memcmp() can return values other than -1, 0, and +1 but tp_compare

must not.

Lib/test/test_types.py

@@ -233,6 +233,9 @@ print 'Buffers'
 try: buffer('asdf', -1)
 except ValueError: pass
 else: raise TestFailed, "buffer('asdf', -1) should raise ValueError"
+cmp(buffer("abc"), buffer("def")) # used to raise a warning: tp_compare didn't return -1, 0, or 1
+
+cmp(buffer('abc'), buffer('def'))
 
 try: buffer(None)
 except TypeError: pass

Misc/NEWS

@@ -12,6 +12,8 @@ What's New in Python 2.5 release candidate 1?
 Core and builtins
 -----------------
 
+- Bug #1536786: buffer comparison could emit a RuntimeWarning.
+
 - Bug #1535165: fixed a segfault in input() and raw_input() when
   sys.stdin is closed.
 

Objects/bufferobject.c

@@ -272,7 +272,7 @@ buffer_compare(PyBufferObject *self, PyBufferObject *other)
 	if (min_len > 0) {
 		cmp = memcmp(p1, p2, min_len);
 		if (cmp != 0)
-			return cmp;
+			return cmp < 0 ? -1 : 1;
 	}
 	return (len_self < len_other) ? -1 : (len_self > len_other) ? 1 : 0;
 }