macos - Enable OpenMP support in clang in Mac OS X (sierra & Mojave)

Question

I am using Mac OS X Sierra, and I found that clang (LLVM version 8.1.0 (clang-802.0.38)) does not support OpenMP: when I run clang -fopenmp program_name.c, I got the following error:

clang: error: unsupported option '-fopenmp'

It seems that clang does not support -fopenmp flag.

I could not find any openmp library in homebrew. According to LLVM website, LLVM already supports OpenMP. But I could not find a way to enable it during compiling.

Does this mean that the default clang in Mac does not support OpenMP? Could you provide any suggestions?

(When I switch to GCC to compile the same program (gcc is installed using brew install gcc --without-multilib), and the compilation is successful.)

Answer 1

1. Try using Homebrew's llvm:

   brew install llvm
   

2. You then have all the llvm binaries in /usr/local/opt/llvm/bin.

   Compile the OpenMP Hello World program. Put omp_hello.c

   /******************************************************************************
        * FILE: omp_hello.c
        * DESCRIPTION:
        *   OpenMP Example - Hello World - C/C++ Version
        *   In this simple example, the master thread forks a parallel region.
        *   All threads in the team obtain their unique thread number and print it.
        *   The master thread only prints the total number of threads.  Two OpenMP
        *   library routines are used to obtain the number of threads and each
        *   thread's number.
        * AUTHOR: Blaise Barney  5/99
        * LAST REVISED: 04/06/05
        ******************************************************************************/
        #include <omp.h>
        #include <stdio.h>
        #include <stdlib.h>
   
        int main (int argc, char *argv[]) 
        {
        int nthreads, tid;
   
        /* Fork a team of threads giving them their own copies of variables */
        #pragma omp parallel private(nthreads, tid)
          {
   
          /* Obtain thread number */
          tid = omp_get_thread_num();
          printf("Hello World from thread = %d
   ", tid);
   
          /* Only master thread does this */
          if (tid == 0) 
            {
            nthreads = omp_get_num_threads();
            printf("Number of threads = %d
   ", nthreads);
            }
   
          }  /* All threads join master thread and disband */
   
        }
   

   in a file and use:

   /usr/local/opt/llvm/bin/clang -fopenmp -L/usr/local/opt/llvm/lib omp_hello.c -o hello
   

   You might also have to set the CPPFLAGS with -I/usr/local/opt/llvm/include.

   The makefile should look like this:

   CPP = /usr/local/opt/llvm/bin/clang
   CPPFLAGS = -I/usr/local/opt/llvm/include -fopenmp
   LDFLAGS = -L/usr/local/opt/llvm/lib
   
   omp_hello: omp_hello.c
        $(CPP) $(CPPFLAGS) $^ -o $@ $(LDFLAGS)
   

Update

In macOS 10.14 (Mojave) you might get an error like

/usr/local/Cellar/llvm/7.0.1/lib/clang/7.0.1/include/omp.h:118:13: fatal error: 'stdlib.h' file not found

If this happens, the macOS SDK headers are missing from /usr/include. They moved into the SDK itself with Xcode 10. Install the headers into /usr/include with

open /Library/Developer/CommandLineTools/Packages/macOS_SDK_headers_for_macOS_10.14.pkg