CMake dependencies on included files

From the question Defining lex include files for (f)lexers we are already knowing that there is no lex distribution known that has an include file facility for e.g. rules into a lex file.

The suggestion there was to write an own pre-processor.

When defining an own pre-processor one generates, of course, dependencies on the included file(s), so when such a file is changed one wants to do a rebuild of the "lex" file. With C / C++ the dependencies are handled (to the best of my knowledge) automatically (probably by some CMake script).

How to accomplish the dependency rules for the file(s) included in a lex file?

Edit

Say I have an original .l file configimpl.l that includes the files aa.inc and bb.inc, by converting the original .l file (${CMAKE_CURRENT_LIST_DIR}/${lex_file}.l) by means of a python script into a new .l (${GENERATED_SRC}/${lex_file}.l) file (and we also have an output file ${GENERATED_SRC}/${lex_file}.corr but that is not relevant here). Of course all "cmake" variables etc. have been set properly.

My code for the preprocessing (i.e. including the include files into the .l file) is:

    add_custom_command(
        COMMAND ${PYTHON_EXECUTABLE} ${CMAKE_CURRENT_LIST_DIR}/pre_lex.py ${CMAKE_CURRENT_LIST_DIR}/${lex_file}.l  ${GENERATED_SRC}/${lex_file}.l ${GENERATED_SRC}/${lex_file}.corr ${CMAKE_CURRENT_LIST_DIR}
        DEPENDS ${CMAKE_CURRENT_LIST_DIR}/pre_lex.py ${CMAKE_CURRENT_LIST_DIR}/${lex_file}.l ${LEX_INC_FILES}
        OUTPUT  ${GENERATED_SRC}/${lex_file}.corr ${GENERATED_SRC}/${lex_file}.l
    )

and this works fine, except when the aa.inc or the bb.inc changes the "generated included" ${GENERATED_SRC}/${lex_file}.l is not rebuild.

Based on the question: Dynamic dependency within custom source files I already tried to create something, but failed.

    add_custom_command(
        COMMAND ${PYTHON_EXECUTABLE} ${CMAKE_CURRENT_LIST_DIR}/dep_lex.py ${CMAKE_CURRENT_LIST_DIR}/${lex_file}.l  ${GENERATED_SRC}/${lex_file}.d ${GENERATED_SRC}/${lex_file}.l ${CMAKE_CURRENT_LIST_DIR}
        DEPENDS ${CMAKE_CURRENT_LIST_DIR}/dep_lex.py ${CMAKE_CURRENT_LIST_DIR}/${lex_file}.l
        OUTPUT  ${GENERATED_SRC}/${lex_file}.d
    )
    set_source_files_properties(${GENERATED_SRC}/${lex_file}.d PROPERTIES GENERATED 1)
    add_custom_command(
        COMMAND ${PYTHON_EXECUTABLE} ${CMAKE_CURRENT_LIST_DIR}/pre_lex.py ${CMAKE_CURRENT_LIST_DIR}/${lex_file}.l  ${GENERATED_SRC}/${lex_file}.l ${GENERATED_SRC}/${lex_file}.corr ${CMAKE_CURRENT_LIST_DIR}
        DEPENDS ${CMAKE_CURRENT_LIST_DIR}/pre_lex.py ${CMAKE_CURRENT_LIST_DIR}/${lex_file}.l
        DEPFILE ${GENERATED_SRC}/${lex_file}.d
        OUTPUT  ${GENERATED_SRC}/${lex_file}.l ${GENERATED_SRC}/${lex_file}.corr
    )

So I first run a dependency detection step that creates a file ${GENERATED_SRC}/${lex_file}.d with as content:

${GENERATED_SRC}/generated_src/configimpl.l: ${CMAKE_CURRENT_LIST_DIR}/aa.inc
${GENERATED_SRC}/generated_src/configimpl.l: ${CMAKE_CURRENT_LIST_DIR}/bb.inc

in this generated file the paths are, of course, set to the full paths. After this the original preprocessing step is run but now with the extra DEPFILE ${GENERATED_SRC}/${lex_file}.d that should signal that there are some extra dependencies.

When I change the aa.inc or the bb.inc the ${GENERATED_SRC}/${lex_file}.l is not rebuild, what did I miss?

Used CMake version: cmake version 3.22.1 used generator: NMake Makefiles

Solution

Make the DEPFILE output as one long line, then it works the best. From memory, I think the parser for the depfile is not smart enough to handle multiple lines. When in doubt - see the output of -MT -MD of gcc, the format should be the same.