CMake Compiling Generated Files

Question

I have a list of files that get generated during the CMake build process. I want to compile these files using "add_library" afterward, but I won't know which files get generated until after they get generated. Is there anyway to build this into a CMake script?

Answer 1

Well, I think it is possible, so I'll share what I've done. My problem was that I had to compile several CORBA idls to use as part of a project's source and I didn't want to manually list every file. I thought it would be better to find the files. So I did it like this:

file(GLOB IDLS "idls/*.idl")
set(ACE_ROOT ${CMAKE_FIND_ROOT_PATH}/ace/ACE-${ACE_VERSION})
foreach(GENERATE_IDL ${IDLS})
   get_filename_component(IDLNAME ${GENERATE_IDL} NAME_WE)
   set(OUT_NAME ${CMAKE_CURRENT_SOURCE_DIR}/idls_out/${IDLNAME})

   list(APPEND IDL_COMPILED_FILES ${OUT_NAME}C.h ${OUT_NAME}C.cpp ${OUT_NAME}S.h ${OUT_NAME}S.cpp)

   add_custom_command(OUTPUT ${OUT_NAME}C.h ${OUT_NAME}C.cpp ${OUT_NAME}S.h ${OUT_NAME}S.cpp
                      COMMAND ${ACE_ROOT}/bin/tao_idl -g ${ACE_ROOT}/bin/ace_gperf -Sci -Ssi -Wb,export_macro=TAO_Export -Wb,export_include=${ACE_ROOT}/include/tao/TAO_Export.h -Wb,pre_include=${ACE_ROOT}/include/ace/pre.h -Wb,post_include=${ACE_ROOT}/include/ace/post.h -I${ACE_ROOT}/include/tao -I${CMAKE_CURRENT_SOURCE_DIR} ${GENERATE_IDL} -o ${CMAKE_CURRENT_SOURCE_DIR}/idls_out/
                      COMMENT "Compiling ${GENERATE_IDL}")
endforeach(GENERATE_IDL)

set_source_files_properties(${IDL_COMPILED_FILES}
                            PROPERTIES GENERATED TRUE)

set(TARGET_NAME ${PROJECT_NAME}${DEBUG_SUFFIX})

add_executable(
   ${TARGET_NAME}
   ${SOURCE} 
   ${IDL_COMPILED_FILES}
)

The GENERATED properties is useful in case one of my idl compilation outputs (*C.cpp, *C.h, *S.cpp and *S.h) is not created, so that the build command doesn't complain that the file doesn't exist.

Answer 2

Well, it is possible to do so with CMake's CMAKE_CONFIGURE_DEPENDS directory property. This forces CMake to reconfigure if any of the given files changed.

Simple solution

The following code shows the approach for a single model file, that is used as input for the code generation:

set(MODEL_FILE your_model_file)
set_directory_properties(PROPERTIES CMAKE_CONFIGURE_DEPENDS ${MODEL_FILE})
set(GENERATED_SOURCE_DIR ${CMAKE_CURRENT_BINARY_DIR}/${MODEL_FILE})

file(REMOVE_RECURSE ${GENERATED_SOURCE_DIR})
file(MAKE_DIRECTORY ${GENERATED_SOURCE_DIR})
execute_process(COMMAND your_code_generation_tool -o ${GENERATED_SOURCE_DIR} ${MODEL_FILE})

file(GLOB LIBGENERATED_FILES ${GENERATED_SOURCE_DIR}/*)
add_library(libgenerated ${LIBGENERATED_FILES})
target_include_directories(libgenerated ${GENERATED_SOURCE_DIR})

With the above approach, each time the model file has changed CMake will reconfigure which results in the model being regenerated.

Advanced solution

The problem with the simple solution is that even for the smallest possible change in the model the entire dependencies of the generated files have to be rebuilt.

The advanced approach uses CMake's copy_if_different feature to let only generated files that are affected by the model change to appear modified which results in better build times. To achieve that we use a staging directory as destination for the generator and sync the contents subsequently with the generator output of the previous compile run:

set(MODEL_FILE your_model_file)
set(GENERATOR_STAGING_DIR ${CMAKE_CURRENT_BINARY_DIR}/${MODEL_FILE}.staging)
set(GENERATOR_OUTPUT_DIR ${CMAKE_CURRENT_BINARY_DIR}/${MODEL_FILE})

set_directory_properties(PROPERTIES CMAKE_CONFIGURE_DEPENDS ${MODEL_FILE})

# Create fresh staging/final output directory
file(REMOVE_RECURSE ${GENERATOR_STAGING_DIR})
file(MAKE_DIRECTORY ${GENERATOR_STAGING_DIR})
file(MAKE_DIRECTORY ${GENERATOR_OUTPUT_DIR})

# Run code generation
execute_process(COMMAND your_code_generation_tool -o ${GENERATOR_STAGING_DIR} "${CMAKE_CURRENT_SOURCE_DIR}/${MODEL_FILE}")

# Remove stale files from final generator output directory
file(GLOB GENERATED_FILES RELATIVE "${GENERATOR_OUTPUT_DIR}/" "${GENERATOR_OUTPUT_DIR}/*")
foreach(FILE ${GENERATED_FILES})
    if(NOT EXISTS "${GENERATOR_STAGING_DIR}/${FILE}")
        file(REMOVE "${GENERATOR_OUTPUT_DIR}/${FILE}")
    endif()    
endforeach()

# Copy modified files from staging to final generator output directory
file(GLOB GENERATED_FILES RELATIVE "${GENERATOR_STAGING_DIR}/" "${GENERATOR_STAGING_DIR}/*")
foreach(FILE ${GENERATED_FILES})
    execute_process(COMMAND ${CMAKE_COMMAND} -E copy_if_different "${GENERATOR_STAGING_DIR}/${FILE}" "${GENERATOR_OUTPUT_DIR}")
endforeach()

file(GLOB LIBGENERATED_FILES "${GENERATOR_OUTPUT_DIR}/*")
add_library(libgenerated ${LIBGENERATED_FILES})
target_include_directories(libgenerated PUBLIC ${GENERATOR_OUTPUT_DIR})