source: CIVL/examples/omp/matProduct2.c

/*
  OpenMP implementation of matrix multiplication. Each thread takes care
  a chunk of rows. 

  Compile with gcc -O3 -fopenmp omp_matrixmult.c -o omp_matrixmult
*/
// Online source: http://users.abo.fi/mats/PP2012/examples/OpenMP/omp_critical.c
// permission obtained

#include <omp.h>
#include <stdio.h>
#include <stdlib.h>

/* Number of threads used */
#define NR_THREADS 4

#ifdef _CIVL
#define DEBUG 1
$input int NRA=10;                 // number of rows in matrix A
$input int NCA=10;                 // number of columns in matrix A
$input int NCB=10;                 // number of columns in matrix B
#else
#define DEBUG 0
#define NRA 1400                 // number of rows in matrix A 
#define NCA 1400                 // number of columns in matrix A
#define NCB 1400                 // number of columns in matrix B
#endif

int main (int argc, char *argv[]) {
  int   tid, nthreads, i, j, k;
  double **a, **b, **c;
  double *a_block, *b_block, *c_block;
  double **res;
  double *res_block;
  double starttime, stoptime;

  a = (double **) malloc(NRA*sizeof(double *)); /* matrix a to be multiplied */
  b = (double **) malloc(NCA*sizeof(double *)); /* matrix b to be multiplied */
  c = (double **) malloc(NRA*sizeof(double *)); /* result matrix c */

  a_block = (double *) malloc(NRA*NCA*sizeof(double)); /* Storage for matrices */
  b_block = (double *) malloc(NCA*NCB*sizeof(double));
  c_block = (double *) malloc(NRA*NCB*sizeof(double));

  /* Result matrix for the sequential algorithm */
  res = (double **) malloc(NRA*sizeof(double *));
  res_block = (double *) malloc(NRA*NCB*sizeof(double));

  for (i=0; i<NRA; i++)   /* Initialize pointers to a */
    a[i] = a_block+i*NRA;

  for (i=0; i<NCA; i++)   /* Initialize pointers to b */
    b[i] = b_block+i*NCA;
  
  for (i=0; i<NRA; i++)   /* Initialize pointers to c */
    c[i] = c_block+i*NRA;

  for (i=0; i<NRA; i++)   /* Initialize pointers to res */
    res[i] = res_block+i*NRA;

  /* A static allocation of the matrices would be done like this */
  /* double a[NRA][NCA], b[NCA][NCB], c[NRA][NCB];  */

  /*** Spawn a parallel region explicitly scoping all variables ***/
#pragma omp parallel shared(a,b,c,nthreads) private(tid,i,j,k) num_threads(NR_THREADS)
  {
    tid = omp_get_thread_num();
    if (tid == 0) {  /* Only thread 0 prints */
      nthreads = omp_get_num_threads();
      printf("Starting matrix multiplication with %d threads\n",nthreads);
      printf("Initializing matrices...\n");
    }
    /*** Initialize matrices ***/
#pragma omp for nowait    /* No need to synchronize the threads before the */
    for (i=0; i<NRA; i++) /* last matrix has been initialized */
      for (j=0; j<NCA; j++)
        a[i][j]= (double) (i+j);
#pragma omp for nowait
    for (i=0; i<NCA; i++)
      for (j=0; j<NCB; j++)
        b[i][j]= (double) (i*j);
#pragma omp for   /* We synchronize the threads after this */
    for (i=0; i<NRA; i++)
      for (j=0; j<NCB; j++)
        c[i][j]= 0.0;

    if (tid == 0) /* Thread zero measures time */
      starttime = omp_get_wtime();  /* Master thread measures the execution time */
    
    /* Do matrix multiply sharing iterations on outer loop */
    /* If DEBUG is TRUE display who does which iterations */
    /* printf("Thread %d starting matrix multiply...\n",tid); */
#pragma omp for
    for (i=0; i<NRA; i++) {
      if (DEBUG) printf("Thread=%d did row=%d\n",tid,i);
      for(j=0; j<NCB; j++) {    
        for (k=0; k<NCA; k++) {
          c[i][j] += a[i][k] * b[k][j];
        }
      }
    }

    if (tid == 0) {
      stoptime = omp_get_wtime();
      printf("Time for parallel matrix multiplication: %3.2f s\n", 
             stoptime-starttime);
    }
  }   /*** End of parallel region ***/
  
  starttime = omp_get_wtime();
  /* Do a sequential matrix multiplication and compare the results */
  for (i=0; i<NRA; i++) {
    for (j=0; j<NCB; j++) {
      res[i][j] = 0.0;
      for (k=0; k<NCA; k++)
        res[i][j] += a[i][k]*b[k][j];
    }
  }
  stoptime = omp_get_wtime();
  printf("Time for sequential matrix multiplication: %3.2f s\n", stoptime-starttime);

  /* Check that the results are the same as in the parallel solution.
     Actually, you should not compare floating point values for equality like this
     but instead compute the difference between the two values and check that it
     is smaller than a very small value epsilon. However, since all values in the
     matrices here are integer values, this will work.
  */
  for (i=0; i<NRA; i++) {
    for (j=0; j<NCB; j++) {
      if (res[i][j] == c[i][j]) {
        /* Everything is OK if they are equal */
      }
      else {
        printf("Different result %5.1f != %5.1f in %d %d\n ", res[i][j], c[i][j], i, j);
      }
    }
  }

  /* If DEBUG is true, print the results. Usa smaller matrices for this */
  if (DEBUG) {
    printf("Result Matrix:\n");
    for (i=0; i<NRA; i++) {
      for (j=0; j<NCB; j++) 
        printf("%6.1f ", c[i][j]);
      printf("\n"); 
    }
  }

  printf ("Done.\n");
  exit(0);
}