Error Enabling Openmp - "Ld: Library Not Found for -Lgomp" and Clang Errors

openmp not linking correctly when compiling with clang

Installing the libiomp5 package and changing -fopenmp to -fopenmp=libiomp5 when compiling has resolved the issue.

How to compile with OpenMP in Qt on macOS?

AppleClang should work just fine. Be sure to call the preprocessor to handle OMP via the .profile.

macx: {
QMAKE_CXXFLAGS += -Xpreprocessor -fopenmp -lomp -I/usr/local/include
}

macx: {
QMAKE_LFLAGS += -lomp
}

macx: {
LIBS += -L /usr/local/lib /usr/local/lib/libomp.dylib
}

Xcode: Undefined symbol: _omp_get_max_threads

This is what you need to do to make a C or C++ project compile and link with OpenMP support in Xcode. Go to Build Settings in the project editor, select the project node and then the All settings filter. Make the following changes:

• Header Search Paths (in the Search Paths group) - set to (or add tp the existing value) /usr/local/include, so that the compiler can find /usr/local/include/omp.h
• Library Search Paths (in the Search Paths group) - set to (or add to the existing value) /usr/local/lib, so that the linker can find /usr/local/lib/libomp.dylib

Select the target executable node and make the following additional changes:

• Other C Flags (in the Apple Clang - Custom Compiler Flags group) - set to (or add) -Xclang -fopenmp to enable processing of OpenMP pragmas by the compiler
• Other Linker Flags (in the Linking group) - set to (or add) -lomp to enable linking with the OpenMP runtime library libomp.dylib

You can also make all four changes in the project node to make them project-wide or in the target node, depending on your project structure.

With C++ projects, one has to put -Xclang -fopenmp in Other C++ Flags.

Compiling NAS Parallel Benchmarks with LLVM Clang gives error

The important error line here is:

/usr/bin/ld: cannot open output file ../bin/sp.S: No such file or directory

Since it is the output file, no such refers to the directory. Create ../bin and it should work.

Using x86 libraries and OpenMP on macOS arm64 architecture

Using an x86 installation of brew solves the problem for me. Here is a minimal set of commands for installing x86 variants of brew and clang, and then compiling my C/C++ code:

# launch x86_64 shell
arch -x86_64 zsh  
# install x86_64 variant of brew 
arch -x86_64 /bin/bash -c "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/master/install.sh)"
# install x86_64 variant of clang
arch -x86_64 /usr/local/bin/brew install llvm 
# compile using x86_64 variant of clang
/usr/local/opt/llvm/bin/clang++ -arch x86_64 omp_ex.cpp

The brew application is now installed in two separate locations on my machine:

# arm64 (default) location
/opt/homebrew/bin/brew
# x86_64 location
/usr/local/bin/brew

and clang is installed in three separate locations:

# Apple arm64 (default) location
/usr/bin/clang
# brew arm64 location
/opt/homebrew/opt/llvm/bin/clang
# brew x86_64 location
/usr/local/opt/llvm/bin/clang

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