CMake dynamically generated header dependencies

Question

I have two static libraries : libfoo and libbar, each one being in its own directory.

libfoo contains a includes/ directory which contains a single file A.h. It also contains a sources/ directory which contains B.gen. A custom rule should create a new header B.h and a new source file B.c.

libbar contains a includes/ directory which contains a single file C.h. It also contains a sources/ directory which contains C.c. This file include the A.h and the B.h from libfoo.

The question is : according to these rules, how can I write the CMakeFiles.txt of the two libraries (from my understanding, the tricky part is to have all the libfoo headers in the same directories, but maybe I'm misleading) ?

File hierarchy :

/
  libfoo/
    includes/
      A.h
    sources/
      A.c
      B.gen
    CMakeLists.txt
  libbar/
    includes/
      C.h
    sources/
      C.c
    CMakeLists.txt

Answer 1

You should not copy any generated files to the source directory. The separation between a source directory and a build directory is intentional and called out-of-source builds to not pollute the source directory during your build with generated files.

What you need to do in this case is:

1. Define a custom command which translates your B.gen to a B.h file using add_custom_command. This will generate the B.h inside the CMAKE_CURRENT_BINARY_DIR, which reflects the source directory your CMakeLists.txt is in relative to the project root CMakeLists.txt. It will look something like this:

   add_custom_command(OUTPUT B.h COMMAND my-gen B.gen B.h DEPENDS B.gen)

2. Add the folder where the file was generated to the include directories using the include_directories function. If all of this was done in a single CMakeLists.txt you could use include_directories(${CMAKE_CURRENT_BINARY_DIR})

The CMake dependency parser will now, once it wants to build C.c, analyze its contents and find out that B.h is required for building it. With the add_custom_command you have told it how to create that header and it will invoke your generator.

You should have a look at how out of source builds work in CMake to correctly understand the folder hierarchy.