multithreading - openmp runs single threaded on my mac -

i trying parallelize program using openmp on mac, can not manage make multi-threaded. i've tried building llvm/clang/openmp 3.7.1 source (after svn co) documented, have tried using prebuild versions of clang , openmp 3.7.0 given llvm project. in each case, resulting compiler works fine -fopenmp flag , produce executable links openmp runtime.

i use following openmp 'hello world' program:

#include <omp.h> #include <stdio.h> #include <stdlib.h> int main (int argc, char *argv[]) {     int nthreads, tid;      /* fork team of threads giving them own copies of variables */     #pragma omp parallel private(nthreads, tid)     {         /* obtain thread number */         tid = omp_get_thread_num();         printf("hello world thread = %d\n", tid);          /* master thread */         if (tid == 0)          {             nthreads = omp_get_num_threads();             printf("number of threads = %d\n", nthreads);         }     } /* threads join master thread , disband */ } 

i compile using:

clang -fopenmp hello.c -o hello 

and run resulting program with:

env omp_num_threads=2 ./hello 

which gives:

hello world thread = 0 number of threads = 1 

any idea?

clang < 3.8.0 requires -fopenmp=libomp generate openmp code. clang >= 3.8.0 supports -fopenmp (-fopenmp=libomp can used also).