c free() function question [duplicate]

Question

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C programming : How does free know how much to free?

Hi,

when i have following code:

void *ptr = malloc(100); // alloc 100 bytes
// do sth
free(ptr);

how does the free() function know how much space has to be freed?

thank you!

--

ok i have found other questions asking the same, please close - sorry

Answer 1

This information is usually contained in some memory area managed by the malloc implementation. The information often preceeds the actual memory handed out to you by malloc, but that's an implementation detail, and you cannot rely on anything here.

Answer 2

It's implementation dependent but usually the underlying system mas a map of addresses to blocks and it knows the size from that memory map.

Here is the striped down code from glibc which shows it doing basically what what I just said.

void fREe(Void_t* mem)
{
  arena *ar_ptr;
  mchunkptr p;
  if (__free_hook != NULL) {
    (*__free_hook)(mem, NULL);
  }

  if (mem == 0)                              /* free(0) has no effect */
    return;

  p = mem2chunk(mem);

  if (chunk_is_mmapped(p))                       /* release mmapped memory. */
  {
    munmap_chunk(p);
    return;
  }

  ar_ptr = arena_for_ptr(p);
  chunk_free(ar_ptr, p);
  (void)mutex_unlock(&ar_ptr->mutex);
}