FP integerOverflow?

[jacob s]

hi,
given this snippet:

#include <stdbool.h>
#include <stdint.h>

#define MSB_SET (1 << 31)

bool nonsense(uint32_t input) {
    return (input & ((uint32_t) MSB_SET)) != 0;
}

cppcheck gives:

test.c:7:33: error: Signed integer overflow for expression '1<<31'. [integerOverflow]
    return (input & ((uint32_t) MSB_SET)) != 0;

is that warning correct with the explicit cast?

thanks, jacob

[CHR]

I think so, since the overflow happens before the cast. This should fix it: #define MSB_SET (1u << 31).

[Daniel Marjamäki]

I agree with CHR ; technically 1 << 31 is undefined behavior.

Relevant quote from ISO C99 (6.5.7/4)

The result of E1 << E2 is E1 left-shifted E2 bit positions; vacated
bits are filled with zeros. If E1 has an unsigned type, the value of
the result is E1 × 2**E2, reduced modulo one more than the maximum
value representable in the result type. If E1 has a signed type and
nonnegative value, and E1 × 2**E2 is representable in the result
type, then that is the resulting value; otherwise, the behavior is
undefined.

In this case the type of E1 will be "signed int".

With 1u there will not be undefined behavior.

[jacob s]

right - thanks for your reply!