re-sizing a 2-dim array

[Brian Scearce]

In comp.lang.c brendan at cs.widener.edu (Brendan Kehoe) writes:
>	LINE **contents = (LINE **) malloc (sizeof(LINE *) * 4096);
>	LINE **new_contents = (LINE **) malloc (sizeof(LINE *) * 8192);
>	bcopy((char *)contents, (char *)new_contents, sizeof(LINE *) * 4096);
>	free(contents);
>	contents = new_contents;

Your idea is sound, so sound in fact that there's already something
in the standard library that basically does it.

  LINE **new_contents;

  if ( (new_contents = realloc(contents, sizeof(LINE *) * 0x2000)) == 0) {
    panic("Out of memory");
  } else {
    contents = new_contents;
  }

The advantages of this method:

1) realloc might find that there's space right after your allocated
   space, so it doesn't do anything except mark that space as
   unavailable.  Corollary: there might not be enough space for
   both malloc'd areas to exist at the same time -- possibly with
   realloc, they won't have to both exist at the same time.  Imagine
   your initial 4K block is followed by 5K of free space, but there
   is no more heap area.  Maybe realloc will be smart enough to
   satisfy the request for 8K, but malloc/bcopy/free will certainly
   fail.

2) One function call (realloc), instead of three (malloc, bcopy, free).

3) In most library implementations, realloc is self-starting: feeding
   it a NULL pointer for the first argument makes it equivalent to
   a call to malloc.  So here's a pretty common idiom:

#define INCREMENT 100
struct element *list = 0;
add(struct element x)
{
  static int listsize = 0;
  static int elements_in_list = 0;

  if (elements_in_list == listsize) {
    listsize += INCREMENT;
    if ( (list = realloc(list, listsize)) == 0) {
      panic("Out of memory");
      /* panic() doesn't return -- assume that out of mem is an
         abort condition, so it's OK to overwrite list with a possible
         NULL since we're giving up anyway. */
    }
  }
  list[elements_in_list++] = x;
}

Hope this helps.

--
     Brian Scearce (bls at robin.svl.cdc.com  -or-  robin!bls at shamash.cdc.com)
    "Don't be surprised when a crack in the ice appears under your feet..."
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