Check if header has been generated using cmake

Question

Let's say I have two files main.cpp and generate_header.cpp. generate_header.cpp generates a header containing data that main.cpp needs to run. based on this answer, I can use the following code to run generate_header.cpp before building main.cpp:

add_custom_target(run_generate_header 
    COMMAND generate_header
    WORKING_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR}
    COMMENT "generating header"
)

add_dependencies(main_target run_generate_header)

However, as generating the header's content is slow, I would like to generate the header content only if the header is empty.

To achieve this, I thought about using a preprocessor macro HEADER_GENERATED, and check_cxx_symbol_exists to generate the header's content only if the macro is not defined.

check_cxx_symbol_exists(HEADER_GENERATED ${HEADER_PATH} HEADER_GENERATED)

if (NOT HEADER_GENERATED)
   add_dependencies(main_target run_generate_header)
endif()

The problem with this approach is that check_cxx_symbol_exists runs during the configuration stage, not the build stage, so if the header is erased after configuration, it will not be regenerated and the build will fail. Is there a way to check if the header contains data during the build stage? Thanks in advance!

Answer 1

Do not use add_custom_target for something that is generated. If something is generated, it's add_custom_command(OUTPUT this_is_generated.h - tell CMake what is generated and where and from what. Usually, you want pairs add_custom_command & add_custom_target. Sole add_custom_target is for executing something, like running a linter or printing file size.

I would like to generate it only if it does not contain data.

So do that. Generate files in cmake current binary dir.

add_custom_command(
    COMMENT "generating header"
    OUTPUT ${CMAKE_CURRENT_BINARY_DIR}/gen/the_generated_header.h
    COMMAND
        sh -xc
        [=[
        mkdir -vp "$(dirname "$2")"
        if it does contains data; then
            "$1" -o "$2"
        else
            echo > "$2"
        fi
        ]=]
        sh
        $<TARGET_FILE:generate_header>
        ${CMAKE_CURRENT_BINARY_DIR}/gen/the_generated_header.h
    DEPENDS generate_header 
    WORKING_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR}
)
add_custom_target(run_generate_header DEPENDS ${CMAKE_CURRENT_BINARY_DIR}/gen/the_generated_header.h)

add_dependencies(main_target run_generate_header)
target_include_directries(main_target PUBLIC ${CMAKE_CURRENT_BINARY_DIR}/gen)

however, it is not cross platform

To be crossplatform, you would write the sh script in cmake. So create a file like:

# script.cmake
# check if the file exists in cmake
execute_process(${ARGV1} -o ${ARGV2})
# etc

Then you would call this cmake script from cmake:

add_custom_command(
   COMMAND cmake -P ./script.cmake 
    $<TARGET_FILE:generate_header>
    ${CMAKE_CURRENT_BINARY_DIR}/gen/the_generated_header.h
  ....