C中的空指针

小灰灰 2022-12-07 13:56 173阅读 0赞

In this tutorial you will learn about null pointer in C with examples.

在本教程中，您将通过示例学习C语言中的空指针。

When we declare a pointer, by default it points to some random memory location. If you will access the pointer then it may give some undesired value or the program may crash.

当我们声明一个指针时，默认情况下它指向某个随机的内存位置。 如果您将访问该指针，则它可能会提供一些不必要的值，否则程序可能会崩溃。

### 什么是C中的空指针？ **(**What is Null Pointer in C?**)** ###

![Null Pointer][]

A pointer pointing to nothing or no memory location is called null pointer. For doing this we simply assign NULL to the pointer.

指向无内容或无存储位置的指针称为空指针。 为此，我们只需将NULL分配给指针。

So while declaring a pointer we can simply assign NULL to it in following way.

因此，在声明指针时，我们可以简单地通过以下方式为其分配NULL。

int *p = NULL;

NULL is a constant which is already defined in C and its value is 0. So instead of assigning NULL to pointer while declaring it we can also assign 0 to it.

NULL是已经在C中定义的常数，其值为0。因此，除了在声明指针时将NULL分配给指针，我们还可以为其分配0。

### 空指针的用法 **(**Usage of Null Pointer**)** ###

1. We can simply check the pointer is NULL or not before accessing it. This will prevent crashing of program or undesired output.

1.我们可以在访问指针之前简单地检查指针是否为NULL。 这样可以防止程序崩溃或不希望的输出。

int *p = NULL;
     
    if(p != NULL) {
    //here you can access the pointer
    }

2. A pointer pointing to a memory location even after its deallocation is called [dangling pointer][]. When we try to access dangling pointer it crashes the program. So to solve this problem we can simply assign NULL to it.

2.即使在释放后仍指向内存位置的[指针][dangling pointer]也称为[悬空指针][dangling pointer] 。 当我们尝试访问悬空指针时，它会使程序崩溃。 因此，要解决此问题，我们只需为其分配NULL。

void function(){
        int *ptr = (int *)malloc(SIZE);
        . . . . . .
        . . . . . . 
        free(ptr);    //ptr now becomes dangling pointer which is pointing to dangling reference
        ptr=NULL;    //now ptr is not dangling pointer
    }

3. We can simply pass NULL to a function if we don’t want to pass a valid memory location.

3.如果我们不想传递有效的内存位置，则可以简单地将NULL传递给函数。

void fun(int *p) {
        //some code
    }
     
    int main() {
     
        fun(NULL);
     
        return 0;
    }

4. Null pointer is also used to represent the end of a linked list.

4.空指针还用于表示链表的结尾。

![Linked List][]

[Image Source][]

[图片来源][Image Source]

Comment below if you found anything incorrect or have queries regarding above tutorial for null pointer in C.

如果发现任何错误或对以上教程的C语言中的空指针有疑问，请在下面评论。

> 翻译自: [https://www.thecrazyprogrammer.com/2017/05/null-pointer-in-c.html][https_www.thecrazyprogrammer.com_2017_05_null-pointer-in-c.html]

[Null Pointer]: /images/20221123/3dc32d9310254152b2d9c4afad6801d3.png
[dangling pointer]: https://www.thecrazyprogrammer.com/2016/03/dangling-pointer-in-c.html
[Linked List]: /images/20221123/10c3fc52bd164fbe9e4baf66a13421a9.png
[Image Source]: http://java-questions.com/linked-list-in-details.html
[https_www.thecrazyprogrammer.com_2017_05_null-pointer-in-c.html]: https://www.thecrazyprogrammer.com/2017/05/null-pointer-in-c.html