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。

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