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* [newlib-cygwin] Expand comments on padding used by nano_malloc
@ 2017-01-13 16:39 Corinna Vinschen
0 siblings, 0 replies; only message in thread
From: Corinna Vinschen @ 2017-01-13 16:39 UTC (permalink / raw)
To: newlib-cvs
https://sourceware.org/git/gitweb.cgi?p=newlib-cygwin.git;h=c0ac2ea2b328ed3e5cf4bbfb4bce2215fc9fdd2a
commit c0ac2ea2b328ed3e5cf4bbfb4bce2215fc9fdd2a
Author: Joe Seymour <joe.s@somniumtech.com>
Date: Fri Jan 13 15:35:26 2017 +0000
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.
Diff:
---
newlib/libc/stdlib/nano-mallocr.c | 51 ++++++++++++++++++++++++++++-----------
1 file changed, 37 insertions(+), 14 deletions(-)
diff --git a/newlib/libc/stdlib/nano-mallocr.c b/newlib/libc/stdlib/nano-mallocr.c
index 457eb88..a0accdd 100644
--- a/newlib/libc/stdlib/nano-mallocr.c
+++ b/newlib/libc/stdlib/nano-mallocr.c
@@ -123,19 +123,24 @@ typedef size_t malloc_size_t;
typedef struct malloc_chunk
{
- /* ------------------
- * chunk->| size (4 bytes) |
- * ------------------
- * | Padding for |
- * | alignment |
- * | holding neg |
- * | offset to size |
- * ------------------
- * mem_ptr->| point to next |
- * | free when freed|
- * | or data load |
- * | when allocated |
- * ------------------
+ /* --------------------------------------
+ * chunk->| size |
+ * --------------------------------------
+ * | Padding for alignment |
+ * | This includes padding inserted by |
+ * | the compiler (to align fields) and |
+ * | explicit padding inserted by this |
+ * | implementation. If any explicit |
+ * | padding is being used then the |
+ * | sizeof (size) bytes at |
+ * | mem_ptr - CHUNK_OFFSET must be |
+ * | 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
CHUNK_OFFSET */
@@ -187,8 +192,13 @@ extern void * nano_pvalloc(RARG size_t s);
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);
- /* 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);
return c;
}
@@ -314,6 +324,19 @@ void * nano_malloc(RARG malloc_size_t s)
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;
}
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2017-01-13 16:39 [newlib-cygwin] Expand comments on padding used by nano_malloc Corinna Vinschen
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