How to compiler OpenMP program using clang?

Question

My gcc compiles well, but clang fails with the following message:

clang -fopenmp=libomp -o main main.c
main.c:5:10: fatal error: 'omp.h' file not found

I also installed libomp5 package and changed flag to -fopenmp=libomp5 , though it didn't help either:

clang -fopenmp=libomp5 -o main main.c
clang: error: unsupported argument 'libomp5' to option 'fopenmp='
clang: error: unsupported argument 'libomp5' to option 'fopenmp='

these recommendations didn't work.

Would be grateful for hints on installing necessary 16.04 specific packages and passing corresponding flags.

Answer 1

I had the same problem.

sudo apt install libomp-dev

Fixed it with Ubuntu 16.10

//test.c
#include "omp.h"
#include <stdio.h>

int main(void) {
  #pragma omp parallel
  printf("thread %d\n", omp_get_thread_num());
}

Then

clang test.c -fopenmp
./a.out
thread 0
thread 5
thread 2
thread 1
thread 7
thread 3
thread 4
thread 6

Also

clant-3.9 test.c -fopenmp

works.

---

GCC and Clang use different OpenMP runtime libraries : libgomp and libomp respectivly.

Clang's runtime is the LLVM OpenMP runtime which in turn is based on the Intel OpenMP runtime (which is open source). https://www.openmprtl.org/

On my system GCC installed omp.h at

/usr/lib/gcc/x86_64-linux-gnu/6/include/omp.h

and libomp-dev insalled omp.h at

/usr/include/omp.h

These are different header files which include different function definitions. It may be okay to use either header file for e.g. omp_get_wtime() but in general I think it's probably better to use the header file that corresponds to the runtime that is linked to.

Answer 2

It seems omp.h file doesn't exist in your system PATH. firstly try to locate omp.h file if you don't know where it is:

find / -name 'omp.h' -type f

And then run this command to compile your code:

clang -o main main.c -I/path/to/omp/folder