printf("%p") 并转换为 (void *) [英] printf("%p") and casting to (void *)

问题描述

在最近的一个问题中，有人提到用 printf 打印指针值时，调用者必须将指针强制转换为 void *，如下所示:

In a recent question, someone mentioned that when printing a pointer value with printf, the caller must cast the pointer to void *, like so:

int *my_ptr = ....

printf("My pointer is: %p", (void *)my_ptr);

对于我的生活，我无法弄清楚为什么.我发现了这个问题，其中几乎是一样的.问题的答案是正确的 - 它解释了整数和指针的长度不一定相同.

For the life of me I can't figure out why. I found this question, which is almost the same. The answer to question is correct - it explains that ints and pointers are not necessarily the same length.

这当然是对的，但是当我已经有一个指针时，就像上面的例子，为什么我要从 int * 转换为 无效 *?什么时候 int * 与 void * 不同?事实上，(void *)my_ptr 什么时候生成任何不同于简单的 my_ptr 的机器码?

This is, of course, true, but when I already have a pointer, like in the case above, why should I cast from int * to void *? When is an int * different from a void *? In fact, when does (void *)my_ptr generate any machine code that's different from simply my_ptr?

更新:多名知识渊博的响应者引用了该标准，称传递错误的类型可能会导致未定义的行为.如何?我希望 printf("%p", (int *)ptr) 和 printf("%p", (void *)ptr) 生成完全相同的堆栈-框架.两个调用什么时候会生成不同的栈帧?

UPDATE: Multiple knowledgeable responders quoted the standard, saying passing the wrong type may result in undefined behavior. How? I expect printf("%p", (int *)ptr) and printf("%p", (void *)ptr) to generate the exact same stack-frame. When will the two calls generate different stack frames?

推荐答案

%p 转换说明符需要 void * 类型的参数.如果不传递 void * 类型的参数，函数调用将调用未定义的行为.

The %p conversion specifier requires an argument of type void *. If you don't pass an argument of type void *, the function call invokes undefined behavior.

来自 C 标准(C11，7.21.6.1p8 格式化输入/输出函数):

From the C Standard (C11, 7.21.6.1p8 Formatted input/output functions):

"p - 参数应是指向 void 的指针."

"p - The argument shall be a pointer to void."

C 中的指针类型不需要具有相同的大小或相同的表示形式.

Pointer types in C are not required to have the same size or the same representation.

具有不同指针类型表示的实现示例是 Cray PVP，其中 void * 和 char * 的指针类型表示为 64 位，但为 32 位对于其他指针类型.

An example of an implementation with different pointer types representation is Cray PVP where the representation of pointer types is 64-bit for void * and char * but 32-bit for the other pointer types.

参见Cray C/C++ 参考手册"，9.1.2.2"中的表 3 http://docs.cray.com/books/004-2179-003/004-2179-003-manual.pdf

See "Cray C/C++ Reference Manual", Table 3. in "9.1.2.2" http://docs.cray.com/books/004-2179-003/004-2179-003-manual.pdf

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