CMake: execute a macro/function as the command of add_custom_command

Question

I'm using an external library which provides a CMake function for automatic code generation, to be used in my CMakeLists. The problem is that whenever I modify a CMakeLists then the function is run again, triggering the recompilation of the newly generated but unchanged sources. I'd need something like add_custom_command with the possibility to specify the CMake function as COMMAND instead of an executable, so that the function is run only if the automatically generated files are not already present. Is this feasible? If not, does it exist another way to obtain the same result? Thanks.

Answer 1

To prevent that function to run, just wrap it into if:

if(NOT EXISTS ${CMAKE_BINARY_DIR}/blah-blah/generated.cpp)
   run_your_provided_command(BLAH_BLAH)
endif()

Easy!

Update: To run it when config file has changed just use little more complicated condition:

if(
   NOT EXISTS ${CMAKE_BINARY_DIR}/blah-blah/generated.cpp OR
   ${CMAKE_SOURCE_DIR}/blah-blah.config IS_NEWER_THAN ${CMAKE_BINARY_DIR}/blah-blah/generated.cpp
  )
...

and use add_dependencies command to make sure your binary will be rebuild in case of config file modifications:

add_executable(
    YourBinary
    ...
    ${CMAKE_BINARY_DIR}/blah-blah/generated.cpp
  )
add_dependencies(YourBinary ${CMAKE_SOURCE_DIR}/blah-blah.config)

Answer 2

Take a look to this SO post.

You can call your function in a separate CMake script, call this script with add_custom_target and cmake -P then add a dependency to your binary :

add_custom_target(run_script COMMAND ${CMAKE_COMMAND} -P separate_script.cmake)
add_executable(your_binary ...)
# or add_library(your_binary ...)
add_dependencies(your_binary run_script)

Answer 3

Is there a way to pass a parameter to the separate_script.cmake?

You can use the cmake variables to pass values when you call the script e.g.

"COMMAND ${CMAKE_COMMAND} -DPARAM=value -P separate_script.cmake"