Verify that all set values of an enum are distinct - c

Question

I just recently found out that when setting values of variables of an enum, several variables can have the same value. For example, the following code is perfectly correct:
enum my_enum
{
A = 3,
B = 4,
C = 3,
D = 5
};
However, in my framework, I have to set manualy each variable, since it is used to communicate error codes to other systems. I cannot change the value of an error code without asking for the other teams to modify it as well.
I regrouped all error codes into an enumeration to be sure we do not reuse the same error code for different reasons (which was the case before). But now, nothing stops anyone to reuse the same value for different errors.
Is there a way I can make the compiler return an error if I assign the same value to different names in my enum?
I can compile with either gcc or clang.

Answer 1

A switch statement cannot have two cases with the same value. So you can have a switch somewhere. Make it with no default: case and gcc will warn you that a member of the enum is not handled in the switch statement if you add a new value. If two values of the enum are the same, compilation will fail.
Some examples:
typedef enum {
ERR_NONE = 0,
ERR_SAME = 0
} err_t;
volatile err_t e = ERR_NONE;
int main( int argc, char* argv[] )
{
switch( e )
{
case ERR_NONE:
break;
case ERR_SAME:
break;
}
return 0;
}
with gcc -Wall switch.c gives:
switch.c: In function 'main':
switch.c:16:9: error: duplicate case value
case ERR_SAME:
^
switch.c:13:9: error: previously used here
case ERR_NONE:
^
and...
typedef enum {
ERR_NONE = 0,
ERR_NEXT,
ERR_MISSING,
} err_t;
volatile err_t e = ERR_NONE;
int main( int argc, char* argv[] )
{
switch( e )
{
case ERR_NONE:
break;
case ERR_NEXT:
break;
}
return 0;
}
with gcc -Wall switch-missing.c gives:
switch-missing.c: In function 'main':
switch-missing.c:12:5: warning: enumeration value 'ERR_MISSING' not handled in switch [-Wswitch]
switch( e )
^