Question

I was writing a matrix multiplication program, and to save code space I created a function make that takes a pointer to a double pointer and dynamically allocates a 2-dimensional array of given size. The arrays a and b are intialised to random values in [-2, 2]. However, when I try to run the code on gcc, I get a seg fault.

I ran the code through gdb, and the error arises when I try to set b[0][0] to a random number. When I try to print b[0][0] in gdb, I get the error:

Cannot access memory at address 0xbfebfdc3f5d77f80

However, I’m actually able to access b[0][0] before this. I can print the array just after allocating it without errors. For some reason, it’s always the array b that seems to cause problems.

I’ve a feeling it’s a pointer issue, but I can’t see where. I’m not that inexperienced a programmer, but I spent the guts of 2 days trying to find out why this error keeps appearing. Can anyone shed some light?

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


void make(double ***a, int m, int n)
{
  int i;
  double *tmp;


  (*a) = (double **) malloc(m*sizeof(double *));
  tmp = (double *) calloc(m*n,sizeof(double));

  if(tmp == NULL)
  {
    printf("Error with malloc.\n");
    exit(EXIT_FAILURE);
  }

  for(i=0;i<m;i++)
    (*a)[i] = &tmp[i*n];


  free(tmp);

  if(a == NULL)
  {
    fprintf(stderr, "Error with the matrix, dimensions: %d, %d. \n", m, n);
    exit(EXIT_FAILURE);
  }
}


int main()
{
  int i, j;
  int l, m, n;
  double **a, **b, **c;

  l = m = n = 8;

  srand48(time(NULL));

  make(&a, l, m);
  make(&b, m, n);
  make(&c, l, n);

  for(i=0; i<l; i++)
    for(j=0; j<m; j++)
      a[i][j] = (drand48()*4.0) - 2.0;

  for(i=0; i<m; i++)
    for(j=0; j<n; j++)
      b[i][j] = (drand48()*4.0) - 2.0;

  free(a);
  free(b);
  free(c);

  return 0;
}

Answer 1

One problem is the call to free(tmp) in make. That is the memory that was allocated for the array. It should not be freed if you plan to continue using it.