Question

We have a set of C macros, here, for using the preprocessor to do bitfield operations, and we run into warnings when attempting to use these macros in visual studio. The problem can be demonstrated very easily:

#define BITFIELD_WIDTHMASK(Width) \
    ((Width) >= 32 ? ~0x0ul : (1ul << (Width)) - 1)

unsigned long foo(void)
{
    return BITFIELD_WIDTHMASK(32);
}

Compiling this with MSVC yields the warning:

test.c(12) : warning C4293: '<<' : shift count negative or too big, undefined behavior

This isn’t a behavior problem – the << operator won’t be used in this case, and that should be detected at compile time. But does anyone have any suggestions about how to rewrite the macro to avoid the warning? Or, failing that, how to redesign the macro interface for this?

Thanks in advance

Answer 1

What about:

#define BITFIELD_WIDTHMASK(Width) \
    ((Width) >= 32 ? ~0x0ul : (1ul << (Width % 32)) - 1)

?