CMake and external dependency

I'd like to add an external dependency to my project. The one I'm trying to add is the Leptonica library as a submodule.

My project has the following directory structure:

In my root CMakeLists.txt file I added
ADD_SUBDIRECTORY($ROOT_DIR/leptonica)

CMakeLists.txt

Unfortunately, CMake is not searching for Configure.cmake in the proper directory:

Configure.cmake

CMake Error at leptonica/CMakeLists.txt:107 (include): include could not find load file: Configure CMake Error: File <root>/cmake/templates/LeptonicaConfig-version.cmake.in does not exist. CMake Error at leptonica/CMakeLists.txt:113 (configure_file): configure_file Problem configuring file

When I build the project by myself, everything goes fine. In my opinion, the problem is with CMAKE_SOURCE_DIR. When using add_subdirectory it has the value of ROOT CMake instead ROOT/leptonica, so it's searching the wrong paths - as you can see in Leptonica CMake, it's used to determinate paths of its files.

CMAKE_SOURCE_DIR

add_subdirectory

ROOT

ROOT/leptonica

What should be the proper way to fix this - should I set CMAKE_SOURCE_DIR to ROOT/leptonica just before calling add_subdirectory and set it back when it's finished, or does some other, more elegant solutions exist?

CMAKE_SOURCE_DIR

ROOT/leptonica

add_subdirectory

2 Answers
2

Not every CMake project is suitable for inclusion via add_subdirectory.

add_subdirectory

Among those are projects which uses CMAKE_SOURCE_DIR or CMAKE_BINARY_DIR variables.

CMAKE_SOURCE_DIR

CMAKE_BINARY_DIR

However, inclusion via ExternalProject_Add (optionally wrapped with execute_process) always works.

ExternalProject_Add

execute_process

Modifying variable CMAKE_SOURCE_DIR (and CMAKE_BINARY_DIR too) is a bad idea: this variable should be changed only by CMake itself. Otherwise you may get weird errors.

CMAKE_SOURCE_DIR

CMAKE_BINARY_DIR

Instead, you may replace (automatically, with some script) all references to the variable with another variable, which is not used in the project. This new variable you may safely set before stepping into the subproject.

$CMAKE_SOURCE_DIR and $CMAKE_BINARY_DIR are set relative to the top-level CMakeLists.txt. If you need something relative to your current CMakeLists.txt (leptonica), use $CMAKE_CURRENT_SOURCE_DIR and $CMAKE_CURRENT_BINARY_DIR.

$CMAKE_SOURCE_DIR

$CMAKE_BINARY_DIR

$CMAKE_CURRENT_SOURCE_DIR

$CMAKE_CURRENT_BINARY_DIR

If you're having trouble finding a cmake file like LeptonicaConfig-version.cmake.in, try appending the appropriate directory to $CMAKE_MODULE_DIR.

LeptonicaConfig-version.cmake.in

$CMAKE_MODULE_DIR

list(APPEND $CMAKE_MODULE_DIR $CMAKE_CURRENT_SOURCE_DIR/cmake/templates)

I prefer to use $CMAKE_CURRENT_SOURCE_DIR over $CMAKE_SOURCE_DIR any day because using the latter will break your build if you try to integrate it into a super-build later. If I need to pass my current top-level directory to subdirectories, then I do the following and use that later down the chain.

$CMAKE_CURRENT_SOURCE_DIR

$CMAKE_SOURCE_DIR

set( LEPTONICA_SOURCE_DIR $CMAKE_CURRENT_SOURCE_DIR )

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