It is a requirement of the C standard that objects with static storage duration are initialised to a "zero of the appropriate type" (which eg. means NULL for po

It is a requirement of the C standard that objects with static storage duration are initialised to a "zero of the appropriate type" (which eg. means NULL for pointers). In most implementations this can take advantange of the BSS section (which is why the requirement came about in the first place), but it's required to work even on eg. MS-DOS. For the second part of your comment, comparisons are always done between the same type. In this case, either the -3 is promoted to unsigned (which gives the result UINT_MAX - 2, a large positive number) or the bitfield is promoted to int. It is the rules for this type promotion that is the heart of the problem.