basic/G-compile-flags /home/matrim/workspace/cmake-examples/01-basic/G-compile-flags/build /home/matrim/workspace/cmake-examples/01-basic/G-compile-flags/build /home/matrim/workspace/cmake-examples/01-basic/G-compile-flags/build/CMakeFiles/cmake_examples_compile_flags.dir/DependInfo.cmake --color= Dependee "/home/matrim/workspace/cmake-examples/01-basic/G-compile-flags/build/CMakeFiles/cmake_examples_compile_flags.dir/DependInfo.cmake" is newer than depender "/home/matrim/workspace/cmake-examples/01-basic/G-compile-flags/build/CMakeFiles/cmake_examples_compile_flags.dir/depend.internal". 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Scanning dependencies of target cmake_examples_compile_flags make[2]: Leaving directory `/home/matrim/workspace/cmake-examples/01-basic/G-compile-flags/build' make -f CMakeFiles/cmake_examples_compile_flags.dir/build.make CMakeFiles/cmake_examples_compile_flags.dir/build make[2]: Entering directory `/home/matrim/workspace/cmake-examples/01-basic/G-compile-flags/build' /usr/bin/cmake -E cmake_progress_report /home/matrim/workspace/cmake-examples/01-basic/G-compile-flags/build/CMakeFiles 1 [100%] Building CXX object CMakeFiles/cmake_examples_compile_flags.dir/main.cpp.o /usr/bin/c++ -DEX2 -o CMakeFiles/cmake_examples_compile_flags.dir/main.cpp.o -c /home/matrim/workspace/cmake-examples/01-basic/G-compile-flags/main.cpp Linking CXX executable cmake_examples_compile_flags /usr/bin/cmake -E cmake_link_script CMakeFiles/cmake_examples_compile_flags.dir/link.txt --verbose=1 /usr/bin/c++ -DEX2 CMakeFiles/cmake_examples_compile_flags.dir/main.cpp.o -o cmake_examples_compile_flags -rdynamic make[2]: Leaving directory `/home/matrim/workspace/cmake-examples/01-basic/G-compile-flags/build' /usr/bin/cmake -E cmake_progress_report /home/matrim/workspace/cmake-examples/01-basic/G-compile-flags/build/CMakeFiles 1 [100%] Built target cmake_examples_compile_flags make[1]: Leaving directory `/home/matrim/workspace/cmake-examples/01-basic/G-compile-flags/build' /usr/bin/cmake -E cmake_progress_start /home/matrim/workspace/cmake-examples/01-basic/G-compile-flags/build/CMakeFiles 0 ----