How to neatly avoid C casts losing truth

Question

I'm quite happy that, in C, things like this are bad code:

(var_a == var_b) ? TRUE : FALSE

However, what's the best way of dealing with this:

/* Header stuff */
#define INTERESTING_FLAG 0x80000000
typedef short int BOOL;

void func(BOOL);

/* Code */
int main(int argc, char *argv[])
{
        unsigned long int flags = 0x00000000;

        ... /* Various bits of flag processing */

        func(flags & INTERESTING_FLAG); /* func never receives a non-zero value
                                         * as the top bits are cut off when the
                                         * argument is cast down to a short 
                                         * int
                                         */
}

Is it acceptable (for whatever value of acceptable you're using) to have (flags & FLAG_CONST) ? TRUE : FALSE?

Answer 1

I would in either case called func with (flags & INTERESTING_FLAG) != 0 as an argument to indicate that a boolean parameter is required and not the arithmetic result of flags & INTERESTING_FLAG.

Answer 2

I'd prefer (flags & CONST_FLAG) != 0. Better still, use the _Bool type if you have it (though it's often disguised as bool).

Answer 3

Set your compiler flags as anally as possible, to warn you of any cast that loses bits, and treat warnings as errors.