4.4.6.2 How to Specify Compilers and Compiler Flags

To define which C and C++ compilers to use, you can define the CC and CXX environment variables. For example:

shell> CC=gcc
shell> CXX=g++
shell> export CC CXX

To specify your own C and C++ compiler flags, for flags that do not affect optimization, use the CMAKE_C_FLAGS and CMAKE_CXX_FLAGS CMake options:

cmake .. -DCMAKE_C_FLAGS=your_c_flags \
  -DCMAKE_CXX_FLAGS=your_c++_flags

When providing your own compiler flags, you might want to specify CMAKE_BUILD_TYPE as well.

For example, to create a 32-bit release build on a 64-bit Linux machine, do this:

cmake .. -DCMAKE_C_FLAGS=-m32 \
  -DCMAKE_CXX_FLAGS=-m32 \
  -DCMAKE_BUILD_TYPE=RelWithDebInfo

If you set flags that affect optimization (-Onumber), you must set the CMAKE_C_FLAGS_build_type and/or CMAKE_CXX_FLAGS_build_type options, where build_type corresponds to the CMAKE_BUILD_TYPE value. To specify a different optimization for the default build type (RelWithDebInfo) set the CMAKE_C_FLAGS_RELWITHDEBINFO and CMAKE_CXX_FLAGS_RELWITHDEBINFO options. For example, to compile on Linux with -O3 and with debug symbols, do this:

cmake .. -DCMAKE_C_FLAGS_RELWITHDEBINFO="-O3 -g" \
  -DCMAKE_CXX_FLAGS_RELWITHDEBINFO="-O3 -g"