How can I install openMP on my new MacBook Pro (with Mac OS Catalina)?

Question

I installed Xcode (and also the command line tools) but terminal says (when I'm compiling):

gcc -o task -fopenmp task.c
clang: error: unsupported option '-fopenmp'

I tried to install openmp via brew but people say that it's not available anymore on homebrew, they suggest to try

brew instal llvm

But I get the same error. I tried also in the boneyard

brew install homebrew/boneyard/clang-omp

but the repository doesn't exist anymore.

Could you help me? I just need to learn openMP, I don't think that is so difficult to install...

Thank you!

Kind regards,

Nico

Answer 1

To build with OpenMP support, you need to make sure you're not invoking Apple's clang from Xcode. Even if you install llvm or gcc via brew, you should try gcc -v and clang -v in your terminal session. Both likely invoke Apple's version.

You can use GNU gcc or LLVM; both are available via brew. If you use LLVM, you'll also need to install libomp .

GNU gcc (currently at version 9)

brew install gcc
gcc-9 -o task -fopenmp task.c

Note that you invoke this version of gcc explicitly by suffixing gcc with a - and the major version number, eg: gcc-9

LLVM

Brew installs LLVM as keg-only so that it doesn't conflict with Apple's version. You'll therefore need to make sure you invoke the right clang . You'll also need to specify where the libomp library is located.

brew install llvm libomp
`brew --prefix llvm`/bin/clang -L`brew --prefix`/lib -o task -fopenmp task.c

Answer 2

This https://iscinumpy.gitlab.io/post/omp-on-high-sierra/ suggest to do the following:

brew install libomp

To install the OpenMP runtime components/libraries.

Then, when compiling:

• use -Xpreprocessor -fopenmp in place of -fopenmp in the compile step ( -c option)
• add -lomp to the linking step

Note that the above page also mention that CMake 3.12 or later will automatically find the right way of adding OpenMP on MacOS:

cmake_minimum_required(VERSION 3.12)
project(openmptest CXX)

add_executable(sample sample.cpp)

find_package(OpenMP REQUIRED)
target_link_libraries(sample PRIVATE OpenMP::OpenMP_CXX)

Note: I didn't test any of this but it sounds relatively sane

Answer 3

brew install gcc

then type gcc in your terminal and hit the tab button twice.

you should see multiple gcc versions eg gcc gcc-10

then find out which one is from Homebrew eg type gcc-10 --version in the terminal should output something like this:

gcc-10 (Homebrew GCC 10.2.0_3) 10.2.0 Copyright (C) 2020 Free Software Foundation, Inc. This is free software; see the source for copying conditions. There is NO warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

Finally use that compiler to compile your eg:

gcc-10 -o task -fopenmp task.c