glibc malloc struct

malloc_struct.c

/* https://github.com/lattera/glibc/blob/895ef79e04a953cac1493863bcae29ad85657ee1/malloc/malloc.c#L1044 */
struct malloc_chunk {

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

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

  /* Only used for large blocks: pointer to next larger size.  */
  struct malloc_chunk* fd_nextsize; /* double links -- used only if free. */
  struct malloc_chunk* bk_nextsize;
};


/*
   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 unallocated (P clear)  |
	    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	    |             Size of chunk, in bytes                     |A|M|P|
      mem-> +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	    |             User data starts here...                          .
	    .                                                               .
	    .             (malloc_usable_size() bytes)                      .
	    .                                                               |
nextchunk-> +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	    |             (size of chunk, but used for application data)    |
	    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	    |             Size of next chunk, in bytes                |A|0|1|
	    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    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 boundaries, so the mem portion
    (which is returned to the user) is also on an even word boundary, and
    thus at least double-word aligned.
    Free chunks are stored in circular doubly-linked lists, and look like this:
    chunk-> +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	    |             Size of previous chunk, if unallocated (P clear)  |
	    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    `head:' |             Size of chunk, in bytes                     |A|0|P|
      mem-> +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	    |             Forward pointer to next chunk in list             |
	    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	    |             Back pointer to previous chunk in list            |
	    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	    |             Unused space (may be 0 bytes long)                .
	    .                                                               .
	    .                                                               |
nextchunk-> +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    `foot:' |             Size of chunk, in bytes                           |
	    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	    |             Size of next chunk, in bytes                |A|0|0|
	    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    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.
    The A (NON_MAIN_ARENA) bit is cleared for chunks on the initial,
    main arena, described by the main_arena variable.  When additional
    threads are spawned, each thread receives its own arena (up to a
    configurable limit, after which arenas are reused for multiple
    threads), and the chunks in these arenas have the A bit set.  To
    find the arena for a chunk on such a non-main arena, heap_for_ptr
    performs a bit mask operation and indirection through the ar_ptr
    member of the per-heap header heap_info (see arena.c).
    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 three 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 M (IS_MMAPPED) set in their size fields.  Because they are
	allocated one-by-one, each must contain its own trailing size
	field.  If the M bit is set, the other bits are ignored
	(because mmapped chunks are neither in an arena, nor adjacent
	to a freed chunk).  The M bit is also used for chunks which
	originally came from a dumped heap via malloc_set_state in
	hooks.c.
     3. Chunks in fastbins are treated as allocated chunks from the
	point of view of the chunk allocator.  They are consolidated
	with their neighbors only in bulk, in malloc_consolidate.
*/

/*
   Bins
    An array of bin headers for free chunks. Each bin is doubly
    linked.  The bins are approximately proportionally (log) spaced.
    There are a lot of these bins (128). This may look excessive, but
    works very well in practice.  Most bins hold sizes that are
    unusual as malloc request sizes, but are more usual for fragments
    and consolidated sets of chunks, which is what these bins hold, so
    they can be found quickly.  All procedures maintain the invariant
    that no consolidated chunk physically borders another one, so each
    chunk in a list is known to be preceeded and followed by either
    inuse chunks or the ends of memory.
    Chunks in bins are kept in size order, with ties going to the
    approximately least recently used chunk. Ordering isn't needed
    for the small bins, which all contain the same-sized chunks, but
    facilitates best-fit allocation for larger chunks. These lists
    are just sequential. Keeping them in order almost never requires
    enough traversal to warrant using fancier ordered data
    structures.
    Chunks of the same size are linked with the most
    recently freed at the front, and allocations are taken from the
    back.  This results in LRU (FIFO) allocation order, which tends
    to give each chunk an equal opportunity to be consolidated with
    adjacent freed chunks, resulting in larger free chunks and less
    fragmentation.
    To simplify use in double-linked lists, each bin header acts
    as a malloc_chunk. This avoids special-casing for headers.
    But to conserve space and improve locality, we allocate
    only the fd/bk pointers of bins, and then use repositioning tricks
    to treat these as the fields of a malloc_chunk*.
 */

typedef struct malloc_chunk *mbinptr;