view release on metacpan or  search on metacpan

src/judy-1.0.5/test/malloc-pre2.8a.c  view on Meta::CPAN

static int dev_zero_fd = -1; /* Cached file descriptor for /dev/zero. */

#define MMAP(addr, size, prot, flags) ((dev_zero_fd < 0) ? \
 (dev_zero_fd = open("/dev/zero", O_RDWR), \
  mmap((addr), (size), (prot), (flags), dev_zero_fd, 0)) : \
   mmap((addr), (size), (prot), (flags), dev_zero_fd, 0))

#else

#define MMAP(addr, size, prot, flags) \
 (mmap((addr), (size), (prot), (flags)|MAP_ANONYMOUS, -1, 0))

#endif


#endif /* HAVE_MMAP */


/*
  -----------------------  Chunk representations -----------------------
*/


/*
  This struct declaration is misleading (but accurate and necessary).
  It declares a "view" into memory allowing access to necessary
  fields at known offsets from a given base. See explanation below.
*/

struct malloc_chunk {

  INTERNAL_SIZE_T      prev_size;  /* Size of previous chunk (if free).  */
  INTERNAL_SIZE_T      size;       /* Size in bytes, including overhead. */

  struct malloc_chunk* fd;         /* double links -- used only if free. */
  struct malloc_chunk* bk;
};


typedef struct malloc_chunk* mchunkptr;

/* conversion from malloc headers to user pointers, and back */
#define chunk2mem(p)   ((Void_t*)((char*)(p) + 2*SIZE_SZ))
#define mem2chunk(mem) ((mchunkptr)((char*)(mem) - 2*SIZE_SZ))

/*
   malloc_chunk details:

    (The following includes lightly edited explanations by Colin Plumb.)

    Chunks of memory are maintained using a `boundary tag' method as
    described in e.g., Knuth or Standish.  (See the paper by Paul
    Wilson ftp://ftp.cs.utexas.edu/pub/garbage/allocsrv.ps for a
    survey of such techniques.)  Sizes of free chunks are stored both
    in the front of each chunk and at the end.  This makes
    consolidating fragmented chunks into bigger chunks very fast.  The
    size fields also hold bits representing whether chunks are free or
    in use.

    An allocated chunk looks like this:


    chunk-> +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
            |             Size of previous chunk, if allocated            | |
            +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
            |             Size of chunk, in bytes                         |P|
      mem-> +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
            |             User data starts here...                          .
            .                                                               .
            .             (malloc_usable_space() bytes)                     .
            .                                                               |
nextchunk-> +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    `foot:' |             Size of chunk                                     |
            +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


    Where "chunk" is the front of the chunk for the purpose of most of
    the malloc code, but "mem" is the pointer that is returned to the
    user.  "Nextchunk" is the beginning of the next contiguous chunk.

    Chunks always begin on even word boundries, so the mem portion
    (which is returned to the user) is also on an even word boundary, and
    thus at least double-word aligned.


    The P (PREV_INUSE) bit, stored in the unused low-order bit of the
    chunk size (which is always a multiple of two words), is an in-use
    bit for the *previous* chunk.  If that bit is *clear*, then the
    word before the current chunk size contains the previous chunk
    size, and can be used to find the front of the previous chunk.
    The very first chunk allocated always has this bit set,
    preventing access to non-existent (or non-owned) memory. If
    prev_inuse is set for any given chunk, then you CANNOT determine
    the size of the previous chunk, and might even get a memory
    addressing fault when trying to do so.

    Note that the `foot' of the current chunk is actually represented
    as the prev_size of the NEXT chunk. This makes it easier to
    deal with alignments etc but can be very confusing when trying
    to extend or adapt this code.

    The two exceptions to all this are

     1. The special chunk `top' doesn't bother using the
        trailing size field since there is no next contiguous chunk
        that would have to index off it. After initialization, `top'
        is forced to always exist.  If it would become less than
        MINSIZE bytes long, it is replenished.

     2. Chunks allocated via mmap, which have the second-lowest-order
        bit (IS_MMAPPED) set in their size fields.  Because they are
        allocated one-by-one, each must contain its own trailing size field.

*/



/*
  --------------- Operations on size fields ---------------
*/