"fatal error: 'omp.h' file not found" using clang on Apple M1

Question

Clang isn't able to find omp.h whenever I try to compile with openMP flag. Here's what I'm trying to do

clang++ -dynamiclib -I/opt/homebrew/Cellar/eigen/3.3.9/include/eigen3/ -Xpreprocessor -fopenmp -o libfoo.dylib foolibrary.cpp -lomp

Although I have omp.h in /opt/homebrew/Cellar/libomp/11.0.1/include/omp.h, while clang is in usr/bin/clang. Is there anything I need to configure so that it's able to find it?

Answer 1

In your terminal:

brew install libomp

Then in your Makefile:

CC=clang++
FLAGS=-g -Wall -std=c++17 -arch arm64 -lomp
LDFLAGS=-L/opt/homebrew/lib
CPPFLAGS=-I/opt/homebrew/include

My Mac & clang:

clang -v

Apple clang version 13.1.6 (clang-1316.0.21.2.5)

Target: arm64-apple-darwin21.5.0

Thread model: posix

Answer 2

When using brew to target the M1 natively, the brew installed clang is not in /usr/bin. The brew instructions show that the preferred location for it is /opt/homebrew. The clang in /usr/bin is a link to the Apple command line tools one; you can see that by using -v.

$ /usr/bin/clang -v
/usr/bin/clang -v
Apple clang version 12.0.0 (clang-1200.0.32.29)
Target: arm64-apple-darwin20.3.0
Thread model: posix
InstalledDir: /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin

To get the brew installed clang you need to follow the instructions brew gave you when you installed it :-

llvm is keg-only, which means it was not symlinked into /opt/homebrew,
because macOS already provides this software and installing another version in
parallel can cause all kinds of trouble.

If you need to have llvm first in your PATH, run:
  echo 'export PATH="/opt/homebrew/opt/llvm/bin:$PATH"' >> ~/.profile

For compilers to find llvm you may need to set:
  export LDFLAGS="-L/opt/homebrew/opt/llvm/lib"
  export CPPFLAGS="-I/opt/homebrew/opt/llvm/include"

Doing that resolves the include issue, but, at least for me, not the library linking one.

/opt/homebrew/opt/llvm/bin/clang -v -fopenmp hello_omp.c
clang version 11.1.0
Target: arm64-apple-darwin20.3.0
Thread model: posix
InstalledDir: /opt/homebrew/opt/llvm/bin
 "/opt/homebrew/Cellar/llvm/11.1.0/bin/clang-11" -cc1 -triple arm64-apple-macosx11.0.0 -Wundef-prefix=TARGET_OS_ -Werror=undef-prefix -Wdeprecated-objc-isa-usage -Werror=deprecated-objc-isa-usage -emit-obj -mrelax-all -disable-free -disable-llvm-verifier -discard-value-names -main-file-name hello_omp.c -mrelocation-model pic -pic-level 2 -mframe-pointer=non-leaf -fno-rounding-math -munwind-tables -fcompatibility-qualified-id-block-type-checking -target-cpu apple-a7 -target-feature +fp-armv8 -target-feature +neon -target-feature +crypto -target-feature +zcm -target-feature +zcz -target-feature +sha2 -target-feature +aes -target-abi darwinpcs -fallow-half-arguments-and-returns -debugger-tuning=lldb -target-linker-version 609.8 -v -resource-dir /opt/homebrew/Cellar/llvm/11.1.0/lib/clang/11.1.0 -isysroot /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk -internal-isystem /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/local/include -internal-isystem /opt/homebrew/Cellar/llvm/11.1.0/lib/clang/11.1.0/include -internal-externc-isystem /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include -fdebug-compilation-dir /Users/jcownie/tmp -ferror-limit 19 -fmessage-length=163 -fopenmp -fopenmp-cuda-parallel-target-regions -stack-protector 1 -fblocks -fencode-extended-block-signature -fregister-global-dtors-with-atexit -fgnuc-version=4.2.1 -fmax-type-align=16 -o /var/folders/lt/nf3dtk8j16qfsl97d_vgv4dw000lts/T/hello_omp-bd4ce3.o -x c hello_omp.c
clang -cc1 version 11.1.0 based upon LLVM 11.1.0 default target arm64-apple-darwin20.3.0
ignoring nonexistent directory "/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/local/include"
ignoring nonexistent directory "/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/Library/Frameworks"
#include "..." search starts here:
#include <...> search starts here:
 /opt/homebrew/Cellar/llvm/11.1.0/lib/clang/11.1.0/include
 /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include
 /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/System/Library/Frameworks (framework directory)
End of search list.
 "/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/ld" -demangle -lto_library /opt/homebrew/Cellar/llvm/11.1.0/lib/libLTO.dylib -no_deduplicate -dynamic -arch arm64 -platform_version macos 11.0.0 0.0.0 -syslibroot /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk -o a.out /var/folders/lt/nf3dtk8j16qfsl97d_vgv4dw000lts/T/hello_omp-bd4ce3.o -lomp -lSystem /opt/homebrew/Cellar/llvm/11.1.0/lib/clang/11.1.0/lib/darwin/libclang_rt.osx.a
ld: library not found for -lomp
clang-11: error: linker command failed with exit code 1 (use -v to see invocation)

If you explicitly pass the recommended $LDFLAGS, then things work...

$ echo $LDFLAGS
-L/opt/homebrew/opt/llvm/lib
$ /opt/homebrew/opt/llvm/bin/clang $LDFLAGS -fopenmp hello_omp.c
/opt/homebrew/opt/llvm/bin/clang $LDFLAGS -fopenmp hello_omp.c
$ ./a.out
./a.out
Hello from thread 0
Hello from thread 5
Hello from thread 4
Hello from thread 1
Hello from thread 3
Hello from thread 2
Hello from thread 7
Hello from thread 6
$ 

My (hacky, but it works) solution to that has been to create my own shell scripts to invoke the compilers with the appropriate library path; you can see this hack in How to build LLVM (clang,clang++) for Apple M1?

Note that at one more point the support for OpenMP tasks was broken in LLVM when targeting the M1. (See "Crash in passing firstprivate args to tasks on Apple M1"). However, that was fixed in March 2021, so be sure to use a more recent LLVM release.