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?

Solution 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.

Solution 2

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

Solution 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.

Comments

[RBerteig]

Casting KEY_F(9) to char might loose information depending on the implementation of that macro. Also, char is either signed or unsigned depending on the platform defaults and often compiler switches.

[RBerteig]

The range of char depends on whether char is signed or unsigned. However, if curses.h is the source of the macro, then it is outside the range either way.

[razzed]

True true, and it's been a while since I played in C. I always used types for string characters anyway and always used unsigned char for the type, so I haven't run into this ever...

[razzed]

Note that casting to char will remove the compiler error, but it will likely not behave correctly!