CMAKECFLAGSDEBUG

CMAKE_C_FLAGS_DEBUG is a CMake cache variable used to specify additional compiler options for the C compiler when building in the Debug configuration. It is the per-configuration counterpart to CMAKE_C_FLAGS, and there are analogous variables for C++ such as CMAKE_CXX_FLAGS_DEBUG. These variables allow you to tailor flags for debugging without altering other configurations.

In practice, the behavior depends on the generator being used. For single-configuration generators (such as Makefiles or Ninja), the Debug configuration is selected by CMAKE_BUILD_TYPE, and CMAKE_C_FLAGS_DEBUG is combined with the base C flags whenever Debug is requested. In multi-configuration generators (such as Visual Studio or Xcode), per-configuration flags are applied when building the Debug configuration, with separate variables like CMAKE_C_FLAGS_DEBUG and CMAKE_C_FLAGS_RELEASE controlling flags for those configurations.

How to use it. You can modify or extend the existing flags in your CMakeLists.txt, for example:

set(CMAKE_C_FLAGS_DEBUG "${CMAKE_C_FLAGS_DEBUG} -g")

or

list(APPEND CMAKE_C_FLAGS_DEBUG "-g")

This approach appends debugging flags to the existing set. Alternatively, you can apply per-target options using generator expressions, such as:

target_compile_options(mytarget PRIVATE $<$<CONFIG:Debug>:-g>)

or combine with other per-config flags as appropriate.

Notes and caveats. On some toolchains (notably some Windows/MSVC setups), the conventional Unix-style flags in CMAKE_C_FLAGS_DEBUG may be ignored or translated differently, so relying on the compiler’s native debug options (or using per-target options) can be more portable. As a best practice, prefer per-target or per-config options via generator expressions when possible to avoid unintended effects across configurations.