C - forward declaration of enums?

Forward declaration of enums in C does not work for me. I searched the internet and Stack Overflow but all of the questions regarding forward declarations of enumerators refer to C++. What do you do for declaring enumerators in C? Put them at the top of each file (or in a header) so that all functions in the 开发者_运维百科file can access them?

Put them in a header so that all files that need them can access the header and use the declarations from it.

When compiled with the options:

$ /usr/bin/gcc -g -std=c99 -Wall -Wextra -c enum.c
$

GCC 4.2.1 (on MacOS X 10.7.1) accepts the following code:

enum xyz;

struct qqq { enum xyz *p; };

enum xyz { abc, def, ghi, jkl };

Add -pedantic and it warns:

$ /usr/bin/gcc -g -std=c99 -Wall -Wextra -pedantic -c enum.c
enum.c:1: warning: ISO C forbids forward references to ‘enum’ types
enum.c:5: warning: ISO C forbids forward references to ‘enum’ types
$

Thus, you are not supposed to try using forward declarations of enumerated types in C; GCC allows it as an extension when not forced to be pedantic.

You can't "forward-declare" enums because the compiler won't know the size of the enum. The C standard says " 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".

I came here having the same error, but there is not really much information provided here on the code/error.

My Makefile-flags are: -Wall -Wextra -Werror -pedantic -std=c17

In my header I have the following enum:

typedef enum 
{
  IS_HEAD = 1, 
  IS_VALUE = 2,
  IS_SIDE
} CoinResult;

The tutorials here and there

Would recommend to use something like this:

enum CoinResult cr;
cr = IS_SIDE;

This results in the error stated by the OP.

Solved by using:

CoinResult cr = IS_SIDE; 

Not sure which C-Standard, Code or reference OP was using, but I somewhat agree: Most tutorials and solutions for this relatively simple issue are kinda ambiguous.

CoinResult isn't an enum, it's a type. If you had

enum CoinResult {
    IS_HEAD = 1,
    IS_VALUE = 2,
    IS_SIDE,
};

then

    enum CoinResult cr;

would be correct.