How can I tell, within a CMakeLists.txt, whether it's used with add_subdirectory()? [duplicate]

Question

I have this code project can be built either independently or as a subproject of a larger repository (checking it our as a sub-repository). In the latter case, I have a top-level CMakeLists.txt for the main project which has

add_subdirectory(${MY_SUBPROJ_SUBDIR})

now, I want the subproject to behave somewhat differently in case it's used via the add_directory(). Obviously, I would be using a large if instruction. But what condition do I check? How can CMake "tell", when running for some CMakeLists.txt, whether it's a subdir file or the main file?

Pankey

Answer 1

1. After the project() call in project's CMakeLists.txt and in the project's subdirectories you may use:

    if(CMAKE_SOURCE_DIR STREQUAL PROJECT_SOURCE_DIR)
        # I am top-level project.
    else()
        # I am called from other project with add_subdirectory().
    endif()
   

   NOTE: As stated in the comments, this approach may to not work on Windows, where CMAKE_SOURCE_DIR contains lower-case version of PROJECT_SOURCE_DIR and thus cannot be compared directly for equality. For that case approach with checking PARENT_DIRECTORY property, as described in that answer to the duplicate question, seems to be more robust.

2. Alternative for use before the project() call:

    if(CMAKE_SOURCE_DIR STREQUAL CMAKE_CURRENT_SOURCE_DIR)
        # I am top-level project.
    else()
        # I am called from other project with add_subdirectory().
    endif()
   

   This alternative can also be used anywhere in project's CMakeLists.txt (but not in subdirectories).

---

Assume you have a project A with two CMakeLists.txt: one in the project's directory, and one in subdirectory src/. Scheme for use approaches described above:

CMakeLists.txt:

cmake_minimum_required(...)
...
<only approach *2* can be used there>
...
project(A)
...
<approach *1* or *2* can be used there>
...
add_subdirectory(src)
...

src/CMakeLists.txt:

...
<only approach *1* can be used there>
...

With given scheme project A may detect, whether it is built standalone (top-level project) or as a part of another project B.

Mccracken

Sounds reasonable. – Pankey

Just a hint: When using PROJEC_SOURCE_DIR it means CMake has already parsed your sub-directories project() command. Alternatively you can find if(CMAKE_SOURCE_DIR STREQUAL CMAKE_CURRENT_SOURCE_DIR) (see e.g. here or here), – Cynthiacynthie

@Florian: I wanted to write the same, but was interrupted. Now the post is updated. Thanks. – Mccracken

The repositories shouldn't really make assumptions about whether or not we will even have a project call, or in which order. – Pankey

None project has assumptions about its caller. By project() call I mean the call from the project itself, not from the callers. I have edited the post, probably you find it more clear. – Mccracken

I needed to use CMAKE_CURRENT_SOURCE_DIR, not CMAKE_SOURCE_DIR – Lumpen

@Lumpen That will always return true in the same file as the project() call. – Apprehensible

This doesn't work on Windows. CMAKE_SOURCE_DIR expands to c:/users/..., but PROJECT_SOURCE_DIR expands to C:/Users/.... Instead of STREQUAL, one has to do a case insensitive comparison (convert both strings to lowercase, then do STREQUAL) – Apprehensible

string(TOLOWER ${CMAKE_SOURCE_DIR} source_dir) string(TOLOWER ${CMAKE_CURRENT_SOURCE_DIR} current_source_dir) if(${source_dir} STREQUAL ${current_source_dir}) message(STATUS "not in subdirectory") else() message(STATUS "in subdirectory") endif() – Chiarra

Newer CMake provides PROJECT_IS_TOP_LEVEL variable – Aegina

@MariuszJaskółka: This variable is already described in that answer to the duplicate question. – Mccracken