bug.pyx

cdef extern from "macros.h":
    # test whether the b'th bit of array a is set:
    unsigned long TESTBIT(unsigned long a[], int b)

cdef unsigned long foo[2]
foo[0] = 281474976710656UL
foo[1] = 0

# what the macro does:
print foo[0] & (1UL << 48)
if (foo[0] & (1UL << 48)):
    print 'bit 48 is set'

# macro works fine on its own:
print TESTBIT(foo, 48)
if TESTBIT(foo, 48) != 0:
    print 'bit 48 is really set'
if TESTBIT(foo, 48):
    print 'bit 48 is set'

print TESTBIT(foo, 48)
if True and TESTBIT(foo, 48) != 0:
    print 'bit 48 is really set'
# bug occurs when combined with other condition:
if True and TESTBIT(foo, 48):
    print 'bit 48 is set'
else:
    print 'bit 48 is NOT set?'

macros.h

#define BITSIZE                 (8*sizeof(long))
#define BITMASK(b)              (1UL << ((b) % BITSIZE))
#define BITSLOT(b)              ((b) / BITSIZE)
#define TESTBIT(a, b)           ((a)[BITSLOT(b)] & BITMASK(b))