OpenMP stands for Open Multiprocessor. It allows you to create C/C++ and Fortran code into something that can be parallelized. Note that this is different from just concurrency.

• Apple’s default Clang: No OpenMP support.
• Homebrew’s Clang: Has OpenMP support and is more configurable for projects that need parallelization tools like OpenMP.

Since we have a Mac the default C compiler is LLVM's Clang Compiler. However, OpenMP is not directly supported in this default version.

Therefore, we must install a different version using Home brew:

brew install llvm

You can check which version of Clang you have by running the following:

clang --version

You know you are using the default version of Clang if your output looks like this:

Apple clang version 13.0.0 (clang-1300.0.29.30)
Target: x86_64-apple-darwin20.3.0
Thread model: posix
InstalledDir: /Library/Developer/CommandLineTools/usr/bin

Once you have Homebrew's Clang you can temporarily set it as your default clang compiler. This will be set back to the default after closing the terminal.

export PATH="/opt/homebrew/opt/llvm/bin:$PATH"

After which if you run clang --version again, your output should look like this:

Homebrew clang version 18.1.8
Target: arm64-apple-darwin23.6.0
Thread model: posix
InstalledDir: /opt/homebrew/opt/llvm/bin

We can check that everything is working by running the following program:

int main() {
  int ID = 0;
  printf("hello(%d)", ID);
  printf(" world(%d) \n", ID);
}

compile it using this:

clang your_file.c -o output_file

Now that we know that Clang is working we can now install OpenMP

brew install libomp

#include <stdio.h>
#include <omp.h>

int main() {
    #pragma omp parallel
    {
        int thread_id = omp_get_thread_num();
        printf("Hello from thread %d\n", thread_id);
    }
    return 0;
}

To compile this program with OpenMP support, you need to use the -fopenmp flag and tell Clang where to find the OpenMP library.

clang -fopenmp -L/opt/homebrew/opt/libomp/lib -I/opt/homebrew/opt/libomp/include -o omp_program omp_program.c

• -fopenmp: Enables OpenMP support.
• -L/opt/homebrew/opt/libomp/lib: Specifies the library path for OpenMP.
• -I/opt/homebrew/opt/libomp/include: Specifies the include path for OpenMP headers.

if you are using the default Clang instead of Homebrew's clang you will probably get an output like this:

clang: error: unsupported option '-fopenmp'
clang: error: unsupported option '-fopenmp'

We can temporarily specify the library path and headers paths using:

export LDFLAGS="-L/opt/homebrew/opt/libomp/lib"
export CPPFLAGS="-I/opt/homebrew/opt/libomp/include"

And now running our program should look like this:

clang -fopenmp our_program.c -o our_program