GCC不会通过位屏蔽警告0或1

Question

Following program working fine on GCC without any warning.

#include <stdio.h>

int func(unsigned int mask)
{
    return (!mask & 0) ? 1 : 0;  // No warning for 0 or 1
                    ^^
                    0 or 1
}

int main() {
        func(1);
        return 0;
}

In this program If I write 0 or 1 . GCC doesn't give any warning.

But, If I write other number like 2 :

 return (!mask & 2) ? 1 : 0; // GCC gives warning

Then GCC give following warning.

 warning: suggest parentheses around operand of ‘!’ or change ‘&’ to ‘&&’ or ‘!’ to ‘~’ [-Wparentheses]
     return (!mask & 2) ? 1 : 0;

GCC warns about other number like 2 , which is good, but why doesn't it warn about 0 or 1 ?

Answer 1

 (!mask & 2)

here you're mixing bitwise operators with boolean ones. That doesn't matter with 0 or 1 since only handling the first bit (boolean operators turn expressions into 0 or 1), but with 2 the result is different if you put parentheses or not, hence the warning.

(another example here which proves that gcc tries hard not to issue warnings unneccessarily)

Answer 2

Because your code is valid C code. And the warning is only a suggestion .

BTW !mask (a logical not operation) is a boolean, so can only be 0 or 1. if you want a bitwise-not operation, use ~mask . Look at the list of operators in C.

As Jean-François Fabre explains having 0 and 1 is licit and common (they are common bitmasks).

GCC is emitting (with gcc -Wall -Wextra ) some warnings (using heuristics), but it has to balance between the case of weird but valid code and the case of improbable code. It tries to avoid emitting useless warnings.

You can extend GCC to give additional warnings if you wanted to. Just spend months to code your own GCC plugin providing those warnings.