nuttx/mm/mm_heap/mm_memalign.c

/****************************************************************************
 * mm/mm_heap/mm_memalign.c
 *
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.  The
 * ASF licenses this file to you under the Apache License, Version 2.0 (the
 * "License"); you may not use this file except in compliance with the
 * License.  You may obtain a copy of the License at
 *
 *   http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
 * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.  See the
 * License for the specific language governing permissions and limitations
 * under the License.
 *
 ****************************************************************************/

/****************************************************************************
 * Included Files
 ****************************************************************************/

#include <nuttx/config.h>

#include <assert.h>

#include <nuttx/mm/mm.h>

/****************************************************************************
 * Public Functions
 ****************************************************************************/

/****************************************************************************
 * Name: mm_memalign
 *
 * Description:
 *   memalign requests more than enough space from malloc, finds a region
 *   within that chunk that meets the alignment request and then frees any
 *   leading or trailing space.
 *
 *   The alignment argument must be a power of two (not checked).  8-byte
 *   alignment is guaranteed by normal malloc calls.
 *
 ****************************************************************************/

FAR void *mm_memalign(FAR struct mm_heap_s *heap, size_t alignment,
                      size_t size)
{
  FAR struct mm_allocnode_s *node;
  size_t rawchunk;
  size_t alignedchunk;
  size_t mask = (size_t)(alignment - 1);
  size_t allocsize;

  /* If this requested alinement's less than or equal to the natural
   * alignment of malloc, then just let malloc do the work.
   */

  if (alignment <= MM_MIN_CHUNK)
    {
      return mm_malloc(heap, size);
    }

  /* Adjust the size to account for (1) the size of the allocated node, (2)
   * to make sure that it is an even multiple of our granule size, and to
   * include the alignment amount.
   *
   * Notice that we increase the allocation size by twice the requested
   * alignment.  We do this so that there will be at least two valid
   * alignment points within the allocated memory.
   *
   * NOTE:  These are sizes given to malloc and not chunk sizes. They do
   * not include SIZEOF_MM_ALLOCNODE.
   */

  size      = MM_ALIGN_UP(size);   /* Make multiples of our granule size */
  allocsize = size + 2*alignment;  /* Add double full alignment size */

  /* Then malloc that size */

  rawchunk = (size_t)mm_malloc(heap, allocsize);
  if (rawchunk == 0)
    {
      return NULL;
    }

  /* We need to hold the MM semaphore while we muck with the chunks and
   * nodelist.
   */

  mm_takesemaphore(heap);

  /* Get the node associated with the allocation and the next node after
   * the allocation.
   */

  node = (FAR struct mm_allocnode_s *)(rawchunk - SIZEOF_MM_ALLOCNODE);

  /* Find the aligned subregion */

  alignedchunk = (rawchunk + mask) & ~mask;

  /* Check if there is free space at the beginning of the aligned chunk */

  if (alignedchunk != rawchunk)
    {
      FAR struct mm_allocnode_s *newnode;
      FAR struct mm_allocnode_s *next;
      size_t precedingsize;

      /* Get the node the next node after the allocation. */

      next = (FAR struct mm_allocnode_s *)((FAR char *)node + node->size);

      /* Make sure that there is space to convert the preceding
       * mm_allocnode_s into an mm_freenode_s.
       * I think that this should always be true
       */

      DEBUGASSERT(alignedchunk >= rawchunk + 8);

      newnode = (FAR struct mm_allocnode_s *)
        (alignedchunk - SIZEOF_MM_ALLOCNODE);

      /* Preceding size is full size of the new 'node,' including
       * SIZEOF_MM_ALLOCNODE
       */

      precedingsize = (size_t)newnode - (size_t)node;

      /* If we were unlucky, then the alignedchunk can lie in such a position
       * that precedingsize < SIZEOF_NODE_FREENODE.  We can't let that happen
       * because we are going to cast 'node' to struct mm_freenode_s below.
       * This is why we allocated memory large enough to support two
       * alignment points.  In this case, we will simply use the second
       * alignment point.
       */

      if (precedingsize < SIZEOF_MM_FREENODE)
        {
          alignedchunk += alignment;
          newnode       = (FAR struct mm_allocnode_s *)
                          (alignedchunk - SIZEOF_MM_ALLOCNODE);
          precedingsize = (size_t)newnode - (size_t)node;
        }

      /* Set up the size of the new node */

      newnode->size = (size_t)next - (size_t)newnode;
      newnode->preceding = precedingsize | MM_ALLOC_BIT;

      /* Reduce the size of the original chunk and mark it not allocated, */

      node->size = precedingsize;
      node->preceding &= ~MM_ALLOC_BIT;

      /* Fix the preceding size of the next node */

      next->preceding = newnode->size | (next->preceding & MM_ALLOC_BIT);

      /* Convert the newnode chunk size back into malloc-compatible size by
       * subtracting the header size SIZEOF_MM_ALLOCNODE.
       */

      allocsize = newnode->size - SIZEOF_MM_ALLOCNODE;

      /* Add the original, newly freed node to the free nodelist */

      mm_addfreechunk(heap, (FAR struct mm_freenode_s *)node);

      /* Replace the original node with the newlay realloaced,
       * aligned node
       */

      node = newnode;
    }

  /* Check if there is free space at the end of the aligned chunk. Convert
   * malloc-compatible chunk size to include SIZEOF_MM_ALLOCNODE as needed
   * for mm_shrinkchunk.
   */

  size = MM_ALIGN_UP(size + SIZEOF_MM_ALLOCNODE);

  if (allocsize > size)
    {
      /* Shrink the chunk by that much -- remember, mm_shrinkchunk wants
       * internal chunk sizes that include SIZEOF_MM_ALLOCNODE.
       */

      mm_shrinkchunk(heap, node, size);
    }

  mm_givesemaphore(heap);
  return (FAR void *)alignedchunk;
}