Why OpenMP program runs only in one thread

Question

I just tried OpenMP with a simple c program

test() {
   for(int i=0;i<100000000;i++);
}
main() {
    printf("Num of CPU: %d\n", omp_get_num_procs());
    #pragma omp parallel for num_threads(4)
    for(int i=0;i<100;i++) test();
}

Compiled with g++ -fopenmp. It can correctly print out that I have 4 CPUs, but all test functions are running at thread 0.

I tried to modify the OMP_NUM_THREADS. But it has no effect also.

I had everything the same as the online examples but why wouldn't I get it to work?

Answer 1

Your problem is here:

#pragma omp parallel for num_thread(4) <---

The correct clause is num_threads(4), not num_thread(4). Incorrect openmp pragmas are ignored and so you ended up with a sequential program. :)

I'm surprised you didn't get a compiler warning, because I did.

Answer 2

I had this problem in visual studio and finally I understood that I had forgotten to enable Open MP support in visual studio. It didn't give me any error but executed the program just for one thread