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lib/os/heap: document the reason behind the header field ordering

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This is not obvious why the order is important when only looking at
the header file.

Signed-off-by: Nicolas Pitre <npitre@baylibre.com>
This commit is contained in:
Nicolas Pitre 2021-03-17 20:44:20 -04:00 • committed by Anas Nashif
commit e8910f629d
1 changed files with 9 additions and 4 deletions
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13
lib/os/heap.h
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@ -33,10 +33,10 @@
* field accessors since we can't use natural syntax. * field accessors since we can't use natural syntax.
* *
* The fields are: * The fields are:
* SIZE_AND_USED: the total size (including header) of the chunk in
* 8-byte units. The bottom bit stores a "used" flag.
* LEFT_SIZE: The size of the left (next lower chunk in memory) * LEFT_SIZE: The size of the left (next lower chunk in memory)
* neighbor chunk. * neighbor chunk.
* SIZE_AND_USED: the total size (including header) of the chunk in
* 8-byte units. The bottom bit stores a "used" flag.
* FREE_PREV: Chunk ID of the previous node in a free list. * FREE_PREV: Chunk ID of the previous node in a free list.
* FREE_NEXT: Chunk ID of the next node in a free list. * FREE_NEXT: Chunk ID of the next node in a free list.
* *
@ -44,15 +44,20 @@
* category. The free list pointers exist only for free chunks, * category. The free list pointers exist only for free chunks,
* obviously. This memory is part of the user's buffer when * obviously. This memory is part of the user's buffer when
* allocated. * allocated.
*
* The field order is so that allocated buffers are immediately bounded
* by SIZE_AND_USED of the current chunk at the bottom, and LEFT_SIZE of
* the following chunk at the top. This ordering allows for quick buffer
* overflow detection by testing left_chunk(c + chunk_size(c)) == c.
*/ */
typedef size_t chunkid_t; typedef size_t chunkid_t;
enum chunk_fields { LEFT_SIZE, SIZE_AND_USED, FREE_PREV, FREE_NEXT };
#define CHUNK_UNIT 8U #define CHUNK_UNIT 8U
typedef struct { char bytes[CHUNK_UNIT]; } chunk_unit_t; typedef struct { char bytes[CHUNK_UNIT]; } chunk_unit_t;
enum chunk_fields { LEFT_SIZE, SIZE_AND_USED, FREE_PREV, FREE_NEXT };
struct z_heap_bucket { struct z_heap_bucket {
chunkid_t next; chunkid_t next;
}; };