macos-openmp.md

Using OpenMP on macOS

If you try to compile a C program with gcc -fopenmp on macOS, you might get the following error:

clang: error: unsupported option '-fopenmp'

This is because the gcc command on macOS actually runs clang, a different C compiler, and for some reason Apple has disabled OpenMP support. The best way to solve this problem is to install a proper version of GCC.

• First, install the Homebrew package manager as discussed here.

• Then run brew install gcc to install GCC.

This will not change the default gcc command. We don't recommend you change it, since other Mac software might expect the default to be in place. You can use the command gcc-14 to run the installed version of GCC. That is, to compile a program, run:

$ gcc-14 foo.c -o foo -fopenmp

In the Makefiles for the code handout, you should change the definition of CC to be gcc-14 instead of gcc. Now do you see the value of defining variables for this?

If the above doesn't work

First, come see TA Rasmus in room 1-0-14 (DIKU) during exercise hours. If you want to debug your self, do the following:

Make sure the X-code command line tools are installed:

$ xcode-select --install

Then install gcc-14 using homebrew and follow the guide above. If this still doesn't work, try the following:

Then prefix every build command with xcrun, e.g.:

$ xcrun make
$ xcrun xcode-select --install

To compile with xcrun + gcc-14 you can use:

$ export CC=gcc-14
$ xcrun make