当它是有效的访问指针＆QUOT;死亡＆QUOT;目的？ [英] When is it valid to access a pointer to a "dead" object?

问题描述

首先，要澄清，我的不可以谈论取消引用无效指针！

First, to clarify, I am not talking about dereferencing invalid pointers!

考虑以下两个例子。

示例1

typedef struct { int *p; } T;

T a = { malloc(sizeof(int) };
free(a.p);  // a.p is now indeterminate?
T b = a;    // Access through a non-character type?

例2

void foo(int *p) {}

int *p = malloc(sizeof(int));
free(p);   // p is now indeterminate?
foo(p);    // Access through a non-character type?

问

执行下列任一上面的例子未定义行为？

Do either of the above examples invoke undefined behaviour?

上下文

这问题是针对提出到<一个href=\"http://stackoverflow.com/questions/17019778/what-will-happen-if-a-stdvector-element-commits-suicide-using-delete-this/17019929#comment24597487_17019929\">this讨论。该建议是，例如，指针参数可能会被传递给通过86段寄存器功能，这可能导致硬件异常。

This question is posed in response to this discussion. The suggestion was that, for example, pointer arguments may be passed to a function via x86 segment registers, which could cause a hardware exception.

从C99的标准，我们了解以下内容（重点煤矿）：

From the C99 standard, we learn the following (emphasis mine):

[3.17] 不确定的值的 - 要么是未指定的值或陷阱重新presentation

[3.17] indeterminate value - either an unspecified value or a trap representation

和则：

[6.2.4 P2] 指针的值变为不确定当
  它所指向的对象达到其生命周期的结束。

[6.2.4 p2] The value of a pointer becomes indeterminate when the object it points to reaches the end of its lifetime.

和则：

[6.2.6.1 P5] 某个对象重新presentations不必再present对象类型的值。如果一个对象的存储值有这样一个重新presentation，由左值前pression不具有字符类型，读这种行为是未定义。如果这样的重presentation是由由不具有字符类型左值前pression修改全部或任何对象的一部分副作用产生的，该行为是不确定的。这样的再presentation叫做陷阱重新presentation

[6.2.6.1 p5] Certain object representations need not represent a value of the object type. If the stored value of an object has such a representation and is read by an lvalue expression that does not have character type, the behavior is undefined. If such a representation is produced by a side effect that modifies all or any part of the object by an lvalue expression that does not have character type, the behavior is undefined. Such a representation is called a trap representation.

把所有的这一起，是什么做限制我们对访问指向死的对象？

Taking all of this together, what restrictions do we have on accessing pointers to "dead" objects?

附录

虽然我已经引述上面的C99标准，我很想知道，如果行为在任何C ++标准是不同的。

Whilst I've quoted the C99 standard above, I'd be interested to know if the behaviour differs in any of the C++ standards.

推荐答案

例2是无效的。在你的问题的分析是正确的。

Example 2 is invalid. The analysis in your question is correct.

实施例1是有效的。结构类型从不保存陷阱重新presentation，即便其成员之一呢。这意味着结构分配，一个系统，其中阱再presentations会造成问题时，必须被实现为按字节拷贝，而不是一个部件逐件拷贝。

Example 1 is valid. A structure type never holds a trap representation, even if one of its members does. This means that structure assignment, on a system where trap representations would cause problems, must be implemented as a bytewise copy, rather than a member-by-member copy.

6.2.6重新$类型p $ psentations

6.2.6.1常规

6 [...]结构或联合对象的值是从来没有一件T说唱
  再presentation，即使结构或联合对象的成员的值可能
  陷阱重新presentation。

6 [...] The value of a structure or union object is never a t rap representation, even though the value of a member of the structure or union object may be a trap representation.

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