What is the meaning of the GCC warning “case label value exceeds maximum value for type”?

Question

My code looks like this:

char * decode_input(char ch)
{
        switch(ch) {
                case 'g':
                        return "get";
                        break;
                case KEY_F(9):
                        return "quit";
                        break;
                default:
                        return "unknown";
                        break;
        }
}

Any clues?

Answer 1

A char is a number between -128 and 127. KEY_F(9) probably is a value outside of that range.

Use:

• unsigned char, or
• int, or
• (char) KEY_F(9)

Or even better, use a debugger and determine sizeof(KEY_F(9)) to make sure it's a byte and not a short.

Answer 2

Well, KEY_F(9) would be 273 (see curses.h) which exceeds the range of char (-128,127).

Answer 3

In this case, KEY_F(9) is evaluating to something outside the range of char. The switch statement is assuming that because its argument is a char, that all case labels will be also. Changing the switch to read switch((unsigned int)ch) will cure it.

Answer 4

It looks like KEY_F(9) must evaluate to something that is outside the range of a char.

Answer 5

What everyone else said regarding the range for char.

I remember this from my early days writing C... you're probably calling decode_input from a loop, right? If the user presses something like F9, you're going to get two bytes in the keyboard buffer - and the first byte will be 0x0.