How do I detect OpenMP support in an R package?

It depends on the target operating system

Most operating systems, including Linux and Windows

In your Makevars, set the PKG_CFLAGS and PKG_LIBS using the Make macros described in Writing R Extensions. If the OpenMP support was detected and enabled during R configuration, your package will use a compatible OpenMP runtime. If not (e.g. disabled by system administrator), OpenMP support will be disabled, so make sure to use #ifdef _OPENMP (or other tests appropriate for your language).

macOS

OpenMP isn't really supported by the Apple toolchain, but with clever hacks you can get it to work. R does not detect OpenMP support by default, so the Make macros described above will be empty.

Nevertheless, starting with version 4.3, R for macOS provides an OpenMP runtime and a way for packages to opt into using it on the CRAN package builder:

the main part is to detect OpenMP even if R doesn't enable it (possibly as an option?) and on macOS also try -Xclang in front of the regular -fopenmp to see if it works

(In addition to testing for -Xclang -fopenmp, the package also needs to link to the OpenMP runtime using the -lomp linker flag.)

This package demonstrates how to achieve that. It is important to test multiple configurations on macOS to make sure that installing from source in weird cases (e.g. custom toolchain that understands -fopenmp) will still work.

As of 2025, newer Xcode versions conflict with the OpenMP runtime shipped with R. Use schedule(dynamic) to check for it and avoid accidentally building a package shared library that will later fail to load. Installing a different version of the OpenMP runtime breaks compatibility with CRAN binaries, so it's up to the administrator to set SHLIB_OPENMP_CFLAGS = -Xclang -fopenmp -Wl,/usr/local/lib/libomp.dylib (untested) and compile all packages from source.

Contents

src/Makevars.win

Adds $(SHLIB_OPENMP_CFLAGS) to the compiler and linker flags on Windows. R will warn about the package having a configure script for Unix-alikes but not for Windows, but we already have src/Makevars.win pre-configured.

See WRE 1.1.5 Package subdirectories for more information.

configure

On Unix-alikes, configure is required to be a POSIX shell script. In theory, it could immediately delegate to an R script, but we will only use it for compile-testing; the rest of the script is more laconic when written in POSIX sh.

A good configure leaves a config.log with detailed information for debugging. POSIX echo is not guaranteed to understand -n or escape characters, so we'll use printf instead.

On macOS (whose uname calls it "Darwin"), we call the compile-test function with different arguments, until one succeeds, or until we reach the last case, which leaves the OpenMP variables empty. On other operating systems, we rely on R-provided flags unconditionally.

The compile-test function runs R in order to dyn.load() the resulting shared library and run code from it. It tests the OpenMP support as completely as it can:

1. Call R CMD SHLIB to compile and link a shared library from the test C file.
2. Load the resulting shared library and run an OpenMP loop from it.

This aims to trigger all of compilation-time, linking-time, loading-time, and runtime problems before they prevent the real package from being installed.

The last step is the autotools-style text replacement that takes the src/Makevars.in file and creates the src/Makevars from it for R CMD SHLIB to consume.

src/Makevars.in

This prototype for src/Makevars contains templates for compiler and linker flags that the configure script will substitute.

cleanup

For best results, configure should be paired with a cleanup script, which removes all files that configure may have created, although it's prudent to also list them in .Rbuildignore. In our case, these files are src/Makevars and config.log.

Bonus points: passing through PKG_CFLAGS, PKG_LIBS

The OpenMP on macOS page recommends:

How you do the latter depends on the package, but if the package does not set these environment variables itself, you can try

PKG_CPPFLAGS='-Xclang -fopenmp' PKG_LIBS=-lomp R CMD INSTALL myPackage

Since our Makevars file sets PKG_CPPFLAGS and PKG_LIBS, Make won't read them from the environment. Solution: read them in the configure script and write them into Makevars, in addition to other variables being set, using the same string substitution as the OpenMP flags. For maximum compatibility, test a configuration with no added compiler or linker flags, making it possible for the user to override OpenMP configuration.

Results

macOS 26.2, R-devel, Apple clang 17.0.0 (macOS builder, R-devel)

* installing *source* package ‘ompdetect’ ...
** this is package ‘ompdetect’ version ‘0.2-0’
** using staged installation
Variables from the environment: PKG_CFLAGS='' PKG_LIBS=''
Guessing OpenMP configuration for macOS
* checking if OpenMP works with PKG_CFLAGS='' PKG_LIBS=''... failed to run
* checking if OpenMP works with PKG_CFLAGS='-Xclang -fopenmp' PKG_LIBS='-lomp'... yes
Adding PKG_CFLAGS='-Xclang -fopenmp', PKG_LIBS='-lomp'
** libs
using C compiler: ‘Apple clang version 17.0.0 (clang-1700.6.3.2)’
using SDK: ‘MacOSX14.4.sdk’
clang -arch arm64 -std=gnu23 -I"/Library/Frameworks/R.framework/Resources/include" -DNDEBUG   -I/opt/R/arm64/include   -Xclang -fopenmp -fPIC  -falign-functions=64 -Wall -g -O2  -c test_omp.c -o test_omp.o
clang -arch arm64 -std=gnu23 -dynamiclib -Wl,-headerpad_max_install_names -undefined dynamic_lookup -L/Library/Frameworks/R.framework/Resources/lib -L/opt/R/arm64/lib -o ompdetect.so test_omp.o -lomp -F/Library/Frameworks/R.framework/.. -framework R
installing to /Volumes/PkgBuild/work/1767639495-8aa9b6af8ed5d0a9/packages/sonoma-arm64/results/4.6/ompdetect.Rcheck/00LOCK-ompdetect/00new/ompdetect/libs

> ### ** Examples
>
>   ompdetect()
OpenMP detected and working.
>   omplimits()
thread_limit  max_threads    num_procs
  2147483647            8            8

macOS 15.7.2, R-4.5.2, Apple clang 17.0.0 (GitHub Actions)

The compiler is too new for the R-bundled OpenMP runtime, and the resulting shared object compiles and links successfully, but fails to load. This can happen in GitHub Actions without extra preparations.

* installing *source* package ‘ompdetect’ ...
** this is package ‘ompdetect’ version ‘0.2-0’
** using staged installation
Variables from the environment: PKG_CFLAGS='' PKG_LIBS=''
Guessing OpenMP configuration for macOS
* checking if OpenMP works with PKG_CFLAGS='' PKG_LIBS=''... failed to run
* checking if OpenMP works with PKG_CFLAGS='-Xclang -fopenmp' PKG_LIBS='-lomp'... failed to build
* checking if OpenMP works with PKG_CFLAGS='$(SHLIB_OPENMP_CFLAGS)' PKG_LIBS='$(SHLIB_OPENMP_CFLAGS)'... failed to run
* checking if OpenMP works with PKG_CFLAGS='-Xclang -fopenmp' PKG_LIBS='/usr/local/lib/libomp.dylib'... failed to build
Couldn't guess a working OpenMP configuration.
Do you need an OpenMP runtime and headers from <https://mac.r-project.org/openmp/>?
Adding PKG_CFLAGS='', PKG_LIBS=''
** libs
using C compiler: ‘Apple clang version 17.0.0 (clang-1700.0.13.5)’
using SDK: ‘MacOSX15.5.sdk’
clang -arch arm64 -std=gnu2x -I"/Library/Frameworks/R.framework/Resources/include" -DNDEBUG   -I/opt/R/arm64/include    -fPIC  -falign-functions=64 -Wall -g -O2  -c test_omp.c -o test_omp.o
clang -arch arm64 -std=gnu2x -dynamiclib -Wl,-headerpad_max_install_names -undefined dynamic_lookup -L/Library/Frameworks/R.framework/Resources/lib -L/opt/R/arm64/lib -o ompdetect.so test_omp.o -F/Library/Frameworks/R.framework/.. -framework R
installing to /Users/runner/work/_temp/Library/00LOCK-ompdetect/00new/ompdetect/libs

$ Rscript -e 'ompdetect::ompdetect(); ompdetect::omplimits()'
OpenMP not detected.
thread_limit  max_threads    num_procs
          -1           -1           -1

macOS 15.7.2, R-4.5.2, Apple clang 17.0.0, compatible OpenMP runtime in /usr/local

Same as above, but with a compatible OpenMP runtime installed:

* installing *source* package ‘ompdetect’ ...
** this is package ‘ompdetect’ version ‘0.2-0’
** using staged installation
Variables from the environment: PKG_CFLAGS='' PKG_LIBS=''
Guessing OpenMP configuration for macOS
* checking if OpenMP works with PKG_CFLAGS='' PKG_LIBS=''... failed to run
* checking if OpenMP works with PKG_CFLAGS='-Xclang -fopenmp' PKG_LIBS='-lomp'... failed to run
* checking if OpenMP works with PKG_CFLAGS='$(SHLIB_OPENMP_CFLAGS)' PKG_LIBS='$(SHLIB_OPENMP_CFLAGS)'... failed to run
* checking if OpenMP works with PKG_CFLAGS='-Xclang -fopenmp' PKG_LIBS='/usr/local/lib/libomp.dylib'... yes
Since an earlier test for -lomp failed, the resulting package is probably unsafe to mix with CRAN binaries!
Adding PKG_CFLAGS='-Xclang -fopenmp', PKG_LIBS='/usr/local/lib/libomp.dylib'
** libs
using C compiler: ‘Apple clang version 17.0.0 (clang-1700.0.13.5)’
using SDK: ‘MacOSX15.5.sdk’
clang -arch arm64 -std=gnu2x -I"/Library/Frameworks/R.framework/Resources/include" -DNDEBUG   -I/opt/R/arm64/include   -Xclang -fopenmp -fPIC  -falign-functions=64 -Wall -g -O2  -c test_omp.c -o test_omp.o
clang -arch arm64 -std=gnu2x -dynamiclib -Wl,-headerpad_max_install_names -undefined dynamic_lookup -L/Library/Frameworks/R.framework/Resources/lib -L/opt/R/arm64/lib -o ompdetect.so test_omp.o /usr/local/lib/libomp.dylib -F/Library/Frameworks/R.framework/.. -framework R
installing to /Users/runner/work/_temp/Library/00LOCK-ompdetect/00new/ompdetect/libs

$ Rscript -e 'ompdetect::ompdetect(); ompdetect::omplimits()'
OpenMP detected and working.
thread_limit  max_threads    num_procs
  2147483647            3            3

macOS 15.7.2, R-4.5.2, Apple clang version 16.0.0, OpenMP headers installed

Switching Xcode version to 16.2 makes it possible to use a compiler version compatible with the OpenMP runtime used by R-4.5.2. Installing the omp.h header is still necessary as it's not bundled with R.

* installing *source* package ‘ompdetect’ ...
** this is package ‘ompdetect’ version ‘0.2-0’
** using staged installation
Variables from the environment: PKG_CFLAGS='' PKG_LIBS=''
Guessing OpenMP configuration for macOS
* checking if OpenMP works with PKG_CFLAGS='' PKG_LIBS=''... failed to run
* checking if OpenMP works with PKG_CFLAGS='-Xclang -fopenmp' PKG_LIBS='-lomp'... yes
Adding PKG_CFLAGS='-Xclang -fopenmp', PKG_LIBS='-lomp'
** libs
using C compiler: ‘Apple clang version 16.0.0 (clang-1600.0.26.6)’
using SDK: ‘MacOSX15.2.sdk’
clang -arch arm64 -std=gnu2x -I"/Library/Frameworks/R.framework/Resources/include" -DNDEBUG   -I/opt/R/arm64/include   -Xclang -fopenmp -fPIC  -falign-functions=64 -Wall -g -O2  -c test_omp.c -o test_omp.o
clang -arch arm64 -std=gnu2x -dynamiclib -Wl,-headerpad_max_install_names -undefined dynamic_lookup -L/Library/Frameworks/R.framework/Resources/lib -L/opt/R/arm64/lib -o ompdetect.so test_omp.o -lomp -F/Library/Frameworks/R.framework/.. -framework R
installing to /Library/Frameworks/R.framework/Versions/4.5-arm64/Resources/library/00LOCK-ompdetect/00new/ompdetect/libs

$ Rscript -e 'ompdetect::ompdetect(); ompdetect::omplimits()'
OpenMP detected and working.
thread_limit  max_threads    num_procs
  2147483647            3            3

macOS 10.15.7, R-4.2.3, Apple clang 12.0.0, compatible OpenMP runtime in /usr/local

Since the bundled OpenMP runtime only appeared in R-4.3, this necessitated a manual installation of the OpenMP runtime (openmp-10.0.0-darwin17-Release.tar.gz).

* installing *source* package ‘ompdetect’ ...
** using staged installation
Variables from the environment: PKG_CFLAGS='' PKG_LIBS=''
Guessing OpenMP configuration for macOS
* checking if OpenMP works with PKG_CFLAGS='' PKG_LIBS=''... failed to run
* checking if OpenMP works with PKG_CFLAGS='-Xclang -fopenmp' PKG_LIBS='-lomp'... yes
Adding PKG_CFLAGS='-Xclang -fopenmp', PKG_LIBS='-lomp'
** libs
clang -mmacosx-version-min=10.13 -I"/Library/Frameworks/R.framework/Resources/include" -DNDEBUG   -I/usr/local/include  -Xclang -fopenmp -fPIC  -Wall -g -O2  -c test_omp.c -o test_omp.o
clang -mmacosx-version-min=10.13 -dynamiclib -Wl,-headerpad_max_install_names -undefined dynamic_lookup -single_module -multiply_defined suppress -L/Library/Frameworks/R.framework/Resources/lib -L/usr/local/lib -o ompdetect.so test_omp.o -lomp -F/Library/Frameworks/R.framework/.. -framework R -Wl,-framework -Wl,CoreFoundation
installing to /Library/Frameworks/R.framework/Versions/4.2/Resources/library/00LOCK-ompdetect/00new/ompdetect/libs

$ Rscript -e 'ompdetect::ompdetect(); ompdetect::omplimits()'
OpenMP detected and working.
thread_limit  max_threads    num_procs
  2147483647            3            3

Flags specified in the environment

Same as above, but the configure script takes the flags manually specified in the environment variables.

$ PKG_CFLAGS='-Xclang -fopenmp' PKG_LIBS='-lomp' R CMD INSTALL ompdetect_0.2-0.tar.gz
* installing to library ‘/Library/Frameworks/R.framework/Versions/4.2/Resources/library’
* installing *source* package ‘ompdetect’ ...
** using staged installation
Variables from the environment: PKG_CFLAGS='-Xclang -fopenmp' PKG_LIBS='-lomp'
Guessing OpenMP configuration for macOS
* checking if OpenMP works with PKG_CFLAGS='-Xclang -fopenmp ' PKG_LIBS='-lomp '... yes
Adding PKG_CFLAGS='-Xclang -fopenmp ', PKG_LIBS='-lomp '
** libs
clang -mmacosx-version-min=10.13 -I"/Library/Frameworks/R.framework/Resources/include" -DNDEBUG   -I/usr/local/include  -Xclang -fopenmp  -fPIC  -Wall -g -O2  -c test_omp.c -o test_omp.o
clang -mmacosx-version-min=10.13 -dynamiclib -Wl,-headerpad_max_install_names -undefined dynamic_lookup -single_module -multiply_defined suppress -L/Library/Frameworks/R.framework/Resources/lib -L/usr/local/lib -o ompdetect.so test_omp.o -lomp -F/Library/Frameworks/R.framework/.. -framework R -Wl,-framework -Wl,CoreFoundation
installing to /Library/Frameworks/R.framework/Versions/4.2/Resources/library/00LOCK-ompdetect/00new/ompdetect/libs

macOS 10.15.7, R-4.5.2 from MacPorts, clang 17.0.6

The toolchain used by MacPorts supports OpenMP natively, so R detects OpenMP support and provides the necessary flags (-fopenmp) in the $(SHLIB_OPENMP_CFLAGS) Make macro:

* installing *source* package ‘ompdetect’ ...
** this is package ‘ompdetect’ version ‘0.2-0’
** using staged installation
Variables from the environment: PKG_CFLAGS='' PKG_LIBS=''
Guessing OpenMP configuration for macOS
* checking if OpenMP works with PKG_CFLAGS='' PKG_LIBS=''... failed to run
* checking if OpenMP works with PKG_CFLAGS='-Xclang -fopenmp' PKG_LIBS='-lomp'... failed to build
* checking if OpenMP works with PKG_CFLAGS='$(SHLIB_OPENMP_CFLAGS)' PKG_LIBS='$(SHLIB_OPENMP_CFLAGS)'... yes
Adding PKG_CFLAGS='$(SHLIB_OPENMP_CFLAGS)', PKG_LIBS='$(SHLIB_OPENMP_CFLAGS)'
** libs
using C compiler: ‘clang version 17.0.6’
using SDK: ‘MacOSX10.15.6.sdk’
/opt/local/bin/clang-mp-17 -std=gnu2x -I"/opt/local/Library/Frameworks/R.framework/Resources/include" -DNDEBUG   -I/opt/local/include -isysroot/Library/Developer/CommandLineTools/SDKs/MacOSX10.15.sdk   -fopenmp -fPIC  -pipe -Os -isysroot/Library/Developer/CommandLineTools/SDKs/MacOSX10.15.sdk -arch x86_64  -c test_omp.c -o test_omp.o
/opt/local/bin/clang-mp-17 -std=gnu2x -dynamiclib -Wl,-headerpad_max_install_names -undefined dynamic_lookup -single_module -multiply_defined suppress -L/opt/local/Library/Frameworks/R.framework/Resources/lib -L/opt/local/lib -Wl,-headerpad_max_install_names -Wl,-rpath,/opt/local/lib/libgcc -Wl,-syslibroot,/Library/Developer/CommandLineTools/SDKs/MacOSX10.15.sdk -arch x86_64 -o ompdetect.so test_omp.o -fopenmp -F/opt/local/Library/Frameworks/R.framework/.. -framework R
installing to /Users/user/Library/R/x86_64/4.5/library/00LOCK-ompdetect/00new/ompdetect/libs

This should work in a similar manner with a Homebrew build of R.