Error when pass "enum" in a function with variable arguments

Question

I'm reading the book Head First C and I am the part about the variable arguments. I wrote the following code: ``` #include #include #include enum drink { MUDSLIDE, FUZZY_NAVEL, MONKEY_GLAND, ZOMBIE }; double price(enum drink d) { switch(d) { case MUDSLIDE: return 6.79; case FUZZY_NAVEL: return 5.31; case MONKEY_GLAND: return 4.82; case ZOMBIE: return 5.89; } return 0; } double calc(int args, ...) { double total = 0; va_list ap; va_start(ap, args); int i; for (i = 0; i < args; i++) { int currentDrink = va_arg(ap, int); total += price((drink) currentDrink); } va_end(ap); return total; } int main() { printf("Price is %.2f ", calc(2, MONKEY_GLAND, MUDSLIDE)); return 0; } ``` The code compiles and works perfectly. But... There are two different lines of my solution with the book. My: ``` int currentDrink = va_arg(ap, int); total += price((drink) currentDrink); ``` Book: ``` enum drink currentDrink = va_arg(ap, enum drink); total += price(currentDrink); ```…

Accepted Answer

Variadic arguments to functions are subject to a conversion called default argument promotions: Everything with a conversion rank smaller than that of `int` gets promoted to `int` before passed to a variadic function. And an `enum` has a smaller conversion rank (and may be represented as a `short` or something else in your case). So what your callee sees is an int and must fetch it by `va_arg` as such.

C99 7.15.1.1 p.2 (emphasis mine) about `va_arg`:

[…] If there is no actual next argument, or if type is not compatible with the type of the actual next argument (as promoted according to the default argument promotions), the behavior is undefined, except for the following cases:

- one type is a signed integer type, the other type is the corresponding unsigned integer type, and the value is representable in both types;

- [sth. about pointer types]

And 6.7.2.2, p.4:

Each enumerated type shall be compatible with char, a signed integer type, or an unsigned integer type. The choice of type is implementation-defined, […]

So, the `va_arg(ap, enum drink)` version has no defined bahavior (as far as the C standard is concerned). At least, if the compiler doesn't specify to always use `int` for `enum`s. Hence the warning from `gcc`.

Some coding-guidelines say to avoid `enum` usage for types entirely, using `int` everywhere and only define `enum` constants:

```
enum {
    MUDSLIDE, FUZZY_NAVEL, MONKEY_GLAND, ZOMBIE
};
```

and

```
double price(int d);
```

HTH

Error when pass "enum" in a function with variable arguments

Solution

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