c - extern 上的这个附加常量是 UB 吗？

Question

我有以下内容:

//h.h file
#pragma once
struct A { int x; };

//a.c file
#include "h.h"
struct A* a;

//b.c
#include "h.h"
extern struct A* const a;
int main() {}

我在 extern 声明中添加了额外的 const 。添加这个 const 会是 UB 吗？

如果不是 UB，那么在 main 中执行下面的操作就会是 UB 吗？

(*(struct A**)&a) = malloc(sizeof(struct A));

Answer 1

是的。来自附件 J.2:

• Two declarations of the same object or function specify types that are not compatible (6.2.7).

和 6.2.7 ¶2:

All declarations that refer to the same object or function shall have compatible type; otherwise, the behavior is undefined.

和 6.7.3 ¶10:

For two qualified types to be compatible, both shall have the identically qualified version of a compatible type; the order of type qualifiers within a list of specifiers or qualifiers does not affect the specified type.