http://takatakamanbou.hatenablog.com/entry/2014/12/18/235900

matrix.c

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

//#define PRINT

int main(int argc, char **argv)
{
  double *Abuf, *Bbuf, *Cbuf;
  double **A, **B, **C;
  double acc;
  int n;
  int i, j, k, t;

  if(argc != 2){
    fprintf(stderr, "usage: %s n\n", argv[0]);
    exit(EXIT_FAILURE);
  }
  n = atoi(argv[1]);

  /*** 行列 A,B,C のメモリを確保，A,Bを乱数で初期化 ***/
  Abuf = malloc(n*n*sizeof(double));
  Bbuf = malloc(n*n*sizeof(double));
  Cbuf = malloc(n*n*sizeof(double));

  srand(1);
  A = malloc(n*sizeof(double *));
  B = malloc(n*sizeof(double *));
  C = malloc(n*sizeof(double *));
  for(i = 0; i < n; i++){
    A[i] = Abuf + i*n;
    B[i] = Bbuf + i*n;
    C[i] = Cbuf + i*n;
    for(j = 0; j < n; j++){
      A[i][j] = drand48(); // [0.0, 1.0) pseudo-random numbers
      B[i][j] = drand48();
    }
  }

  /***** C = A x B *****/
  for(t = 0; t < 10; t++){
    printf("t = %d\n", t);
    for(i = 0; i < n; i++){
      for(j = 0; j < n; j++){
	acc = 0.0;
	for(k = 0; k < n; k++){
	  acc += A[i][k] * B[k][j];
	}
	C[i][j] = acc;
      }
    }
  }

  /***** 出力 *****/
#ifdef PRINT
  printf("A = \n");
  for(i = 0; i < n; i++){
    for(j = 0; j < n; j++){
      printf(" %f", A[i][j]);
    }
    printf("\n");
  }
  printf("B = \n");
  for(i = 0; i < n; i++){
    for(j = 0; j < n; j++){
      printf(" %f", B[i][j]);
    }
    printf("\n");
  }
  printf("C = \n");
  for(i = 0; i < n; i++){
    for(j = 0; j < n; j++){
      printf(" %f", C[i][j]);
    }
    printf("\n");
  }
#else
  printf("%f\n", C[n-1][n-1]);
#endif

  return 0;
}