CMake run custom command before build?

Question

Example source of a binary I want to run before each build, once per add_executable:

#include <stdio.h>

int main(int argc, char *argv[]) {
    for(int i=0; i<argc; ++i)
        printf("argv[%d] = %s\n", i, argv[i]);
    fclose(fopen("foo.hh", "a"));
}

CMakeLists.txt

cmake_minimum_required(VERSION 3.5)
project(foo_proj)

set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++14")
set(SOURCE_FILES main.cpp)
# ---- this line changes ----
add_executable(foo_proj ${SOURCE_FILES})

Attempts:

add_custom_target(create_foo_hh COMMAND /tmp/bin/create_foo_hh)
add_dependencies(${SOURCE_FILES} create_foo_hh)

Error:Cannot add target-level dependencies to non-existent target "main.cpp". The add_dependencies works for top-level logical targets created by the add_executable, add_library, or add_custom_target commands. If you want to add file-level dependencies see the DEPENDS option of the add_custom_target and add_custom_command commands.

execute_process(COMMAND /tmp/bin/create_foo_hh main.cpp)

No error, but foo.hh isn't created.

How do I automate the running of this command?

Answer 1

execute_process() is invoked at configuration time.

You can use add_custom_command():

add_custom_command(
  OUTPUT foo.hh
  COMMAND /tmp/bin/create_foo_h main.cpp
  DEPENDS ${SOURCE_FILES} /tmp/bin/create_foo_hh main.cpp
  WORKING_DIRECTORY ${CMAKE_CURRENT_BINARY_DIR}
)
include_directories(${CMAKE_CURRENT_BINARY_DIR})
add_executable(foo_proj ${SOURCE_FILES} foo.hh)

That way, foo.hh is a dependency of foo_proj: and your command will be invoked when building foo_proj. It depends on ${SOURCE_FILES} and /tmp/bin/create_foo_hh main.cpp so that it is generated again if one of those files changes.

Regarding paths, add_custom_command() is configured to run in the current build directory to generate the file there, and include_directories() is used to add the build directory to the include dirs.