取消引用一个NULL指针＆QUOT;？;究竟由＆QUOT意思 [英] What EXACTLY is meant by "de-referencing a NULL pointer"?

问题描述

我是一个完整的新手到C，和我的大学工作中我遇到在code评论说，往往是指取消引用一个NULL指针。我有在C＃中的背景下，我已经得到通过，这可能是类似于的NullReferenceException你是在净得到的，但现在我有严重的怀疑。

I am a complete novice to C, and during my university work I've come across comments in code that often refer to de-referencing a NULL pointer. I do have a background in C#, I've been getting by that this might be similar to a "NullReferenceException" that you get in .Net, but now I am having serious doubts.

有人请给我解释一下在深入浅出而言究竟这是什么，以及为什么它是坏的？

Can someone please explain to me in laymans terms exactly what this is and why it is bad?

推荐答案

A NULL 指针指向一个不存在的内存。这可能是地址 00000000 或任何其他实现定义值（只要它不能是一个真正的地址）。解除引用意味着试图访问无论是由指针指向的。在 * 运算符是对其操作：

A NULL pointer points to memory that doesn't exist. This may be address 0x00000000 or any other implementation-defined value (as long as it can never be a real address). Dereferencing it means trying to access whatever is pointed to by the pointer. The * operator is the dereferencing operator:

int a, b, c; // some integers
int *pi;     // a pointer to an integer

a = 5;
pi = &a; // pi points to a
b = *pi; // b is now 5
pi = NULL;
c = *pi; // this is a NULL pointer dereference

这正是在C＃同样的事情，作为一个的NullReferenceException ，除了在C指针可以指向任何数据对象，数组内的偶数元素。

This is exactly the same thing as a NullReferenceException in C#, except that pointers in C can point to any data object, even elements inside an array.

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