<omp.h>

Most C compilers now support OpenMP, usually no special support needed.

gcc -Wall -fopenmp -o openmphello –O3 openmphello.c

(Listed in increasing priority)

1. Environment variable in shell

   can be changed by environment variable OMP_NUM_THREADSintcshell/scshell/bash

   #!/bin/bash
   setenv OMP_NUM_THREADS t

2. Explicit setting by library call

   omp_set_num_threads(t);

3. Clause num_threads(t) in #pragma omp parallel

Master thread always has number 0.

#include <omp.h>void omp_set_num_threads(int num_threads);
int omp_get_num_threads(void);
int omp_get_max_threads(void);
int omp_get_thread_num(void);
int omp_get_num_procs(void);

• Example: Set number of threads and print thread ID

  #include <stdio.h>
  #include <stdlib.h>
  #include <omp.h> // OpenMP headerint main(int argc, char *argv[]) {
  int threads, myid;
  int i;
  threads = 1; // shared variable
  // read from commandline
  for (i=1; i<argc && argv[i][0]=='-'; i++) {
  if (argv[i][1]=='t') {
  sscanf(argv[++i],"%d",&threads);
  }
  }printf("Maximum number of threads possible is %d\n", omp_get_max_threads());// check if below max, else set to max
  if (threads < omp_get_max_threads()) {
  if (threads<1) {
  threads = 1;
  }
  omp_set_num_threads(threads);
  } else {
  threads = omp_get_max_threads();
  }
  #pragma omp parallel num_threads(threads) {
  int myid = omp_get_thread_num(); // private variable
  printf("Thread %d of %d active\n", myid, threads);
  }return 0;
  }

used for performance evaluation:

Wall-clock time in seconds since some time in the past returned by omp_get_wtime() .

#include <omp.h>double omp_get_wtime(void);
double omp_get_wtick(void);