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Expand comments on padding used by nano_malloc 

This patch adds further comments to nano-mallocr.c, to more comprehensively
explain how padding works in the malloc_chunk structure.

It was originally discussed in the following thread:
  https://sourceware.org/ml/newlib/2017/msg00031.html

2017-01-13  Joe Seymour  <joe.s@somniumtech.com>

        newlib/
        * libc/stdlib/nano-mallocr.c (malloc_chunk, get_chunk_from_ptr)
        (nano_malloc): Add comments.
This commit is contained in:
Joe Seymour 2017-01-13 15:35:26 +00:00 committed by Corinna Vinschen
parent 677ffdc247
commit c0ac2ea2b3
1 changed files with 37 additions and 14 deletions
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51
newlib/libc/stdlib/nano-mallocr.c
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@ -123,19 +123,24 @@ typedef size_t malloc_size_t;
typedef struct malloc_chunk typedef struct malloc_chunk
{ {
/* ------------------ /* --------------------------------------
* chunk->| size (4 bytes) | * chunk->| size |
* ------------------ * --------------------------------------
* | Padding for | * | Padding for alignment |
* | alignment | * | This includes padding inserted by |
* | holding neg | * | the compiler (to align fields) and |
* | offset to size | * | explicit padding inserted by this |
* ------------------ * | implementation. If any explicit |
* mem_ptr->| point to next | * | padding is being used then the |
* | free when freed| * | sizeof (size) bytes at |
* | or data load | * | mem_ptr - CHUNK_OFFSET must be |
* | when allocated | * | initialized with the negative |
* ------------------ * | offset to size. |
* --------------------------------------
* mem_ptr->| When allocated: data |
* | When freed: pointer to next free |
* | chunk |
* --------------------------------------
*/ */
/* size of the allocated payload area, including size before /* size of the allocated payload area, including size before
CHUNK_OFFSET */ CHUNK_OFFSET */
@ -187,8 +192,13 @@ extern void * nano_pvalloc(RARG size_t s);
static inline chunk * get_chunk_from_ptr(void * ptr) static inline chunk * get_chunk_from_ptr(void * ptr)
{ {
/* Assume that there is no explicit padding in the
chunk, and that the chunk starts at ptr - CHUNK_OFFSET. */
chunk * c = (chunk *)((char *)ptr - CHUNK_OFFSET); chunk * c = (chunk *)((char *)ptr - CHUNK_OFFSET);
/* Skip the padding area */
/* c->size being negative indicates that there is explicit padding in
the chunk. In which case, c->size is currently the negative offset to
the true size. */
if (c->size < 0) c = (chunk *)((char *)c + c->size); if (c->size < 0) c = (chunk *)((char *)c + c->size);
return c; return c;
} }
@ -314,6 +324,19 @@ void * nano_malloc(RARG malloc_size_t s)
if (offset) if (offset)
{ {
/* Initialize sizeof (malloc_chunk.size) bytes at
align_ptr - CHUNK_OFFSET with negative offset to the
size field (at the start of the chunk).
The negative offset to size from align_ptr - CHUNK_OFFSET is
the size of any remaining padding minus CHUNK_OFFSET. This is
equivalent to the total size of the padding, because the size of
any remaining padding is the total size of the padding minus
CHUNK_OFFSET.
Note that the size of the padding must be at least CHUNK_OFFSET.
The rest of the padding is not initialized. */
*(long *)((char *)r + offset) = -offset; *(long *)((char *)r + offset) = -offset;
} }