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)
{
  if (mem)
    {
      size_t size;
      struct heap_free_area *fa;

      mem -= MALLOC_ALIGNMENT;
      size = *(size_t *)mem;

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

      __malloc_lock ();

      fa = __heap_free (&__malloc_heap, mem, size);

      /* Now we check to see if FA has grown big enough that it should be
	 unmapped.  */
      if (HEAP_FREE_AREA_SIZE (fa) < MALLOC_UNMAP_THRESHOLD)
	/* Nothing left to do, just release the lock.  */
	__malloc_unlock ();
      else
	/* Try to unmap FA.  */
	{
	  unsigned long start, end;
#ifndef MALLOC_USE_SBRK
	  unsigned long unmap_start, unmap_end;
#endif

	  end = (unsigned long)HEAP_FREE_AREA_END (fa);

#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 (%d bytes)\n",
			    (unsigned long)HEAP_FREE_AREA_START (fa),
			    (unsigned long)HEAP_FREE_AREA_END (fa),
			    fa->size);
	      __malloc_unlock_sbrk ();
	      __malloc_unlock ();
	      return;
	    }
#endif

	  start = (unsigned long)HEAP_FREE_AREA_START (fa);

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

	  /* Remove FA from the heap.  */
	  __heap_unlink_free_area (&__malloc_heap, fa);

	  if (!fa->next && !fa->prev)
	    /* 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 (&__malloc_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 (&__malloc_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 (&__malloc_heap, (void *)unmap_end, end - unmap_end);
	    }

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

	  if (unmap_end > unmap_start)
	    munmap ((void *)unmap_start, unmap_end - unmap_start);

#endif /* MALLOC_USE_SBRK */
	}
    }
}