From 8d998a7b3b741304df860b5dc843826257633237 Mon Sep 17 00:00:00 2001
From: Rich Felker <dalias@aerifal.cx>
Date: Mon, 25 Aug 2014 22:47:27 -0400
Subject: [PATCH] add malloc_usable_size function and non-stub malloc.h

this function is needed for some important practical applications of
ABI compatibility, and may be useful for supporting some non-portable
software at the source level too.

I was hesitant to add a function which imposes any constraints on
malloc internals; however, it turns out that any malloc implementation
which has realloc must already have an efficient way to determine the
size of existing allocations, so no additional constraint is imposed.

for now, some internal malloc definitions are duplicated in the new
source file. if/when malloc is refactored to put them in a shared
internal header file, these could be removed.

since malloc_usable_size is conventionally declared in malloc.h, the
empty stub version of this file was no longer suitable. it's updated
to provide the standard allocator functions, nonstandard ones (even if
stdlib.h would not expose them based on the feature test macros in
effect), and any malloc-extension functions provided (currently, only
malloc_usable_size).
---
 include/malloc.h                | 25 +++++++++++++++++++++++++
 src/malloc/malloc_usable_size.c | 17 +++++++++++++++++
 2 files changed, 42 insertions(+)
 create mode 100644 src/malloc/malloc_usable_size.c

diff --git a/include/malloc.h b/include/malloc.h
index e69de29b..35f8b19c 100644
--- a/include/malloc.h
+++ b/include/malloc.h
@@ -0,0 +1,25 @@
+#ifndef _MALLOC_H
+#define _MALLOC_H
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+#define __NEED_size_t
+
+#include <bits/alltypes.h>
+
+void *malloc (size_t);
+void *calloc (size_t, size_t);
+void *realloc (void *, size_t);
+void free (void *);
+void *valloc (size_t);
+void *memalign(size_t, size_t);
+
+size_t malloc_usable_size(void *);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif
diff --git a/src/malloc/malloc_usable_size.c b/src/malloc/malloc_usable_size.c
new file mode 100644
index 00000000..8cccd9d8
--- /dev/null
+++ b/src/malloc/malloc_usable_size.c
@@ -0,0 +1,17 @@
+#include <malloc.h>
+
+void *(*const __realloc_dep)(void *, size_t) = realloc;
+
+struct chunk {
+	size_t psize, csize;
+	struct chunk *next, *prev;
+};
+
+#define OVERHEAD (2*sizeof(size_t))
+#define CHUNK_SIZE(c) ((c)->csize & -2)
+#define MEM_TO_CHUNK(p) (struct chunk *)((char *)(p) - OVERHEAD)
+
+size_t malloc_usable_size(void *p)
+{
+	return CHUNK_SIZE(MEM_TO_CHUNK(p)) - OVERHEAD;
+}
-- 
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