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

macos clang llvm openmp

59,275

Solution 1

Try using Homebrew's llvm:
```
brew install llvm
```

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\n", tid);

       /* Only master thread does this */
       if (tid == 0) 
         {
         nthreads = omp_get_num_threads();
         printf("Number of threads = %d\n", 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

Solution 2

Other people have given one solution (using Homebrew llvm). You can also use OpenMP with Apple Clang and Homebrew libomp (brew install libomp). Just replace a command like clang -fopenmp test.c with clang -Xpreprocessor -fopenmp test.c -lomp.

Solution 3

MacOS Mojave with CMake

Install LLVM with openmp and libomp with brew
```
 brew update
 brew install llvm libomp
```

add include directories and link directories in CMakeList.txt

 include_directories("/usr/local/include" "/usr/local/opt/llvm/include")
 link_directories("/usr/local/lib" "/usr/local/opt/llvm/lib")

run CMake with the new compilers

 cmake -DCMAKE_C_COMPILER="/usr/local/opt/llvm/bin/clang" -DCMAKE_CXX_COMPILER="/usr/local/opt/llvm/bin/clang++" ..

The clang version is 7.0.1 at time of writing

59,275

Starry

Updated on September 16, 2021

Comments

Starry over 2 years

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.)
- Anton Korobeynikov about 7 years
  
  Indeed, Apple-provided clang does not support OpenMP.
- cbrnr almost 7 years
  
  brew install llvm should install the latest LLVM version, i.e. 4.0.0. Does this fix the problem?
- Increasingly Idiotic about 6 years
  
  Apple-provided clang does not support OpenMP by default. It is possible to enable the feature in Apple-provided clang and also possible to install a more recent version of clang that does support OpenMP by default.
- MarcusJ about 5 years
  
  @IncreasinglyIdiotic How do we enable it? do we just need to compile and install the openmp runtime?
- Increasingly Idiotic about 5 years
  
  @MarcusJ you should just need to brew install llvm libomp and then make sure to use the new clang to compile with the -fopenmp flag
Increasingly Idiotic about 6 years

You may also need brew install libomp
Ryan H. almost 5 years

Indeed, with llvm@8, I had to install libomp
Jacob almost 5 years

After trying to build Blender on macOS (which has libomp as a dependency) for 3 days, this is the solution that worked for me, though my symptoms were a bit different. For me the compilation completed, but there were linker errors about missing x86_64 symbols from the libomp library. This worked for Homebrew's GCC 9.1.0 as of this comment.
Py-ser almost 5 years

Where is the CMakelists.txt ? I have at least 6 files named this way.
SBlincov over 4 years

Thank you so much! It works for me in Clion and Mac OS 10.14.6
rgov about 4 years

An improvement on this answer would be to use brew --prefix llvm rather than assuming /usr/local/opt/llvm. Something like execute_process(COMMAND brew --prefix llvm OUTPUT_VARIABLE LLVM_PREFIX)
Walter almost 4 years

I tried that, but it wouldn't find omp.h for inclusion.
Yongwei Wu almost 4 years

If you are sure you have install libomp under Homebrew, you can ask on the Homebrew forum discourse.brew.sh. In my case, omp.h is installed under /usr/local/Cellar/libomp/10.0.0/include, symlinked to /usr/local/include.
user76284 almost 4 years

Is it known why Apple clang requires the -Xpreprocessor flag?
Dmitry Grebenyuk over 3 years

You can add set(CMAKE_C_COMPILER "/usr/local/opt/llvm/bin/clang") set(CMAKE_CXX_COMPILER "/usr/local/opt/llvm/bin/clang++") in CMakeList.txt, so you can run cmake without -D
Dmitry Grebenyuk over 3 years

This is a more modern solution stackoverflow.com/a/51448364/7604852
rocksNwaves about 3 years

This resulted in a "missing separator" error for me. Apparently, that means tabs have to be "hard". So I found the "soft" tab and replaced it, and then I got the error: *** No rule to make target omp_hello.c', needed by omp_hello'. Stop. Damned if I do, damned if I don't.
Ed. about 3 years

@rocksNwaves The instructions given here are how to compile a file called omp_hello.c. Make is telling you that file doesn't exist and it doesn't know what to do. Solution: provide that file.
Dirk about 3 years

@Ed. Thanks for noting, I included the file in my answer.
Rob almost 3 years

This worked for me (macOS 11.4). For future readers, if you that have Xcode installed, you might need to do xcode-select --install to install the command line utilities. When you do brew install llvm, it will let you know if you need to do this.