uclibc-ng/libc/stdlib/malloc/free.c

/*
 * libc/stdlib/malloc/free.c -- free function
 *
 *  Copyright (C) 2002  NEC Corporation
 *  Copyright (C) 2002  Miles Bader <miles@gnu.org>
 *
 * This file is subject to the terms and conditions of the GNU Lesser
 * General Public License.  See the file COPYING.LIB in the main
 * directory of this archive for more details.
 * 
 * Written by Miles Bader <miles@gnu.org>
 */

#include <stdlib.h>
#include <unistd.h>
#include <sys/mman.h>

#include "malloc.h"
#include "heap.h"


void
free (void *mem)
{
  size_t size;
  struct heap_free_area *fa;
  struct heap *heap = &__malloc_heap;

  /* Check for special cases.  */
  if (unlikely (! mem))
    return;

  /* Normal free.  */

  MALLOC_DEBUG ("free: 0x%lx (base = 0x%lx, total_size = %d)\n",
		(long)mem, (long)MALLOC_BASE (mem), MALLOC_SIZE (mem));

  size = MALLOC_SIZE (mem);
  mem = MALLOC_BASE (mem);

  __malloc_lock ();

  /* Put MEM back in the heap, and get the free-area it was placed in.  */
  fa = __heap_free (heap, mem, size);

  /* See if the free-area FA has grown big enough that it should be
     unmapped.  */
  if (HEAP_FREE_AREA_SIZE (fa) < MALLOC_UNMAP_THRESHOLD)
    /* Nope, nothing left to do, just release the lock.  */
    __malloc_unlock ();
  else
    /* Yup, try to unmap FA.  */
    {
      unsigned long start = (unsigned long)HEAP_FREE_AREA_START (fa);
      unsigned long end = (unsigned long)HEAP_FREE_AREA_END (fa);
#ifndef MALLOC_USE_SBRK
      unsigned long unmap_start, unmap_end;
#endif

#ifdef MALLOC_USE_SBRK
      /* Get the sbrk lock so that the two possible calls to sbrk below
	 are guaranteed to be contiguous.  */
      __malloc_lock_sbrk ();
      /* When using sbrk, we only shrink the heap from the end.  It would
	 be possible to allow _both_ -- shrinking via sbrk when possible,
	 and otherwise shrinking via munmap, but this results in holes in
	 memory that prevent the brk from every growing back down; since
	 we only ever grow the heap via sbrk, this tends to produce a
	 continuously growing brk (though the actual memory is unmapped),
	 which could eventually run out of address space.  Note that
	 `sbrk(0)' shouldn't normally do a system call, so this test is
	 reasonably cheap.  */
      if ((void *)end != sbrk (0))
	{
	  MALLOC_DEBUG ("  not unmapping: 0x%lx - 0x%lx (%ld bytes)\n",
			start, end, end - start);
	  __malloc_unlock_sbrk ();
	  __malloc_unlock ();
	  return;
	}
#endif

      MALLOC_DEBUG ("  unmapping: 0x%lx - 0x%lx (%ld bytes)\n",
		    start, end, end - start);

      /* Remove FA from the heap.  */
      __heap_delete (heap, fa);

      if (__heap_is_empty (heap))
	/* We want to avoid the heap from losing all memory, so reserve
	   a bit.  This test is only a heuristic -- the existance of
	   another free area, even if it's smaller than
	   MALLOC_MIN_SIZE, will cause us not to reserve anything.  */
	{
	  /* Put the reserved memory back in the heap; we asssume that
	     MALLOC_UNMAP_THRESHOLD is greater than MALLOC_MIN_SIZE, so
	     we use the latter unconditionally here.  */
	  __heap_free (heap, (void *)start, MALLOC_MIN_SIZE);
	  start += MALLOC_MIN_SIZE;
	}

#ifdef MALLOC_USE_SBRK

      /* Release the main lock; we're still holding the sbrk lock.  */
      __malloc_unlock ();
      /* Lower the brk.  */
      sbrk (start - end);
      /* Release the sbrk lock too; now we hold no locks.  */
      __malloc_unlock_sbrk ();

#else /* !MALLOC_USE_SBRK */

      /* MEM/LEN may not be page-aligned, so we have to page-align them,
	 and return any left-over bits on the end to the heap.  */
      unmap_start = MALLOC_ROUND_UP_TO_PAGE_SIZE (start);
      unmap_end = MALLOC_ROUND_DOWN_TO_PAGE_SIZE (end);

      /* We have to be careful that any left-over bits are large enough to
	 return.  Note that we _don't check_ to make sure there's room to
	 grow/shrink the start/end by another page, we just assume that
	 the unmap threshold is high enough so that this is always safe
	 (i.e., it should probably be at least 3 pages).  */
      if (unmap_start > start)
	{
	  if (unmap_start - start < HEAP_MIN_FREE_AREA_SIZE)
	    unmap_start += MALLOC_PAGE_SIZE;
	  __heap_free (heap, (void *)start, unmap_start - start);
	}
      if (end > unmap_end)
	{
	  if (end - unmap_end < HEAP_MIN_FREE_AREA_SIZE)
	    unmap_end -= MALLOC_PAGE_SIZE;
	  __heap_free (heap, (void *)unmap_end, end - unmap_end);
	}

      /* Release the malloc lock before we do the system call.  */
      __malloc_unlock ();

      if (unmap_end > unmap_start)
	/* Finally, actually unmap the memory.  */
	munmap ((void *)unmap_start, unmap_end - unmap_start);

#endif /* MALLOC_USE_SBRK */
    }
}