C - Why can't I use auto, extern, static or register variables in a union?

Question

Try this code with all four storage classes:

#include <stdio.h>

union Data{
int a;
double b;
auto int c;
};

int main() {
union Data data;
printf("%lu",sizeof(data));
return 0;
}

They all give the same error. Why?

Answer 1

In a union all variables are stored at the same address. If a has the storage class auto and b register they would be stored at different addresses but this is not allowed in a union.

Answer 2

Well, this one is interesting. These keywords mean different things in C than in C++. In C, they are all "storage-class specifiers".

https://devdocs.io/c/language/storage_duration

Why can't storage-class specifiers be used in unions (and structs), to declare their members?

"For any struct or union declared with a storage-class specifier, the storage duration (but not linkage) applies to their members, recursively."

The C language dictates that the storage duration of a member within an union cannot be defined separately from other members. What we can do is to define the storage duration of the union instance. E.g.

static union Data data;
auto union Data data2;