是＆QUOT;结构黑客＆QUOT;技术上未定义的行为？ [英] Is the "struct hack" technically undefined behavior?

问题描述

我要问的是著名的struct的最后一个成员具有可变长度的把戏。它是这样的：

What I am asking about is the well known "last member of a struct has variable length" trick. It goes something like this:

struct T {
    int len;
    char s[1];
};

struct T *p = malloc(sizeof(struct T) + 100);
p->len = 100;
strcpy(p->s, "hello world");

的由于该结构在内存布局方式，我们能够覆盖在结构上比必要块较大和治疗的最后一个成员，就好像它是比 1字符<大/ code>指定。

Because of the way that the struct is laid out in memory, we are able to overlay the struct over a larger than necessary block and treat the last member as if it were larger than the 1 char specified.

所以，问题是：这是技术在技术上未定义的行为？。我希望它是，但很好奇的标准说这个问题。

So the question is: Is this technique technically undefined behavior?. I would expect that it is, but was curious what the standard says about this.

PS：我知道C99的方法来此，我想答案是上面列出专门坚持把戏的版本

PS: I am aware of the C99 approach to this, I would like the answers to stick specifically to the version of the trick as listed above.

推荐答案

由于Ç常见问题解答说：

目前还不清楚，如果是合法的或便携式的，但它是相当受欢迎的。

It's not clear if it's legal or portable, but it is rather popular.

和

...官方间pretation已经认定它没有严格遵守C标准一致，虽然它似乎在所有已知的实施工作。 （其中仔细检查数组边界的编译器可能会发出警告。）

... an official interpretation has deemed that it is not strictly conforming with the C Standard, although it does seem to work under all known implementations. (Compilers which check array bounds carefully might issue warnings.)

背后的严格符合位的基本原理是在规范中，部分的 J.2未定义行为，其中包括未定义行为的列表：

The rationale behind the 'strictly conforming' bit is in the spec, section J.2 Undefined behavior, which includes in the list of undefined behavior:

  
• 数组下标超出范围，即使对象是与给定的下标显然是可访问的（如在左值前pression A [1] [7] 给出的声明 int类型的[4] [5] ）（6.5.6）。

  

• An array subscript is out of range, even if an object is apparently accessible with the given subscript (as in the lvalue expression a[1][7] given the declaration int a[4][5]) (6.5.6).

节第8 6.5.6加法运算符有超出定义数组边界访问未定义再次提及：

Paragraph 8 of Section 6.5.6 Additive operators has another mention that access beyond defined array bounds is undefined:

如果这两个指针操作数和结果指向相同的数组对象，或者一个过去的数组对象的最后一个元素的元素，该评估也不得产生溢出;否则，行为是不确定的。

If both the pointer operand and the result point to elements of the same array object, or one past the last element of the array object, the evaluation shall not produce an overflow; otherwise, the behavior is undefined.

这篇关于是＆QUOT;结构黑客＆QUOT;技术上未定义的行为？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！