将整数常量转换为指针类型 [英] Casting integer constant to a pointer type

问题描述

UB是否将任意整数常量转换为指向对象/函数类型的指针(例如在单元测试中使用)?

Is it UB to cast an arbitrary integer constant to a pointer to a object/function type (to use in unit tests for example)?

struct helper; //opqaue, creation the structure is complicated

struct my_struct{
   struct helper *h_ptr;
   char another_member;
};

static inline struct my_struct *create_my_struct(struct helper *h_ptr, char am){
    struct my_struct *m_ptr = malloc(sizeof(*m_ptr));
    m_ptr->h_ptr = h_ptr;
    m_ptr->another_member = am;
    return m_ptr;
}

我要为此编写单元测试:

I want to write unit test for it as follows:

uintptr_t helper_ptr = (uintptr_t) 2; //How about this?
char am = 'a';
struct my_struct *ms_ptr = create_my_struct((struct helper *) helper_ptr, am);
//do asserts

我不确定的是(struct helper *) helper_ptr.是UB吗?如果(uintptr_t) 2对齐不正确怎么办?

The thing that I'm not sure about is (struct helper *) helper_ptr. Is it UB? What if (uintptr_t) 2 is not correctly aligned?

推荐答案

根据C11标准6.3.2.3指针的第5段:

From the C11 Standard 6.3.2.3 Pointers, paragraph 5:

整数可以转换为任何指针类型.除非先前指定，否则结果是实现定义的，可能未正确对齐，可能未指向引用类型的实体，并且可能是陷阱表示.

An integer may be converted to any pointer type. Except as previously specified, the result is implementation-defined, might not be correctly aligned, might not point to an entity of the referenced type, and might be a trap representation.

因此，只要不取消引用它就可以做到这一点，而且它不是UB，但是发生什么情况取决于平台.

So you can do it as long as you do not dereference it, and it is not UB, but whatever happens will depend on the platform.

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