Re: Adding a realloc() usage note to the malloc(3) manual page

[Paul Eggert <eggert@cs.ucla.edu>]

On 8/31/21 12:07 AM, Florian Weimer wrote:
> the somewhat common idiom of adjusting internal pointers in the
> allocation to point to the new allocation is invalid.

Good point. Also, the example call to malloc should check the return value.

Something like this, perhaps:

   char *ptr = malloc(origsize);
   if (ptr == NULL)
     return NULL;
   char *p = ptr + some_random_value();

   /* In the following, we presume 'newsize' is not 0.
      (If 'newsize' is zero, realloc() may return NULL,
      and that is not an error.) */

   ptrdiff_t p_offset = p - ptr;
   char *nptr = realloc(ptr, newsize);
   if (nptr == NULL) {
     /* Handle error; the block pointed to by 'ptr' is
        still usable. */
   } else {
     /* realloc() succeeded; update 'ptr' and 'p' to point to
        the (possibly moved) block.  'p += nptr - ptr; ptr = nptr;'
        would be invalid here, since 'ptr' is invalid immediately
        after the successful realloc().  */
     ptr = nptr;
     p = nptr + p_offset;
   }