How to define a C++ preprocessor macro through the command line with CMake?

Question

I try to set a preprocessor macro in the command line of CMake. I\'ve tried:

set generator=\"Visual Studio 8 2005\"
set params=-D MY_MACRO=1
cmake.exe -G %ge         


        

Answer 1

Try this: -D CMAKE_CXX_FLAGS=/DMY_MACRO=1

Answer 2

Unless you have a good reason not to, you should use ADD_DEFINITIONS(<name>=<value>[, ...]).

Just add the following line to your CMakeLists.txt:

ADD_DEFINITIONS("MY_MACRO=1")

CMake will take care of the syntax of the switches (be it -D<name>=<value>, or /D<name>=<value>).

Answer 3

The motivation behind the question was to batch build 3rd party libraries, which is why I wanted to avoid modifying CMakeLists. So years later, even though I don't need that anymore, I figured out that it's easily achievable by means external to CMake:

• Invoke CMake as usual, no special flags.

• Then:

  • With MSVC: The compiler reads the CL environment variable to get extra command line arguments. So

      set CL=/DMY_MACRO=1 %CL%
    

    then invoke MSBuild to do its job.

  • With Makefiles: The generated makefiles use the CFLAGS and CXX_FLAGS variables as makefiles are expected to do. So the build can be started by

      make CXX_FLAGS=-DMY_MACRO=1
    

    or by setting the corresponding environment variables.

Answer 4

A good alternative would be to define a cmake option:

OPTION(DEFINE_MACRO "Option description" ON) # Enabled by default

Followed by a condition:

IF(DEFINE_MACRO)
    ADD_DEFINITIONS(-DMACRO)
ENDIF(DEFINE_MACRO)

Then you can turn that option ON/OFF via command line with cmake using the -D flag. Example:

cmake -DDEFINE_MACRO=OFF ..

To make sure the compiler is receiving the definition right, you can call make in verbose mode and check for the macro being defined or not:

make VERBOSE=1

This is a good solution also because make will recompile your code when any of cmake options changes.