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macoscmakeversiontarget

Figuring out the target MacOS SDK version in CMake?

I have code that I want to conditionally compile depending on the target SDK version, so that I can use newer features if possible. This can be done by checking the __MAC_OS_X_VERSION_MIN_REQUIRED macro, which tells us the target SDK version, in the source code.

Typically, performing conditional compilation on that macro suffices, and everything is nice and good. However, when targeting a newer SDK version, I would like to use features from a new library (specifically the UniformTypeIdentifiers library) that is not available on an older SDK version. This means that the target_link_libraries() in CMake has to account for this, i.e. link the new library if targeting a newer SDK version, and skip the new library otherwise.

I can't seem to find any way to figure out the target SDK version in CMake. What is the proper way to do this?

Things I have tried:

  1. Checking CMAKE_OSX_DEPLOYMENT_TARGET. This flag is what CMake uses to pass the -mmacosx-version-min flag to clang, that makes it use a particular deployment target. However, it turns out that this may not necessarily be defined. When not defined, clang doesn't get the flag, but since clang is given something like -isysroot /Applications/Xcode-13.3.1.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX12.3.sdk, the framework sets __MAC_OS_X_VERSION_MIN_REQUIRED to something default (12.3), and so the source code is able to still perform conditional compilation.
  2. Checking if the library exists by using find_library() (e.g. find_library(UNIFORMTYPEIDENTIFIERS_LIBRARY UniformTypeIdentifiers)). This does not work when users want to target a lower version (e.g. by setting CMAKE_OSX_DEPLOYMENT_TARGET), since the new library will still exist.

Essentially, I would like a way for CMake to figure out the deployment target that ultimately gets chosen (i.e. something equivalent to __MAC_OS_X_VERSION_MIN_REQUIRED), so that it can decide whether or not to link with the new library.

How should I do this?

Context:

I'm trying to resolve an issue in my library here.

I also have a testing branch set up to check CMAKE_OSX_DEPLOYMENT_TARGET and the compiler command in the CI builds: GitHub CI builds MacOS 10.15 (without the new library, with CMAKE_OSX_DEPLOYMENT_TARGET) and 11 (with the new library, with CMAKE_OSX_DEPLOYMENT_TARGET), and Circle CI builds MacOS 12 (with the new library, but without CMAKE_OSX_DEPLOYMENT_TARGET).

Solution

  • I couldn't find a way to determine this via some CMake variable, so I rolled by own compilation test for it:

    include(CheckCXXSourceCompiles)
check_cxx_source_compiles(
  "
  #include <Availability.h>
  #if !defined(__MAC_OS_X_VERSION_MIN_REQUIRED) || !defined(__MAC_11_0) || __MAC_OS_X_VERSION_MIN_REQUIRED < __MAC_11_0
  static_assert(false);
  #endif
  int main() { return 0; }
  "
  NFD_USE_ALLOWEDCONTENTTYPES
)

    This ensures full consistency with the check in the source code, at the cost of a slightly longer time to configure the build.