bpo-36389: Change PyMem_SetupDebugHooks() constants (GH-12782)

Modify CLEANBYTE, DEADDYTE and FORBIDDENBYTE constants: use 0xCD,
0xDD and 0xFD, rather than 0xCB, 0xBB and 0xFB, to use the same byte
patterns than Windows CRT debug malloc() and free().

2 files changed, 10 insertions, 8 deletions

diff --git a/Objects/object.c b/Objects/object.c
index c9aa479..3fad73c 100644
--- a/Objects/object.c
+++ b/Objects/object.c
@@ -415,13 +415,12 @@ _Py_BreakPoint(void)
 }
 
 
-/* Heuristic checking if the object memory has been deallocated.
-   Rely on the debug hooks on Python memory allocators which fills the memory
-   with DEADBYTE (0xDB) when memory is deallocated.
+/* Heuristic checking if the object memory is uninitialized or deallocated.
+   Rely on the debug hooks on Python memory allocators:
+   see _PyMem_IsPtrFreed().
 
    The function can be used to prevent segmentation fault on dereferencing
-   pointers like 0xdbdbdbdbdbdbdbdb. Such pointer is very unlikely to be mapped
-   in memory. */
+   pointers like 0xDDDDDDDDDDDDDDDD. */
 int
 _PyObject_IsFreed(PyObject *op)
 {
diff --git a/Objects/obmalloc.c b/Objects/obmalloc.c
index e919fad..be43c7a 100644
--- a/Objects/obmalloc.c
+++ b/Objects/obmalloc.c
@@ -1915,13 +1915,16 @@ _Py_GetAllocatedBlocks(void)
 /* Special bytes broadcast into debug memory blocks at appropriate times.
  * Strings of these are unlikely to be valid addresses, floats, ints or
  * 7-bit ASCII. If modified, _PyMem_IsPtrFreed() should be updated as well.
+ *
+ * Byte patterns 0xCB, 0xBB and 0xFB have been replaced with 0xCD, 0xDD and
+ * 0xFD to use the same values than Windows CRT debug malloc() and free().
  */
 #undef CLEANBYTE
 #undef DEADBYTE
 #undef FORBIDDENBYTE
-#define CLEANBYTE      0xCB    /* clean (newly allocated) memory */
-#define DEADBYTE       0xDB    /* dead (newly freed) memory */
-#define FORBIDDENBYTE  0xFB    /* untouchable bytes at each end of a block */
+#define CLEANBYTE      0xCD    /* clean (newly allocated) memory */
+#define DEADBYTE       0xDD    /* dead (newly freed) memory */
+#define FORBIDDENBYTE  0xFD    /* untouchable bytes at each end of a block */
 
 static size_t serialno = 0;     /* incremented on each debug {m,re}alloc */