CMake macOS Xcode Environment

Related: GCC / Clang header clash on macOS

When macOS, Xcode, or Command Line Tools upgrades, build directories (including for CMake-based projects) often need to be refreshed. If the user has set custom environment variables concerning Xcode, they may need to be updated as well. Here are some important environment variables and CMake variables to check if problems occur after upgrading.

For a simple CMake project on macOS, CMakeCache.txt might include:



Having multiple directories under /Library/Developer/CommandLineTools/SDKs and /Applications/ is fine, CMake can select the appropriate one. The user can select the SDK:

xcode-select --switch

The XCode versions that Homebrew targets can be found in xcode.rb.


If Homebrew GCC breaks after upgrading Xcode or Command Line Tools, try specifying an older SDK. Note that the SDK version corresponds to macOS version, not the XCode version. For example, if the latest SDK is MacOSX14.4.sdk, try using MacOSX13.3.sdk in “~/”:

export CC=gcc-13 CXX=g++-13 FC=gfortran-13

export SDKROOT=/Library/Developer/CommandLineTools/SDKs/MacOSX13.3.sdk/

and then source ~/ before running cmake with a fresh build directory.

If a CMake build step fails, try copy-pasting the command and removing the -isysroot portion of the command. This is a clear clue the older SDK is (at least temporarily) needed till Homebrew updates its GCC formula.

GCC will tell where included files are coming from by adding the gcc -H flag. This tells what to specify for environment variable SDKROOT.