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:

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?

Answer 1

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

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.

Answer 3

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")

Answer 4

You must change the cmake C/CXX 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