malloc: Update comments about chunk layout
This commit is contained in:
Florian Weimer
parent
09472915dd
commit
ae9166f2b8
|
|
@ -1,3 +1,8 @@
|
|||
2016-10-28 Florian Weimer <fweimer@redhat.com>
|
||||
|
||||
* malloc/malloc.c: Update chunk layout comments.
|
||||
(chunk_main_arena): Update comment.
|
||||
|
||||
2016-10-28 Florian Weimer <fweimer@redhat.com>
|
||||
|
||||
* crypt/sha256.c, crypt/sha256-block.c,
|
||||
|
|
|
|||
|
|
@ -1070,18 +1070,19 @@ struct malloc_chunk {
|
|||
|
||||
|
||||
chunk-> +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|
||||
| Size of previous chunk, if allocated | |
|
||||
| Size of previous chunk, if unallocated (P clear) |
|
||||
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|
||||
| Size of chunk, in bytes |M|P|
|
||||
| Size of chunk, in bytes |A|M|P|
|
||||
mem-> +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|
||||
| User data starts here... .
|
||||
. .
|
||||
. (malloc_usable_size() bytes) .
|
||||
. |
|
||||
nextchunk-> +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|
||||
| Size of chunk |
|
||||
| (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
|
||||
|
|
@ -1094,9 +1095,9 @@ nextchunk-> +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|
|||
Free chunks are stored in circular doubly-linked lists, and look like this:
|
||||
|
||||
chunk-> +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|
||||
| Size of previous chunk |
|
||||
| Size of previous chunk, if unallocated (P clear) |
|
||||
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|
||||
`head:' | Size of chunk, in bytes |P|
|
||||
`head:' | Size of chunk, in bytes |A|0|P|
|
||||
mem-> +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|
||||
| Forward pointer to next chunk in list |
|
||||
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|
||||
|
|
@ -1108,6 +1109,8 @@ nextchunk-> +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|
|||
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
|
||||
|
|
@ -1120,12 +1123,21 @@ nextchunk-> +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|
|||
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 two exceptions to all this are
|
||||
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
|
||||
|
|
@ -1135,8 +1147,16 @@ nextchunk-> +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|
|||
|
||||
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.
|
||||
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.
|
||||
*/
|
||||
|
||||
/*
|
||||
|
|
@ -1215,7 +1235,7 @@ nextchunk-> +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|
|||
the chunk to the user, if necessary. */
|
||||
#define NON_MAIN_ARENA 0x4
|
||||
|
||||
/* check for chunk from non-main arena */
|
||||
/* Check for chunk from main arena. */
|
||||
#define chunk_main_arena(p) (((p)->mchunk_size & NON_MAIN_ARENA) == 0)
|
||||
|
||||
/* Mark a chunk as not being on the main arena. */
|
||||
|
|
|
|||
Loading…
Reference in New Issue