CMake: GCC preprocessor IMACROS file change does not trigger rebuild - gcc

Question

I'm using the -imacros option for GCC in order to set all preprocessor defines (Options) for a project.
Before imacros I have been using a raw file with the preprocessor defines names and with a regular expression in CMAKE I was creating the list of -D to put in the CMAKE_C_FLAGS.
This works fine but ugly to see in the text editor. So to enhance that, I have changed to -imacros.
CMAKE_C_FLAGS will contain -imacros "path to configuration header"
This works fine, but if I change some configuration item in the configuration header the CMAKE do not recompile the file (don't see changes). In the old version - as you can expect - if some -D was changed all the files will be recompiled.
Any help?

Answer 1

An simple Approach
You can use OBJECT_DEPENDS source file property. But that needs to be set for all source files with something like:
set_source_files_properties(
${sources}
PROPERTIES
OBJECT_DEPENDS "path to configuration header"
)
Alternatives for all Source Files in Project
Officially CMake recommends to put all your definitions in a header file that is included by all your source files. The header could e.g. be generated from a template using configure_file().
But to follow your line of thought with using -imacros compiler flag, here are two alternative approaches for triggering a rebuild of all source files if "path to configuration header" file changes:
You can extend the scope of OBJECT_DEPENDS to all targets and their source files in the current directory with define_property(... INHERITED ...):
If the INHERITED option then the get_property() command will chain up to the next higher scope when the requested property is not set in the scope given to the command. DIRECTORY scope chains to GLOBAL. TARGET, SOURCE, and TEST chain to DIRECTORY.
So in your case this translates to:
define_property(
SOURCE
PROPERTY OBJECT_DEPENDS
INHERITED
BRIEF_DOCS "brief-doc"
FULL_DOCS "full-doc"
)
set_directory_properties(
PROPERTIES
OBJECT_DEPENDS "path to configuration header"
)
If I understand correctly, you anyway have to re-run CMake if your "configuration header" should/would change. Then you can simply add one definition outside your "configuration header" that keeps track of the header with something like:
file(TIMESTAMP "path to configuration header" _timestamp)
add_definitions(-DIMACROS_TIMESTAMP=${_timestamp})
Now every time your header gets a new timestamp, the definitions for all targets are changing and your build system will rebuild all source files.