Why does the C compiler allow silent casting from uint16_t to an enum type?

user_44330 0
Viewed 124

I was under the impression that it was an error or at least a warning for an integer type to be passed into a function that requires an enum argument. But I tried it in Compiler Explorer with -Wall -Wextra -Werror and there are no warnings. What am I missing? Does the C standard allow this?

Perhaps more importantly to my immediate use, is there a compiler option for gcc or clang to warn for this? The intent of having an enum type as a formal argument is as a contract to restrict to valid enum values.

#include <stdint.h>

typedef enum {
    FLAG_A = 1,
    FLAG_B = 2,
    FLAG_C = 4
} FLAG;

uint16_t flags = 0;

void clearFlags(FLAG flag)
{
    flags &= ~flag;
}

void doit()
{
    clearFlags(flags);   // this should not be ok; flags is type uint16_t
}
language-lawyer enums c
2 Answers

Enumerated types are compatible with integer types, so it is valid to pass an integer to a function that expects an enum as an argument.

Section 6.7.2.2p4 of the C standard regarding Enumeration Specifiers states:

Each enumerated type shall be compatible with char, a signed integer type, or an unsigned integer type. The choice of type is implementation-defined, but shall be capable of
representing the values of all the members of the
enumeration. The enumerated type is incomplete until immediately after the } that terminates the list of enumerator declarations, and complete thereafter.

The definition of compatible types is in section 6.2.7p1:

Two types have compatible type if their types are the same.

So this means that an enumerated type actually has the same type as one of the integer types.

user_16871190

It's legal (as already explained in other answers). The compiler may (but is not required to) warn:

A value is given to an object of an enumerated type other than by assignment of an enumeration constant that is a member of that type, or an enumeration object that has the same type, or the value of a function that returns the same enumerated type

— Annex I, Common warnings.

user_1529480
Related