pkg_check_modules cannot find *.pc.cmake

Question

I am using a 3rd party library rbdl, which contains rbdl.pc.cmake, which 'I suppose' is included for using pkg_check_modules in a cmake file.

I update PKG_CONFIG_PATH to point at the rbdl folder

set(ENV{PKG_CONFIG_PATH} "$ENV{PKG_CONFIG_PATH}:${CMAKE_BINARY_DIR}/externals/rbdl")
pkg_check_modules(RBDL rbdl)

but pkg_check_modules says it cannot find the module.

When I manually duplicate rbdl.pc.cmake, rename the copy into rbdl.pc and run pkg-config --cflags --libs rbdl in terminal, then my cmake also start working!

Interestingly, now even if I delete rbdl.pc, rbdl module if perfectly found by rbdl.pc.cmake!

So my questions are:

1. What is the difference between *.pc and *.pc.cmake?
2. How do I correctly setup my cmake to work with original rbdl.pc.cmake?
3. Why rbdl.pc.cmake starts to be accepted by pkg_check_modules after that tweak with duplicating it, renaming the copy and running pkg-config manually?

Answer 1

You understand it wrong! rbdl.pc.cmake is just a template file. It is not supposed to be used by you! Take a look into CMakeLists.txt line 160 -- configure_file() used to render variables ("quoted" by @ in template file) and produce a rbdl.pc (a real pkg-config) file. Latter should be installed (some way) and then will be available to pkg-config hence can be used in your project.

pkg-config is stupid do not interpret or validate compiler/linker flags any way, so your renamed file "works" (yeah, producing invalid command line for compiler/linker).

I wish you to read CMake documentation before trying to code something using it! It'll save your time and give you a necessary knowledge which stops you from doing stupid things like you described in your question ;-)