Is there a way to make an enum unsigned in the C90 standard? (MISRA-C 2004 compliant)

c, enums, misra, standards

Solution

There is no standard C way to control the type chosen for an `enum`. You can do it in implementation specific ways sometimes, like by adding a value to the enumeration that forces the type to be unsigned:

enum {
  x1,
  x2,
  x3,
  giant_one_for_forcing_unsigned = 0x80000000;
};

But that's not even standard C, either (since the value provided won't fit in an `int`). Unfortunately, you're pretty much out of luck. Here's the relevant bit from the standard:

6.7.2.2 Enumeration specifiers, paragraph 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, 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.

You might be better off using `#define` rather than `enum` to make your constants:

#define x1 0U
#define x2 1U
#define x3 2U

uint8_t x = x2;

Problem

I'm trying to find a way to make an enum "unsigned". ``` enum{ x1 = 0, x2, x3 }; uint8_t = x2; /* <--- PC-LINT MISRA-C 2004 will complain about mixing signed and unsigned here */ ``` Of course, I can add a typecast to get rid of the error, that is time consuming and error prone. ``` uint8_t = (uint8_t)x2; /* This works, but is a lot of extra work over the course of 1000s of lines of code*/ ``` So, is there a way to make a specific enum unsigned that MISRA-C 2004 will like?

Original source