The following code compiles without complaints:
struct s {
const int a;
};
static const struct s *s = &(const struct s) {
.a = 5
};
int main(void) {
(void) s;
return 0;
}
However, if we move the definition of s
to the body of main
, i.e.:
struct s {
const int a;
};
int main(void) {
static const struct s *s = &(const struct s) {
.a = 5
};
(void) s;
return 0;
}
we get the error:
error: initializer element is not constant
static const struct s* s = &(const struct s) {
^
Since, in both cases, we deal with static (i.e. compile-time) initialization, why is the second use case illegal? Is there any way to make it legal in the context of a function?
(I have checked this with GCC 7.3.0 and clang 6.0.0, and they both report this as an error)
Per C 2018 6.6 (“Constant expressions”) 7:
More latitude is permitted for constant expressions in initializers. Such a constant expression shall be, or evaluate to, one of the following: … an address constant, or…
Per 6.6 9:
An address constant is a null pointer, a pointer to an lvalue designating an object of static storage duration, or a pointer to a function designator…
Then, in 6.5.2.5 (“Compound literals”) 5, we have:
… If the compound literal occurs outside the body of a function, the object has static storage duration; otherwise, it has automatic storage duration associated with the enclosing block.
Thus, in the first case, the compound literal is an object with static storage duration, and a pointer to it is an address constant. In the second case, the compound literal is an object with automatic storage duration, and a pointer to it is not an address constant.