Why enums have names in C?

Question

Let's take the following example:

#include <stdio.h>


enum fruit  {APPLE, ORANGE, BANANA};
enum planet {EARTH, MARS, NEPTUNE};


void foo(enum fruit f)
{
    printf("%ld", f);
}


int main()
{
    enum planet p = MARS;

    foo(p); /* compiler doesn't complain */

    return 0;
}

What's the point of enums having names, if that code would compile?
Here is my command line for gcc:

gcc file.c -ansi -Wall -Wextra

Answer 1

AFAIK, in C99, the "tag" (what you call the name, e.g. fruit) of enum is optional.

So

enum { BLACK, RED, GREEN, BLUE, YELLOW, WHITE };

is acceptable (and quite useful)

and you can even give values integral constants

enum { RFCHTTP =2068, RFCSMTP= 821 };

which is a more modern way than

#define RFCHTTP 2068
#define RFCSMTP 821

(in particular, such enum values are known to the debugger, when you compile with debug info)

But in C enum are mostly integral values. Things are different in C++.

Regarding using %ld format control conversion for printf with an enum argument it is probably some undefined behavior (so you should be scared). Probably gcc -Wall -Wextra might warn you.

BTW, GCC has many options. Read its chapter on Invoking GCC.

I recommend (in 2017) coding for C99 or C11, not old 1989 ANSI C.

Answer 2

Even if your code will compile, the following will not

enum fruit *f = 0;
enum planet *p = f;

since enum fruit and enum planet are two different unrelated types.

What you observe in your code sample is a mere implicit conversion between two enum types in value context. In exactly the same way you can implicitly convert a double value to int type. Yet, I hope, you will not use that to conclude that there's no reason to distinguish between double and int.

Your question would be more justified if you asked "Why do we have implicit conversions between unrelated enum types?" As for enum types having distinctive names... there are perfectly good reasons for that.