CMake: how to build additional files for an existing target? - cmake

Question

I need to generate extra files for an existing CMake target that is already defined with an add_executable(); I don't know how many files there are in advance, and in addition those files are not compiled/part of the executable itself. These files should be build or updated whenever I build that target, but only if their dependent files have been updated.
These extra files are generated and/or updated from an existing file with a Python script. So the natural choices are add_custom_target() and add_custom_command(), but I run into two issues with these:
add_custom_target() works and I can add that as an additional depency of the main target, but the scripts are always executed.
add_custom_command() has proper dependency tracking, but I cannot add the files as dependencies of the main target, CMake simply won't allow it.
So what does not work:
function(register_translation_files)
## determine TARGET and INPUT_FILES ...
foreach (LANG IN LISTS TRANSLATION_LANGUAGES)
message ("Add translation '${LANG}' for target ${TARGET}")
set (XLF_FILE "${TARGET}_${LANG}.xlf")
add_custom_command (
OUTPUT ${XLF_FILE}
COMMAND scripts/cet2xlf.py --language ${LANG} ${XLF_FILE} ${INPUT_FILES}
DEPENDS ${INPUT_FILES}
WORKING_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR}
)
add_dependencies (${TARGET} ${XLF_FILE}) <<--- fails with '('the dependency target of TARGET does not exist')
endforeach()
endfunction()
(....)
add_executable (MainTarget foo.cpp bla.cpp)
register_translation_files (TARGET MainTarget INPUT file1 file2)
add_custom_target works but is always executed (as CMake considers it always outdated):
function(register_translation_files)
## determine TARGET and INPUT_FILES ...
foreach (LANG IN LISTS TRANSLATION_LANGUAGES)
message ("Add translation '${LANG}' for target ${TARGET}")
set (XLF_FILE "${TARGET}_${LANG}.xlf")
add_custom_target (
${XLF_FILE}
COMMAND scripts/cet2xlf.py --language ${LANG} ${XLF_FILE} ${INPUT_FILES}
DEPENDS ${INPUT_FILES}
WORKING_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR}
)
add_dependencies (${TARGET} ${XLF_FILE}) <<--- builds, but script is executed every time!
endforeach()
endfunction()
(....)
add_executable (MainTarget foo.cpp bla.cpp)
register_translation_files (TARGET MainTarget INPUT file1 file2)
I tried all kinds if variations, including a custom_target with dependencies on custom_command output, but I either end up with 'this dependency does not exist' or a script that is always executed.
Surely, one can add files with add_depencies()?

Answer 1

You could combine both add_custom_target and add_custom_command to accomplish this:
List the files generated by add_custom_command in via the DEPENDS option of add_custom_target and then use add_dependencies.
I strongly recommend keeping the source directories free of any files generated during build/cmake configuration. Not doing this prevents projects you set up based on the same source directory from working properly.
function(register_translation_files)
## determine TARGET and INPUT_FILES ...
set(OUTPUT_DIR ${CMKAE_CURRENT_BINARY_DIR}/generated_translation_files)
file(MAKE_DIRECTORY ${OUTPUT_DIR})
set(GENERATED_FILES)
foreach (LANG IN LISTS TRANSLATION_LANGUAGES)
message ("Add translation '${LANG}' for target ${TARGET}")
set (XLF_FILE "${OUTPUT_DIR}/${TARGET}_${LANG}.xlf")
add_custom_command (
OUTPUT ${XLF_FILE}
COMMAND "${CMAKE_CURRENT_SOURCE_DIR}/scripts/cet2xlf.py" --language ${LANG} ${XLF_FILE} ${INPUT_FILES}
DEPENDS ${INPUT_FILES}
WORKING_DIRECTORY ${OUTPUT_DIR}
)
list(APPEND GENERATED_FILES ${XLF_FILE})
endforeach()
add_custom_target("${TARGET}_generate_translations" DEPENDS ${GENERATED_FILES})
add_dependencies(${TARGET} "${TARGET}_generate_translations")
endfunction()
Surely, one can add files with add_depencies()?
No, this is not possible. This is hinted at in the documentation of add_dependencies alongside hinting at the solution presented above:
add_dependencies( [<target-dependency>]...)
[...]
See the DEPENDS option of add_custom_target() and add_custom_command() commands for adding file-level dependencies in custom rules.