空指针的解引用
NULL指针的访问异常是C程序员常见的bug，事实上部分情况下空指针可以解引用，下面的thread，列出了几个例子。
http://stackoverflow.com/questions/6793262/why-dereferencing-a-null-pointer-is-undefined-behaviour
On the PDP-11, it happened that address 0 always contained the value 0, so dereferencing a null pointer also gave the value 0. Quite a few people who used these machines felt that since they were the original machine C had been written on/used to program, that this should be considered canonical behavior for C on all machines (even though it originally happened quite accidentally).
On some other machines (Interdata comes to mind, though my memory could easily be wrong) address 0 was put to normal use, so it could contain other values.
There was also some hardware on which address 0 was actually some memory-mapped hardware, so reading/writing it did special things -- not at all equivalent to reading/writing normal memory at all.
空指针解引用是否导致异常应该是硬件设备和OS组合决定的。以前在VXwork下工作，空指针也可以解引用，可以访问内存0地址，还可以修改内容。这种情况下，为了便于程序员debug，印象中我们大概是采用了对于0地址内容监控，如果内容有改动则报告或者crash。
建议继续学习：
- Linus：利用二级指针删除单向链表 (阅读：12812)
- C语言结构体里的成员数组和指针 (阅读：5925)
- 通过引用计数解决野指针的问题(C&C++) (阅读：4633)
- cpp智能指针的简单实现 (阅读：3951)
- C 语言中统一的函数指针 (阅读：4001)
- 重构发现：指针操作问题 (阅读：3378)
- 一起空指针引发的程序问题的排查过程 (阅读：2791)
- 关于类成员函数指针的正确写法 (阅读：2482)
扫一扫订阅我的微信号：IT技术博客大学习
- 作者：appleleaf 来源： kernelchina blogs
- 标签： 指针
- 发布时间：2012-09-06 23:46:40
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