How to make gcc warn about passing wrong enum to a function

Question

gcc doesn\'t seem to produce a warning with the following code. How can I get it to produce a warning?

typedef enum
{
    REG8_A,
    REG8_B,
    REG8_C
}REG         


        

Answer 1

As others have pointed out, C does not differentiate between an enumerated type and the underlying integer type. (Some compilers might include type-checking for enums or typedefs as extensions; YMMV.)

To get type-checking in C, you can use structs, but then you lose use of the built-in comparison operators and the ability to switch on a variable. You might try something like this, though:

typedef struct {
    enum {
        reg8_val_A,
        reg8_val_B,
        reg8_val_C,
    } val;
} reg8;
#define reg8_A (reg8){.val = reg8_val_A}
#define reg8_B (reg8){.val = reg8_val_B}
#define reg8_C (reg8){.val = reg8_val_C}
…
bool
is_A_or_B(reg8 reg) {
    if reg.val == reg8_A.val    // one way to compare
        return true;
    switch (reg.val) {
        case reg8_val_B:        // the other way to compare; note that
            return true;        // “case reg8_B.val:” will *not* work
        case reg8_val_C:
            return false;
        default:
            fprintf(stderr, "bad reg value %d\n", reg.val);
            abort();
    }
}

(Uses some C99 features.)