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macosclangllvmopenmp
Starry
Starry

Reputation: 805

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

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.)

Upvotes: 56

Views: 79104

Answers (4)

Dirk
Dirk

Reputation: 2425

  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\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

Upvotes: 40

merv
merv

Reputation: 76700

Conda-Based Compilation Environment

Conda uses clang for OSX compilation (umbrella package cxx-compiler), but I hit similar issues with using llvm-openmp and the -fopenmp flag throwing errors. Solution is rather similar to other answers, but I am including here in case others have more exactly this issue.

Specific solution was to include the Conda environment's include/ directory in the CFLAGS, namely:

CFLAGS="-I${CONDA_PREFIX}/include"

Note, I also needed to add -lstdc++ -Wl,-rpath ${CONDA_PREFIX}/lib -L${CONDA_PREFIX}/lib when linking, similar to this GitHub Issue.

Upvotes: 1

Yongwei Wu
Yongwei Wu

Reputation: 5582

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.

Upvotes: 33

Bruno Grieder
Bruno Grieder

Reputation: 29814

MacOS Mojave with CMake

  1. Install LLVM with openmp and libomp with brew

     brew update
 brew install llvm libomp

  2. 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")

  3. 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

Upvotes: 16

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