Index: Modules/_ctypes/cfield.c
===================================================================
--- Modules/_ctypes/cfield.c	(revision 74022)
+++ Modules/_ctypes/cfield.c	(working copy)
@@ -427,11 +427,7 @@
 #define NUM_BITS(x) ((x) >> 16)
 
 /* This seems nore a compiler issue than a Windows/non-Windows one */
-#ifdef MS_WIN32
-#  define BIT_MASK(size) ((1 << NUM_BITS(size))-1)
-#else
-#  define BIT_MASK(size) ((1LL << NUM_BITS(size))-1)
-#endif
+#define BIT_MASK(v, size) (sizeof(v)*8 == NUM_BITS(size) ? ~0U : ((1U << NUM_BITS(size))-1))
 
 /* This macro CHANGES the first parameter IN PLACE. For proper sign handling,
    we must first shift left, then right.
@@ -445,7 +441,7 @@
 /* This macro RETURNS the first parameter with the bit field CHANGED. */
 #define SET(x, v, size)							\
 	(NUM_BITS(size) ?						\
-	 ( ( x & ~(BIT_MASK(size) << LOW_BIT(size)) ) | ( (v & BIT_MASK(size)) << LOW_BIT(size) ) ) \
+	 ( ( x & ~(BIT_MASK(v, size) << LOW_BIT(size)) ) | ( (v & BIT_MASK(v, size)) << LOW_BIT(size) ) ) \
 	 : v)
 
 /* byte swapping macros */