什么时候可以返回NULL作为C函数的返回值? [英] When are you able to return NULL as the returning value of a C function?

问题描述

我想知道是否可以告诉我何时可以返回NULL，这是C函数的结果.

I was wondering if you could tell me when you are able to return NULL, as the result of a function in C.

例如int lenght()不能返回NULL，因为它在return语句中期望int.

For instance int lenght() can't return NULL because is is expecting an int in the return statement.

但是函数struct node* find(int key)在使用链表时允许我返回NULL.

But the function struct node* find(int key), when working with linked lists allows me to return NULL.

推荐答案

NULL是一个指针值-或更确切地说是一个空指针值.

NULL is a pointer value - or rather a null-pointer value.

NULL表示该函数找不到指针应指向的位置-例如，如果您要打开文件，但不起作用，文件指针将以NULL的形式返回.因此，您可以测试指针的值并检查它是否起作用.

NULL means that the function can't find where your pointer should point to - for example if you want to open a file, but it doesn't work your file pointer is returned as NULL. So you can test the value of a pointer and check to see if it worked or not.

如果您正在编写例程

int length()

然后，如果length无法读取所发送内容的长度，则可以返回负值-这是一种指示错误的方式，因为通常length永远不会为负....

then you could return a negative value if length is unable to read the length of whatever you send it - this would be a way of indicating an error, because normally lengths can never be negative....

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