c - Char array subscript warning

Question

when I use char array subscript as in this example:

int main(){
    char pos=0;
    int array[100]={};

    for(pos=0;pos<100;pos++)
        printf("%i
", array[pos]);

    return 0;
}

I am getting warning that I am using char array subscript:

warning: array subscript has type ‘char’ [-Wchar-subscripts]

Which is OK, because I have this warning enabled.

GCC manual says:

-Wchar-subscripts Warn if an array subscript has type "char". This is a common cause of error, as programmers often forget that this type is signed on some machines. This warning is enabled by -Wall.

So this warning should prevent of using negative array index. My question is, why is this warning active only on char and not also on other signed types?

Thank you.

Answer 1

This is because int is always signed.

char doesn't have to.

char can be signed or unsigned, depending on implementation. (there are three distinct types - char, signed char, unsigned char)

But what's the problem? I can just use values from 0 to 127. Can this hurt me silently?

Oh, yes it can.

//depending on signedess of char, this will
//either be correct loop,
//or loop infinitely and write all over the memory
char an_array[50+1];
for(char i = 50; i >= 0; i--)
{
    an_array[i] = i;
    // if char is unsigned, the i variable can be never < 0
    // and this will loop infinitely
}