malloc\malloc.c

static void* _int_malloc(mstate av, size_t bytes)
{
  INTERNAL_SIZE_T nb;             /* normalized request size */
  mchunkptr       victim;         /* inspected/selected chunk */
  INTERNAL_SIZE_T size;           /* its size */
  mchunkptr       remainder;      /* remainder from a split */
  unsigned long   remainder_size; /* its size */

  checked_request2size(bytes, nb);

  [...]

  victim = av->top;
  size = chunksize(victim);
  if ((unsigned long)(size) >= (unsigned long)(nb + MINSIZE))
  {
    remainder_size = size - nb;
    remainder = chunk_at_offset(victim, nb);
    av->top = remainder;
    set_head(victim, nb | PREV_INUSE | (av!=&main_arena ? NON_MAIN_ARENA : 0));
    set_head(remainder, remainder_size | PREV_INUSE);

    check_malloced_chunk(av, victim, nb);
    void *p = chunk2mem(victim);
    if (__builtin_expect (perturb_byte, 0))
      alloc_perturb (p, bytes);
    return p;
  }

  [...]
}