if (rows <= 0)		/* this chunk might be past end of file! */
      break;
    byte_count = rows * bytesperrow;
    if (writing)
      (*ptr->b_s_info.write_backing_store) (cinfo, & ptr->b_s_info,
					    (void FAR *) ptr->mem_buffer[i],
					    file_offset, byte_count);
    else
      (*ptr->b_s_info.read_backing_store) (cinfo, & ptr->b_s_info,
					   (void FAR *) ptr->mem_buffer[i],
					   file_offset, byte_count);
    file_offset += byte_count;
  }
}


METHODDEF(JSAMPARRAY)
access_virt_sarray (j_common_ptr cinfo, jvirt_sarray_ptr ptr,
		    JDIMENSION start_row, JDIMENSION num_rows,
		    boolean writable)
/* Access the part of a virtual sample array starting at start_row */
/* and extending for num_rows rows.  writable is true if  */
/* caller intends to modify the accessed area. */
{
  JDIMENSION end_row = start_row + num_rows;
  JDIMENSION undef_row;

  /* debugging check */
  if (end_row > ptr->rows_in_array || num_rows > ptr->maxaccess ||
      ptr->mem_buffer == NULL)
    ERREXIT(cinfo, JERR_BAD_VIRTUAL_ACCESS);

  /* Make the desired part of the virtual array accessible */
  if (start_row < ptr->cur_start_row ||
      end_row > ptr->cur_start_row+ptr->rows_in_mem) {
    if (! ptr->b_s_open)
      ERREXIT(cinfo, JERR_VIRTUAL_BUG);
    /* Flush old buffer contents if necessary */
    if (ptr->dirty) {
      do_sarray_io(cinfo, ptr, TRUE);
      ptr->dirty = FALSE;
    }
    /* Decide what part of virtual array to access.
     * Algorithm: if target address > current window, assume forward scan,
     * load starting at target address.  If target address < current window,
     * assume backward scan, load so that target area is top of window.
     * Note that when switching from forward write to forward read, will have
     * start_row = 0, so the limiting case applies and we load from 0 anyway.
     */
    if (start_row > ptr->cur_start_row) {
      ptr->cur_start_row = start_row;
    } else {
      /* use long arithmetic here to avoid overflow & unsigned problems */
      long ltemp;

      ltemp = (long) end_row - (long) ptr->rows_in_mem;
      if (ltemp < 0)
	ltemp = 0;		/* don't fall off front end of file */
      ptr->cur_start_row = (JDIMENSION) ltemp;
    }
    /* Read in the selected part of the array.
     * During the initial write pass, we will do no actual read
     * because the selected part is all undefined.
     */
    do_sarray_io(cinfo, ptr, FALSE);
  }
  /* Ensure the accessed part of the array is defined; prezero if needed.
   * To improve locality of access, we only prezero the part of the array
   * that the caller is about to access, not the entire in-memory array.
   */
  if (ptr->first_undef_row < end_row) {
    if (ptr->first_undef_row < start_row) {
      if (writable)		/* writer skipped over a section of array */
	ERREXIT(cinfo, JERR_BAD_VIRTUAL_ACCESS);
      undef_row = start_row;	/* but reader is allowed to read ahead */
    } else {
      undef_row = ptr->first_undef_row;
    }
    if (writable)
      ptr->first_undef_row = end_row;
    if (ptr->pre_zero) {
      size_t bytesperrow = (size_t) ptr->samplesperrow * SIZEOF(JSAMPLE);
      undef_row -= ptr->cur_start_row; /* make indexes relative to buffer */
      end_row -= ptr->cur_start_row;
      while (undef_row < end_row) {
	jzero_far((void FAR *) ptr->mem_buffer[undef_row], bytesperrow);
	undef_row++;
      }
    } else {
      if (! writable)		/* reader looking at undefined data */
	ERREXIT(cinfo, JERR_BAD_VIRTUAL_ACCESS);
    }
  }
  /* Flag the buffer dirty if caller will write in it */
  if (writable)
    ptr->dirty = TRUE;
  /* Return address of proper part of the buffer */
  return ptr->mem_buffer + (start_row - ptr->cur_start_row);
}


METHODDEF(JBLOCKARRAY)
access_virt_barray (j_common_ptr cinfo, jvirt_barray_ptr ptr,
		    JDIMENSION start_row, JDIMENSION num_rows,
		    boolean writable)
/* Access the part of a virtual block array starting at start_row */
/* and extending for num_rows rows.  writable is true if  */
/* caller intends to modify the accessed area. */
{
  JDIMENSION end_row = start_row + num_rows;
  JDIMENSION undef_row;

  /* debugging check */
  if (end_row > ptr->rows_in_array || num_rows > ptr->maxaccess ||
      ptr->mem_buffer == NULL)
    ERREXIT(cinfo, JERR_BAD_VIRTUAL_ACCESS);

  /* Make the desired part of the virtual array accessible */
  if (start_row < ptr->cur_start_row ||
      end_row > ptr->cur_start_row+ptr->rows_in_mem) {
    if (! ptr->b_s_open)
      ERREXIT(cinfo, JERR_VIRTUAL_BUG);
    /* Flush old buffer contents if necessary */
    if (ptr->dirty) {
      do_barray_io(cinfo, ptr, TRUE);
      ptr->dirty = FALSE;
    }
    /* Decide what part of virtual array to access.
     * Algorithm: if target address > current window, assume forward scan,
     * load starting at target address.  If target address < current window,
     * assume backward scan, load so that target area is top of window.
     * Note that when switching from forward write to forward read, will have
     * start_row = 0, so the limiting case applies and we load from 0 anyway.
     */
    if (start_row > ptr->cur_start_row) {
      ptr->cur_start_row = start_row;
    } else {
      /* use long arithmetic here to avoid overflow & unsigned problems */
      long ltemp;

      ltemp = (long) end_row - (long) ptr->rows_in_mem;
      if (ltemp < 0)
	ltemp = 0;		/* don't fall off front end of file */
      ptr->cur_start_row = (JDIMENSION) ltemp;
    }
    /* Read in the selected part of the array.
     * During the initial write pass, we will do no actual read
     * because the selected part is all undefined.
     */
    do_barray_io(cinfo, ptr, FALSE);
  }
  /* Ensure the accessed part of the array is defined; prezero if needed.
   * To improve locality of access, we only prezero the part of the array
   * that the caller is about to access, not the entire in-memory array.
   */
  if (ptr->first_undef_row < end_row) {
    if (ptr->first_undef_row < start_row) {
      if (writable)		/* writer skipped over a section of array */
	ERREXIT(cinfo, JERR_BAD_VIRTUAL_ACCESS);
      undef_row = start_row;	/* but reader is allowed to read ahead */
    } else {
      undef_row = ptr->first_undef_row;
    }
    if (writable)
      ptr->first_undef_row = end_row;
    if (ptr->pre_zero) {
      size_t bytesperrow = (size_t) ptr->blocksperrow * SIZEOF(JBLOCK);
      undef_row -= ptr->cur_start_row; /* make indexes relative to buffer */
      end_row -= ptr->cur_start_row;
      while (undef_row < end_row) {
	jzero_far((void FAR *) ptr->mem_buffer[undef_row], bytesperrow);
	undef_row++;
      }
    } else {
      if (! writable)		/* reader looking at undefined data */
	ERREXIT(cinfo, JERR_BAD_VIRTUAL_ACCESS);
    }
  }
  /* Flag the buffer dirty if caller will write in it */
  if (writable)
    ptr->dirty = TRUE;
  /* Return address of proper part of the buffer */
  return ptr->mem_buffer + (start_row - ptr->cur_start_row);
}


/*
 * Release all objects belonging to a specified pool.
 */

METHODDEF(void)
free_pool (j_common_ptr cinfo, int pool_id)
{
  my_mem_ptr mem = (my_mem_ptr) cinfo->mem;
  small_pool_ptr shdr_ptr;
  large_pool_ptr lhdr_ptr;
  size_t space_freed;

  if (pool_id < 0 || pool_id >= JPOOL_NUMPOOLS)
    ERREXIT1(cinfo, JERR_BAD_POOL_ID, pool_id);	/* safety check */

#ifdef MEM_STATS
  if (cinfo->err->trace_level > 1)
    print_mem_stats(cinfo, pool_id); /* print pool's memory usage statistics */
#endif

  /* If freeing IMAGE pool, close any virtual arrays first */
  if (pool_id == JPOOL_IMAGE) {
    jvirt_sarray_ptr sptr;
    jvirt_barray_ptr bptr;

    for (sptr = mem->virt_sarray_list; sptr != NULL; sptr = sptr->next) {
      if (sptr->b_s_open) {	/* there may be no backing store */
	sptr->b_s_open = FALSE;	/* prevent recursive close if error */
	(*sptr->b_s_info.close_backing_store) (cinfo, & sptr->b_s_info);
      }
    }
    mem->virt_sarray_list = NULL;
    for (bptr = mem->virt_barray_list; bptr != NULL; bptr = bptr->next) {
      if (bptr->b_s_open) {	/* there may be no backing store */
	bptr->b_s_open = FALSE;	/* prevent recursive close if error */
	(*bptr->b_s_info.close_backing_store) (cinfo, & bptr->b_s_info);
      }
    }
    mem->virt_barray_list = NULL;
  }

  /* Release large objects */
  lhdr_ptr = mem->large_list[pool_id];
  mem->large_list[pool_id] = NULL;

  while (lhdr_ptr != NULL) {
    large_pool_ptr next_lhdr_ptr = lhdr_ptr->hdr.next;
    space_freed = lhdr_ptr->hdr.bytes_used +
		  lhdr_ptr->hdr.bytes_left +
		  SIZEOF(large_pool_hdr);
    jpeg_free_large(cinfo, (void FAR *) lhdr_ptr, space_freed);
    mem->total_space_allocated -= space_freed;
    lhdr_ptr = next_lhdr_ptr;
  }

  /* Release small objects */
  shdr_ptr = mem->small_list[pool_id];
  mem->small_list[pool_id] = NULL;

  while (shdr_ptr != NULL) {
    small_pool_ptr next_shdr_ptr = shdr_ptr->hdr.next;
    space_freed = shdr_ptr->hdr.bytes_used +
		  shdr_ptr->hdr.bytes_left +
		  SIZEOF(small_pool_hdr);
    jpeg_free_small(cinfo, (void *) shdr_ptr, space_freed);
    mem->total_space_allocated -= space_freed;
    shdr_ptr = next_shdr_ptr;
  }
}


/*
 * Close up shop entirely.
 * Note that this cannot be called unless cinfo->mem is non-NULL.
 */

METHODDEF(void)
self_destruct (j_common_ptr cinfo)
{
  int pool;

  /* Close all backing store, release all memory.
   * Releasing pools in reverse order might help avoid fragmentation
   * with some (brain-damaged) malloc libraries.
   */
  for (pool = JPOOL_NUMPOOLS-1; pool >= JPOOL_PERMANENT; pool--) {
    free_pool(cinfo, pool);
  }

  /* Release the memory manager control block too. */
  jpeg_free_small(cinfo, (void *) cinfo->mem, SIZEOF(my_memory_mgr));
  cinfo->mem = NULL;		/* ensures I will be called only once */

  jpeg_mem_term(cinfo);		/* system-dependent cleanup */
}


/*
 * Memory manager initialization.
 * When this is called, only the error manager pointer is valid in cinfo!
 */

GLOBAL(void)
jinit_memory_mgr (j_common_ptr cinfo)
{
  my_mem_ptr mem;
  long max_to_use;
  int pool;
  size_t test_mac;

  cinfo->mem = NULL;		/* for safety if init fails */

  /* Check for configuration errors.
   * SIZEOF(ALIGN_TYPE) should be a power of 2; otherwise, it probably
   * doesn't reflect any real hardware alignment requirement.
   * The test is a little tricky: for X>0, X and X-1 have no one-bits
   * in common if and only if X is a power of 2, ie has only one one-bit.
   * Some compilers may give an "unreachable code" warning here; ignore it.
   */
  if ((SIZEOF(ALIGN_TYPE) & (SIZEOF(ALIGN_TYPE)-1)) != 0)
    ERREXIT(cinfo, JERR_BAD_ALIGN_TYPE);
  /* MAX_ALLOC_CHUNK must be representable as type size_t, and must be
   * a multiple of SIZEOF(ALIGN_TYPE).
   * Again, an "unreachable code" warning may be ignored here.
   * But a "constant too large" warning means you need to fix MAX_ALLOC_CHUNK.
   */
  test_mac = (size_t) MAX_ALLOC_CHUNK;
  if ((long) test_mac != MAX_ALLOC_CHUNK ||
      (MAX_ALLOC_CHUNK % SIZEOF(ALIGN_TYPE)) != 0)
    ERREXIT(cinfo, JERR_BAD_ALLOC_CHUNK);

  max_to_use = jpeg_mem_init(cinfo); /* system-dependent initialization */

  /* Attempt to allocate memory manager's control block */
  mem = (my_mem_ptr) jpeg_get_small(cinfo, SIZEOF(my_memory_mgr));

  if (mem == NULL) {
    jpeg_mem_term(cinfo);	/* system-dependent cleanup */
    ERREXIT1(cinfo, JERR_OUT_OF_MEMORY, 0);
  }

  /* OK, fill in the method pointers */
  mem->pub.alloc_small = alloc_small;
  mem->pub.alloc_large = alloc_large;
  mem->pub.alloc_sarray = alloc_sarray;
  mem->pub.alloc_barray = alloc_barray;
  mem->pub.request_virt_sarray = request_virt_sarray;
  mem->pub.request_virt_barray = request_virt_barray;
  mem->pub.realize_virt_arrays = realize_virt_arrays;
  mem->pub.access_virt_sarray = access_virt_sarray;
  mem->pub.access_virt_barray = access_virt_barray;
  mem->pub.free_pool = free_pool;
  mem->pub.self_destruct = self_destruct;

  /* Initialize working state */
  mem->pub.max_memory_to_use = max_to_use;

  for (pool = JPOOL_NUMPOOLS-1; pool >= JPOOL_PERMANENT; pool--) {
    mem->small_list[pool] = NULL;
    mem->large_list[pool] = NULL;
  }
  mem->virt_sarray_list = NULL;
  mem->virt_barray_list = NULL;

  mem->total_space_allocated = SIZEOF(my_memory_mgr);

  /* Declare ourselves open for business */
  cinfo->mem = & mem->pub;

  /* Check for an environment variable JPEGMEM; if found, override the
   * default max_memory setting from jpeg_mem_init.  Note that the
   * surrounding application may again override this value.
   * If your system doesn't support getenv(), define NO_GETENV to disable
   * this feature.
   */
#ifndef NO_GETENV
  { char * memenv;

    if ((memenv = getenv("JPEGMEM")) != NULL) {
      char ch = 'x';

      if (sscanf(memenv, "%ld%c", &max_to_use, &ch) > 0) {
	if (ch == 'm' || ch == 'M')
	  max_to_use *= 1000L;
	mem->pub.max_memory_to_use = max_to_use * 1000L;
      }
    }
  }
#endif

}