CMake: How to run a add_custom_command before ever

Question

I have a custom command

add_custom_command(
    OUTPUT config.h
    PRE_BUILD
    COMMAND ${CMAKE_CURRENT_SOURCE_DIR}/mk_config_h.py ${CMAKE_CURRENT_BINARY_DIR}/config.h
)

I'm trying to run it before everything else and I generate unix Makefiles.

However PRE_BUILD is only supported for VS2010 which means that config.h is build before linking.

how do I make a custom command before cmake starts compiling sources.

Answer 1

You should use add_custom_target instead and add_dependencies to make your normal target depend on it:

add_custom_target(
    myCustomTarget
    COMMAND ${CMAKE_CURRENT_SOURCE_DIR}/mk_config_h.py ${CMAKE_CURRENT_BINARY_DIR}/config.h
)
add_dependencies(myTarget myCustomTarget)

This should ensure that the command is run before compiling the sources of myTarget.

Answer 2

Usualy build systems which are based on makefiles do not need to mention headers as part of a target. Further more, adding headers to target is considered to be an antipattern. But generating a header is an exception from this rule.

http://voices.canonical.com/jussi.pakkanen/2013/03/26/a-list-of-common-cmake-antipatterns/

You should set your ${CMAKE_CURRENT_BINARY_DIR}/config.h as part of some other compilled target. For example:

set(PROGRAM1_SOURCES main_program1.cpp)
set(PROGRAM1_HEADERS main_program1.h ${CMAKE_CURRENT_BINARY_DIR}/config.h)
add_executable(program1 ${PROGRAM1_SOURCES} ${PROGRAM1_HEADERS})

So that, CMake will know which target actually needs the generated config.h and will be able to insert the generation commands into a right build steps.

Also, see an example at https://cmake.org/cmake-tutorial/#s5