realloc help

[nappaji]

The following code gives me a segmentation fault. I thinks its an issue with the syntax of realloc. Please advise.

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

main()
{
int **arr;
int i,k, j;

arr = (int **) malloc (3);
arr[0] = (int *) malloc (2);
arr[1] = (int *) malloc (2);
arr[2] = (int *) malloc (2);

arr[0][0] = 9;
arr[0][1] = 21;

arr[1][0] = 44;
arr[1][1] = -9;

arr[2][0] = 12;
arr[2][1] = 22;

for (i=0;i<3;i++)
for (k=0;k<2;k++)
printf("\narr[%d][%d] = %d\n", i,k, arr[k]);

arr[0] = (int *) realloc (arr[0],(sizeof(arr[0]) + 1));

arr[0][2] = 234;

printf("\narr[0][2] = %d\n", arr[0][2]);
}

 

[Salem]

> arr = (int **) malloc (3);
1. Don't cast the result of malloc - doing so only hides a potential serious problem if you fail to include stdlib.h.
If your compiler is complaining about void* casts in C++, then stop compiling your code with a C++ compiler, and use a C compiler.

2. You're not allocating enough memory.
The template for using any malloc/realloc is as follows
[tt]T *ptr = malloc( num * sizeof *ptr );[/tt]
T is any type - say int
ptr is the name of your pointer, say arr
num is how many you want, say 3

So your first line would be
[tt]arr = malloc(3 * sizeof *arr);[/tt]


> arr[0] = (int *) realloc (arr[0],(sizeof(arr[0]) + 1));
sizeof() can't tell you how much memory you previously allocated. You need to save this in another variable, and adjust it accordingly.

Also, you're open to a memory leak using realloc in that way. If realloc fails, it returns NULL, but then you've lost your only handle to the memory you did have
This is how you should use realloc

void *temp = realloc( arr[0], (max+1)*sizeof(*arr[0]) );
if ( temp != NULL ) {
  // success, update max and our original pointer
  max += 1;
  arr[0] = temp;
} else {
  // oops, it failed
  // don't change max, and do something with arr[0]
  // which still points to the original memory we had
}

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