不（为size_t）（（的char *）0）以往任何时候都不能计算为0？ [英] Does (size_t)((char *)0) ever not evaluate to 0?

问题描述

根据的回应为什么减去offsetof（）空指针？（和我的K＆放阅读; R），C标准并不要求（为size_t）（（的char *）0）== 0 。不过，我从来没有见过，其中铸造一个空指针到一个整数类型的计算结果为别的情况。

According to the responses in "Why subtract null pointer in offsetof()?" (and my reading of K&R), the C standard doesn't require that (size_t)((char *)0) == 0. Still, I've never seen a situation where casting a null pointer to an integer type evaluates to anything else.

如果有一个编译器或场景，其中（为size_t）（（的char *）0）！= 0 ，是什么呢？

If there is a compiler or scenario where (size_t)((char *)0) != 0, what is it?

推荐答案

好了，你也知道，给定类型的空指针的物理重新presentation不一定全零的位模式。当强制转换指针（任何指针）的值为整型，其结果是所定义的实施，但通常（并且是这样的意图）的指针的数值 - 数字地址 - 保持不变，如果可能的话。这意味着，如果在给定的平台类型的空指针的char * 重新由 0xBAADF00D psented $ P $格局（例如），上述前pression将评估为 0xBAADF00D ，而不是零。当然，对于你需要用非零空指针的平台。我个人从来没有这样的平台合作，虽然我听说了一些像在那里（如，在嵌入式平台领域它不是不寻常的东西）。

Well, as you know, the physical representation of null pointer of a given type is not necessarily all-zero bit pattern. When you forcefully convert a pointer (any pointer) value to integer type, the result is implementation defined, but normally (and that's the intent) the numerical value of the pointer - the numerical address - remains unchanged, if possible. This means that if on a given platform a null pointer of type char * is represented by 0xBAADF00D pattern (for example), the above expression will evaluate to 0xBAADF00D, and not to zero. Of course, for that you'd need a platform with non-zero null-pointers. I personally never worked with such platforms, although I heard about a number of real platforms like that out there (like, in the realm of embedded platforms it is not something unusual).

此外，作为额外的音符，不同类型的空指针值可以具有不同的物理重新presentations，这意味着在理论上可以从获得不同的值（为size_t）（为（int *） 0），（为size_t）（（的char *）0）和（为size_t）（（双*）0） 。但是，这将是一个颇为奇特的情况，虽然完全可以从查看摘要C语言的地步。

Moreover, as an additional note, null pointer values of different types can have different physical representations, meaning that in theory you can get different values from (size_t) ((int *) 0), (size_t) ((char *) 0) and (size_t) ((double *) 0). But that would be a rather exotic situation, albeit perfectly possible from the point of view of abstract C language.

P.S。阅读这里（C FAQ）与非零空指针的实际平台的一些例子。

P.S. Read here (C FAQ) for some examples of actual platforms with non-zero null pointers.

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