Cause CMake to force recompile a file, but only if project is otherwise recompiled?

Question

I have generally the same question as in Can CMake always force the compilation/build of a specific file?

I have a C++ file using __DATE__ to display the build date of my app. But if this file is not modified, it will not be rebuilt and the date will not be updated.

Can CMake always rebuild that specific file?

... except I want something slightly different:

In the CMake project I have (for C, transpiles to Makefile which I use), sometimes there are no actual changes to the code when I run make, which is detected nicely, in the sense that there is no recompilation (or relinking) of the program.

Obviously, in this case, I do not want to update the timestamp, and end up with a new executable, which is otherwise identical to the previous one - apart from the build date.

I have seen in the quoted post, that one simply has to ensure a changed timestamp on the file, to force a recompilation. So, assuming my __DATE__ usage is in use_date.c, what I'd want, is that the timestamp of use_date.c is updated (forcing recompilation), only if any other file in the project (say, main.c) has been changed, so it forces project recompilation and linking (obviously, this should also work if I just change use_date.c manually, and no other file).

So, assuming my project just generates an executable (no libraries):

add_executable(my_project use_date.c other_file.c main.c)

... is it possible to add a CMake step, that updates the timestamp of use_date.c (and thus causes its recompilation), only if otherwise the project is getting recompiled and relinked?

Answer 1

OK, found a way: it seems kind of a squeaky solution - hopefully someone more knowledgeable in CMake will post a proper solution eventually. But in the meantime:

I've found that add_custom_command with POST_BUILD runs the custom command only if a new binary (.elf for me) is generate, and otherwise does not run the custom command.

So, basically, we could have a small bash script in the custom command:

• If this bash script is called, we can assume that POST_BUILD has called it, meaning that a new .elf (for whatever other reasons) has been built
• So we can touch the use_date.c, and while in the bash script still, descend into the build directory, and explicitly run make again - which should recompile only use_date.c, and then link with the rest from the previous build.

This essentially works - except, it causes an infinite loop.

This can get solved with creating a temp file:

• If this bash script is called, we can assume that POST_BUILD has called it, meaning that a new .elf (for whatever other reasons) has been built
• check for a temp file:
  • if it does not exist, create it - and then we can touch the use_date.c, and while in the bash script still, descend into the build directory, and explicitly run make again - which should ...
  • if it does exist, then we've been called from the second .elf. build (for the touched use_date.c), so we want to remove the temp file, and just exit - stopping the infinite loop

This seems to work fine:

add_custom_command(TARGET ${PROJECT_NAME}
                   POST_BUILD
                   DEPENDS ALL
                   #COMMAND bash ARGS -c "touch ${CMAKE_CURRENT_SOURCE_DIR}/use_date.c && cd build && make" ## infinite loop!
                   COMMAND bash ARGS -c "if [ ! -f .refresh ]; then echo '.refresh' | tee .refresh && touch ${CMAKE_CURRENT_SOURCE_DIR}/use_date.c && cd build && make; else rm -v .refresh; fi" # OK!
                   WORKING_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR}
                   COMMENT "remake with refreshed __DATE__ in use_date.c (${CMAKE_CURRENT_SOURCE_DIR})"
                   VERBATIM
)