Executing multiple post build commands in CMake depending on condition

Question

So I have some cmake definition of ADD_EXTRA_STEP that can be either true or false and is set depending on what the user wants. Then, at post build I have to execute a few commands. I currently have something like this:

    add_custom_command(TARGET ${some_target}
        POST_BUILD
        COMMAND <command to generate FOO.out>
        COMMAND <command that uses FOO.out and generates FOO2.out>
        COMMENT <some comment>
        VERBATIM
    )
    add_custom_command(TARGET ${some_target}
        POST_BUILD
        COMMAND <some other command>
        COMMENT <some other comment>
        VERBATIM
    )

Now in between the two add_custom_command I need to execute another command if and only if ADD_EXTRA_STEP is set to true. The problem is that for this to work, I need to guarantee that FOO2.out exists. My Idea so far is to do something like this:

    add_custom_command(TARGET ${some_target}
        POST_BUILD
        COMMAND <command to generate FOO.out>
        COMMAND <command that uses FOO.out and generates FOO2.out>
        COMMENT <some comment>
        VERBATIM
    )
    if(${ADD_EXTRA_STEP})
        add_custom_command(TARGET ${some_target}
            POST_BUILD
            COMMAND <extra step command that uses FOO2.out>
            COMMENT <some extra comment>
            VERBATIM
        )
    endif()
    add_custom_command(TARGET ${some_target}
        POST_BUILD
        COMMAND <some other command>
        COMMENT <some other comment>
        VERBATIM
    )

However, I am in doubt whether this would work and whether it is a 'good' practice to implement it in this way. In other words, is it guaranteed that when the command in the add_custom_command is executed the command in the previous add_custom_command (i.e. the one that generates FOO2.out) will have already executed?

Thanks in advance!

Answer 1

You need to combine add_custom_command with add_custom_target

The main idea is to use the OUTPUT form of the add_custom_command on the first step with OUTPUT being equal to FOO2.out and then specify FOO2.out as DEPENDS in the conditional target/command. Whether to use add_custom_command or add_custom_target depends on the actual actions in the command and how the results of the command are used in later steps. Consider this simple example:

add_executable(some_executable some_executable_source.c)
add_custom_command(OUTPUT FOO2.out
   DEPENDS some_executable
   COMMAND ${CMAKE_COMMAND} -E touch FOO2.out
   COMMAND ${CMAKE_COMMAND} -E echo "creating FOO2.out"
)

if (${ADD_EXTRA_STEP})
  add_custom_target(run ALL
    ${CMAKE_COMMAND} -E echo "executing extra step"
    DEPENDS FOO2.out
  )
endif()

In this case extra step, if needed, will always be executed after FOO2.out is created