[SOLVED] C

CincinnatiKid · 01-26-2014, 09:46 AM

Can anyone tell me what is wrong with this code? When I run it I get a seg fault:

Code:

#include <stdlib.h>

int main (int argc, char **argv)
{
	char **board;
	
	board = (char **) malloc (sizeof (char *) * 8);
	
	board [0][0] = 'a';
	
	free (board);

	return 0;
}

firstfire · 01-26-2014, 10:08 AM

Hi.

You allocate a vector of pointers, but each pointer in this vector should also point to allocated memory. Something like this:

Code:

char **board;
int N = 8, M = 8;
board = (char **) malloc (sizeof (char *) * N);
int i;
for(i=0; i<N; i++)
    board[i] = malloc(sizeof(char)*M); // NOTE: char, not char* here

To deallocate memory:

Code:

for(i=0; i<N; i++) free(board[i]);
free(board);

Alternative approach would be to allocate an array of length N*M (malloc(sizeof(char)*N*M)

and access its ij-th element using index i*M+j, i=0..N-1, j=0..M-1.

CincinnatiKid · 01-26-2014, 10:23 AM

That worked for me. I think I understand. So what I did was allocate memory for a certain number of pointers, but did not allocate the memory of the data that the pointers would point to right?

firstfire · 01-26-2014, 11:36 AM

Right you are!