的printf（QUOT;％P＆QUOT;）和铸造（无效*） [英] printf("%p") and casting to (void *)

问题描述

在最近的问题，有人提到，与打印printf的指针值时，调用者必须转换指针为void *，像这样：

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);

有关我的生活我想不出为什么。我发现<一个href=\"http://stackoverflow.com/questions/7290923/when-printf-is-an-address-of-a-variable-why-use-void\">this问题时，这几乎是相同的。这个问题的答案的问题是正确的 - 这解释了整数和指针不一定相同长度

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.

这是当然的，真实的，但是当我的已的有一个指针，就像上面的例子中，我为什么要自投为int * 到无效* ？当从一个void *不同的一个int *？事实上，当确实（无效*）my_ptr 产生的任何机器code这是不同于简单的 my_ptr ？

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：
多懂行反应援引的标准，并称传递错误的类型可能会导致不确定的行为。怎么样？我期待的printf（％P，为（int *）PTR）和的printf（％P（无效*）PTR）来产生完全相同的堆栈帧。当将两个调用生成不同的堆栈帧？

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

在C语言中所有指针类型都在重新presentation可能不同。所以，是的，为int * 是不同的void * 。一个真实的平台，可以说明这种差异可能很难（甚至不可能）找到，但在概念层次上的差异仍然存在。

In C language all pointer types are potentially different in representation. So, yes, int * is different from void *. A real-life platform that would illustrate this difference might be difficult (or impossible) to find, but at the conceptual level the difference is still there.

在换句话说，在一般的情况下，不同的指针类型在重新presentation不同。 为int * 是不同的void * ，不同于双* 。该平台使用相同的重新presentation为事实上无效* 和为int * 无非是多巧合的是，就C语言而言。

In other words, in general case different pointer types are different in representation. int * is different from void * and different from double *. The fact that your platform uses the same representation for void * and int * is nothing more than a coincidence, as far as C language is concerned.

语言指出，一些指针类型都需要有相同的再presentations，其中包括无效* 与的char * ，指向不同的结构类型，或者说，为int * 和 const int的* 。但这些都是从一般规则公正的例外情况。

The language states that some pointer types are required to have identical representations, which includes void * vs. char *, pointers to different struct types or, say, int * and const int *. But these are just exceptions from the general rule.

这篇关于的printf（QUOT;％P＆QUOT;）和铸造（无效*）的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！