Dynamic Memory Allocation - offset pointers
 

Hi all,
I am just learning DMA, and am supposed to write 2 functions utilizing the offset technique, such that a 2-D array starts and ends the index of each dimension as the user specifies.

matrix(): creates a 2D array of float type numbers, indexing from -n to n for each dimension. A proper pointer is to be returned so that after invocation of this function, a 2D array of float type numbers and the specified starting and ending indices can be used.
free_matrix(): releases the memory allocated for the 2D array created by matrix().

I am using C, and use a UNIX-based compiler to run my program. Below is what I have:

#include <stdio.h>

#include <stdlib.h>

float** matrix(float **x, int n);

void free_matrix(float **x, int n);

int main(void)

{

   float **x;

   int i, j, n;

   scanf("%d", &n);

   x = matrix(x, n);

   for (i = -n ; i <= n ; i++)

   {

       for (j = -n ; j <= n ; j++)

          x[i][j] = i + j;

   }

   for (i = -n ; i <= n ; i++)

   {

       for (j = -n ; j <= n ; j++)

       printf("%f ", x[i][j]);

       printf("\n");

   }   

   free_matrix(x, n);

   return 0;

}

 

float** matrix(float **x, int n)

{

   int i;

   x = (float**) malloc(2 * n * sizeof(float*));

   for (i = 0 ; i < n ; i++)

   {

      x[i] = (float*) malloc(2 * n * sizeof(float));

      x[i] -= n;

   }

   x -= n;

   return x;

}



void free_matrix(float **x, int n)

{

   int i;

   x += n;

   for (i = 0 ; i < n ; i++)

      free(x[i] + n);

   free(x);

}

I keep getting the error "Segmentation fault". What is wrong with my code?

All those in blue were given in the question, so I can't change any of that. I followed the code to create the array closely with that given in my notes, which showed how to create a 2D array.

On a similar matter, I was able to create a 1D array (with starting and ending indices as -n and n respectively), but had problems freeing the memory. My function to free the memory was
void free_vector(float *x, int n)
{
free(x + n);
}
but I got the same "Segmentation Fault" error. What is wrong?

Thank you.

Regards,
Rayne

Re: Dynamic Memory Allocation - offset pointers
 

x = (float**) malloc(2 * n * sizeof(float*));

That's the problem I belive, why the 2? ,just put n like

x = (float**) malloc(n * sizeof(float*));

Then try.......

Also put in debug lines following each step,that way you can quickly find out where the problems are....


And this is really weird for (i = -n ; i <= n ; i++), I belive it's a mistake, it should be for (i = 0 ; i < n ; i++)