How can I control build order in cmake without adding static dependencies?

Question

I'm using OpenGL Loader Generator to generate GL function loading code for my project. It generates gl_loader.h and gl_loader.c. Some of my other source files depend on gl_loader.h, but not all of them, so I need cmake to (re)run glLoadGen if necessary, before compiling the rest of the code, otherwise the build stops with errors because it's trying to compile a file that depends on gl_loader.h before gl_loader.h has been built. I have added the GENERATED property to gl_loader.h/.c but this isn't quite the problem the property is designed to solve and it didn't work.

In this case I can simply add an OBJECT_DEPENDS property to all of my source files, because I will hardly ever need to rerun glLoadGen. But in other situations this would cause too much unnecessary recompilation, such as when several C source and header files are generated by GObjectBuilder (gob2). Manually working out which C files depend on which generated header is impractical.

Another possibility in this case is to run glLoadGen at configure time instead of build time, but that wouldn't be appropriate for the other scenario with gob2.

Is there any other way to tell cmake that it has to run glLoadGen before compiling the other code?

Answer 1

This seems like the ideal scenario for the GENERATED source file property. It basically tells CMake to not worry about the source file not existing at configure time (when CMake runs); it will be available when it's needed at build time (when make/vc/etc. runs).

I'm not familiar with OpenGL or its Loader Generator, but if you use the OUTPUT form of add_custom_command to invoke glLoadGen, the GENERATED property is automatically applied to the output files:

add_custom_command(OUTPUT gl_loader.h gl_loader.c
                   COMMAND glLoadGen [whatever args are needed]
                   COMMENT "Generating gl_loader.h and gl_loader.c")
...
add_executable(MyExe ${OtherSources} gl_loader.h gl_loader.c)

WIth this setup, when you build MyExe, the custom command should execute first (if required) producing the appropriate sources (gl_loader.h/.c)