Set CFLAGS and CXXFLAGS options using CMake

Question

I just want to debug some code running on Linux and I need a debug build (-O0 -ggdb). So I added these things to my CMakeLists.txt file:

set(CMAKE_BUILD_TYPE DEBUG)
set(CMAKE_C_FLAGS "-O0 -ggdb")
set(CMAKE_C_FLAGS_DEBUG "-O0 -ggdb")
set(CMAKE_C_FLAGS_RELEASE "-O0 -ggdb")
set(CMAKE_CXX_FLAGS "-O0 -ggdb")
set(CMAKE_CXX_FLAGS_DEBUG "-O0 -ggdb")
set(CMAKE_CXX_FLAGS_RELEASE "-O0 -ggdb")

When I tried to compile I turned verbose on using make VERBOSE=1 And I observed the output, like this

... /usr/bin/c++ -D_BSD_SOURCE **-O0 -ggdb** -Wnon-virtual-dtor 
-Wno-long-long -ansi -Wundef -Wcast-align -Wchar-subscripts -Wall -W 
-Wpointer-arith -Wformat-security -fno-exceptions -DQT_NO_EXCEPTIONS 
-fno-check-new -fno-common -Woverloaded-virtual -fno-threadsafe-statics 
-fvisibility=hidden -fvisibility-inlines-hidden **-g -O2** 
-fno-reorder-blocks -fno-schedule-insns -fno-inline ...

Apparently the code is compiled with "-g -O2" and this is not what I want. How can I force it to use "-O0 -ggdb" only?

Solemn

If you want a debuggable build, just do a debug configure at the command line. "cmake -DCMAKE_BUILD_TYPE=Debug". The resulting build will have the debug flags on for the given build system. No reason to modify the cmake file itself. You can also send in the CMAKE_C_FLAGS value with another -D argument. – Niemeyer

Answer 1

You need to set the flags after the project command in your CMakeLists.txt.

Also, if you're calling include(${QT_USE_FILE}) or add_definitions(${QT_DEFINITIONS}), you should include these set commands after the Qt ones since these would append further flags. If that is the case, you maybe just want to append your flags to the Qt ones, so change to e.g.

set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -O0 -ggdb")

Industrial

I did put those flags after the project command. But the output is the same. – Solemn

I just noticed the QT_NO_EXCEPTIONS in your flags, so I've extended my answer a bit. I'm guessing that ${QT_DEFINITIONS} is appending the -g -O2. – Industrial

Yes, This is it. Putting those set commands after the Qt ones solves the problem. Thank you very much. – Solemn

Thanks a lot for the hint about the project() command! This solved a problem where we want set the CMAKE_CXX_FLAGS for all projects via a common include but did so after the project command. – Steading

I am getting "cl : Command line warning D9002 : ignoring unknown option '-ggdb'" – Borlow

Answer 2

The easiest solution working fine for me is this:

export CFLAGS=-ggdb
export CXXFLAGS=-ggdb

CMake will append them to all configurations' flags. Just make sure to clear CMake cache.

Ihs

Setting the flags in the environment like this is the only way to do it without modifying your CMakeLists.txt. Note that the environment value is only read on first configuration and is then put into the cache, so if you want to change this you need to clear the CMake cache and re-run the configure step with the new values in the env. Docs: cmake.org/cmake/help/latest/envvar/CFLAGS.html cmake.org/cmake/help/latest/envvar/CXXFLAGS.html – Alanealanine

That's not "easiest", that's the only right way to do it. – Vestibule

Minor catch, if the generator is configuring a specific flag for a specific build type (e.g. in CMAKE_CXX_FLAGS_DEBUG etc.), then including it in CXXFLAGS (and thereby injecting it into CMAKE_CXX_FLAGS) doesn't actually suffice for passing the option to the compiler. The per build type configuration takes precedence over user input. – Advertence

But just how do you clear the CMake cache? If you're building into a subdir like _build, erase and recreate it. – Vaishnava

Answer 3

You must change the CMake CFLAGS/CXXFLAGS default flags.

According to CMAKE_BUILD_TYPE={DEBUG/MINSIZEREL/RELWITHDEBINFO/RELEASE}, put in the main CMakeLists.txt one of:

For C

set(CMAKE_C_FLAGS_DEBUG "put your flags")
set(CMAKE_C_FLAGS_MINSIZEREL "put your flags")
set(CMAKE_C_FLAGS_RELWITHDEBINFO "put your flags")
set(CMAKE_C_FLAGS_RELEASE "put your flags")

For C++

set(CMAKE_CXX_FLAGS_DEBUG "put your flags")
set(CMAKE_CXX_FLAGS_MINSIZEREL "put your flags")
set(CMAKE_CXX_FLAGS_RELWITHDEBINFO "put your flags")
set(CMAKE_CXX_FLAGS_RELEASE "put your flags")

This will override the values defined in CMakeCache.txt.

Dipetalous

that overrides your cmake flags that are current. so it breaks debugging unless you add the -g flag yourself (gcc) – Excrete

Is there the same think using "Modern Cmake" syntax? – Flaunch

Answer 4

On Unix systems, for several projects, I added these lines into the CMakeLists.txt and it was compiling successfully because base (/usr/include) and local includes (/usr/local/include) go into separated directories:

set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -I/usr/local/include -L/usr/local/lib")
set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -I/usr/local/include")
set(CMAKE_EXE_LINKER_FLAGS "${CMAKE_EXE_LINKER_FLAGS} -L/usr/local/lib")

It appends the correct directory, including paths for the C and C++ compiler flags and the correct directory path for the linker flags.

Note: C++ compiler (c++) doesn't support -L, so we have to use CMAKE_EXE_LINKER_FLAGS

Washwoman

Isnt it better to use include_directories instead of the -I flag? – Trotline

Thank you so much. I was placing my linker flags in CMAKE_CXX_FLAGS, which caused them to be ignored. Now that I placed them in CMAKE_EXE_LINKER_FLAGS based on your answer, they are working just fine. – Tolentino