source: CIVL/examples/omp/vecAdd_fix.c@ afc300c

/******************************************************************************
* FILE: omp_bug5fix.c
* DESCRIPTION:
*   The problem in omp_bug5.c is that the first thread acquires locka and then
*   tries to get lockb before releasing locka. Meanwhile, the second thread
*   has acquired lockb and then tries to get locka before releasing lockb.
*   This solution overcomes the deadlock by using locks correctly.
* AUTHOR: Blaise Barney  01/29/04
* LAST REVISED: 04/06/05
******************************************************************************/
/**
* Fixes the deadlock in vecAdd_deadlock.c.
* Online source:
* https://computing.llnl.gov/tutorials/openMP/samples/C/omp_bug5fix.c
**/
#include <omp.h>
#include <stdio.h>
#include <stdlib.h>
#define N 100
#define PI 3.1415926535
#define DELTA .01415926535

int main (int argc, char *argv[]) 
{
int nthreads, tid, i;
float a[N], b[N];
omp_lock_t locka, lockb;

/* Initialize the locks */
omp_init_lock(&locka);
omp_init_lock(&lockb);

/* Fork a team of threads giving them their own copies of variables */
#pragma omp parallel shared(a, b, nthreads, locka, lockb) private(tid)
  {

  /* Obtain thread number and number of threads */
  tid = omp_get_thread_num();
  #pragma omp master
    {
    nthreads = omp_get_num_threads();
    printf("Number of threads = %d\n", nthreads);
    }
  printf("Thread %d starting...\n", tid);
  #pragma omp barrier

  #pragma omp sections nowait
    {
    #pragma omp section
      {
      printf("Thread %d initializing a[]\n",tid);
      omp_set_lock(&locka);
      for (i=0; i<N; i++)
        a[i] = i * DELTA;
      omp_unset_lock(&locka);
      omp_set_lock(&lockb);
      printf("Thread %d adding a[] to b[]\n",tid);
      for (i=0; i<N; i++)
        b[i] += a[i];
      omp_unset_lock(&lockb);
      }

    #pragma omp section
      {
      printf("Thread %d initializing b[]\n",tid);
      omp_set_lock(&lockb);
      for (i=0; i<N; i++)
        b[i] = i * PI;
      omp_unset_lock(&lockb);
      omp_set_lock(&locka);
      printf("Thread %d adding b[] to a[]\n",tid);
      for (i=0; i<N; i++)
        a[i] += b[i];
      omp_unset_lock(&locka);
      }
    }  /* end of sections */
  }  /* end of parallel region */

}