[SOLVED] CMake - Alternate option to FetchContent when the source file already exists locally

CMake - Alternate option to FetchContent when the source file already exists locally

I tried building the ORTools github package locally using cmake and it builds without any errors. However the environment which we are planning to ultimately use does not have an outbound network connection. The problem I see is that https://github.com/google/or-tools/blob/v9.4/cmake/dependencies/CMakeLists.txt performs a Git Clone to download the dependencies and add them. Since there is no outbound network access this step fails and I'm unable to build the dependency. To circumvent this we are planning to manually download the dependencies and add them to https://github.com/google/or-tools/blob/v9.4/cmake/dependencies/ folder. I'm pretty new to CMake and I'm not sure what changes have to be made to accommodate this.

For example, what would the following snippet need to be changed to if I cloned Zlib v1.2.11 repository and added it to https://github.com/google/or-tools/blob/v9.4/cmake/dependencies/?

# ##############################################################################
# ZLIB
# ##############################################################################
if(BUILD_ZLIB)
  message(CHECK_START "Fetching ZLIB")
  list(APPEND CMAKE_MESSAGE_INDENT "  ")
  FetchContent_Declare(
    zlib
    GIT_REPOSITORY "https://github.com/madler/ZLIB.git"
    GIT_TAG "v1.2.11"
    PATCH_COMMAND git apply --ignore-whitespace "${CMAKE_CURRENT_LIST_DIR}/../../patches/ZLIB.patch")
  FetchContent_MakeAvailable(zlib)
  list(POP_BACK CMAKE_MESSAGE_INDENT)
  message(CHECK_PASS "fetched")
endif()

Can FetchContent_Declare be used to point to a directory that already contains the source files? What's the alternative?

Solution

You could specify SOURCE_DIR parameter for FetchContent_Declare and omit download options:

FetchContent_Declare(
    zlib
    SOURCE_DIR <path/to/existing/directory>
    PATCH_COMMAND git apply --ignore-whitespace "${CMAKE_CURRENT_LIST_DIR}/../../patches/ZLIB.patch"
)

This works in the same way as in ExternalProject_Add command, which options are accepted by FetchContent_Declare.