How to specify the compiler

We can specify the compilers like this:

$ cd build
$ env FC=gfortran CC=gcc CXX=g++ cmake ..
$ make

Or by exporting the corresponding environment variables:

$ cd build
$ export FC=gfortran
$ export CC=gcc
$ export CXX=g++
$ cmake ..
$ make

Or by setting CMake variables:

$ cmake -D CMAKE_Fortran_COMPILER=gfortran -D CMAKE_C_COMPILER=gcc -D CMAKE_CXX_COMPILER=g++ ..

Controlling compiler flags

• We can define compiler flags for different compilers and build types

if(CMAKE_Fortran_COMPILER_ID MATCHES Intel)
    set(CMAKE_Fortran_FLAGS         "${CMAKE_Fortran_FLAGS} -Wall"
    set(CMAKE_Fortran_FLAGS_DEBUG   "-g -traceback")
    set(CMAKE_Fortran_FLAGS_RELEASE "-O3 -ip -xHOST")
endif()

if(CMAKE_Fortran_COMPILER_ID MATCHES GNU)
    set(CMAKE_Fortran_FLAGS         "${CMAKE_Fortran_FLAGS} -Wall")
    set(CMAKE_Fortran_FLAGS_DEBUG   "-O0 -g3")
    set(CMAKE_Fortran_FLAGS_RELEASE "-Ofast -march=native")
endif()

...

Similarly you can set CMAKE_C_FLAGS and CMAKE_CXX_FLAGS.

This is how we set preprocessor definitions:

target_compile_options(sometarget PUBLIC -DVAR_SOMETHING -DENABLE_DEBUG -DTHIS_DIMENSION=137)

Controlling the build type

CMake distinguishes the following build types:

• Debug
• Release
• RelWithDebInfo
• MinSizeRel

We can select the build type on the command line:

$ cd build
$ cmake -D CMAKE_BUILD_TYPE=Debug ..
$ make

It is often useful to set:

# we default to Release build type
if(NOT CMAKE_BUILD_TYPE)
    set(CMAKE_BUILD_TYPE "Release")
endif()

What are the actual compiler flags and link line?

The default compilation output is nice and compact. But sometimes we want to see the current compiler flags and the gory compiler output:

$ make clean
$ make VERBOSE=1

The link line is saved in CMakeFiles/<target>.dir/link.txt.