如何将值赋给指向NULL的指针(How to asign a value to a pointer that points to NULL)

 据我所知，一个星号代表C中的一个正常指针。例如，你可以分配内存并使指针指向新分配的区域。 你也可以指向堆栈中的变量，如： 
 
int x = 10;
int *p = &x;
 
 但是在某些情况下，您想要将指针传递给函数，并使函数能够将指针更改为指向另一个地址。 现在我们不是在谈论通过指针或者指向它的内存来改变变量的内容，而是通过文字将指针信息（这是一个内存地址）指向另一个地方。 在这种情况下，我们使用一个指针指向一个由双星号表示的指针。 
 
int x, y;
int *p;
int **pp;

pp = &p;  //pp points to pointer p
*pp = &x; //pp changes pointer p to point to variable x
*p = 10;  //p (now points to x) assigns value 10 to x
*pp = &y; //pp changes pointer p again assigning the address of y
*p = 20;  //p assigns 20 to y;
**pp = 25;// that's tricky, the first * give access to the address pointed by p (&y) the second changes the content of that address to 25. so now y ends with 25
 
 最后x = 10和y = 25 
 
 现在看起来尤其如此，该函数调用看起来像一个将从链接列表中删除节点的函数。 这是有道理的，因为你有一个指向列表头部的外部指针。 当你删除最后一个元素时，你的函数必须能够将指针从当前地址改为null。 你可以使用全局指针来完成它，或者像你的情况那样，传递头指针的地址。 

As far as I know one asterisk represents a normal pointer in C. For instance you can allocate memory and make the pointer point the that new allocated area. You can also point to variables in the stack like:
 
int x = 10;
int *p = &x;
 
But there are some situations where you want to pass a pointer to a function and make the function able to change the pointer to point to another address. Now we are not talking about change the contents of a variable through a pointer nor a memory pointed by it but literary change the pointer information (which is a mem. address) to point to another places. In this case we use a pointer to a pointer which is represented by double asterisk. 
 
int x, y;
int *p;
int **pp;

pp = &p;  //pp points to pointer p
*pp = &x; //pp changes pointer p to point to variable x
*p = 10;  //p (now points to x) assigns value 10 to x
*pp = &y; //pp changes pointer p again assigning the address of y
*p = 20;  //p assigns 20 to y;
**pp = 25;// that's tricky, the first * give access to the address pointed by p (&y) the second changes the content of that address to 25. so now y ends with 25
 
In the end x = 10 and y = 25
 
Now looking particularly in you case, that function call looks like a function that will remove a node from a linked list. And this makes sense because you have an external pointer pointing to the head of the list. When you remove the last element your function must be able to change the pointer from the current address to null. You can do it using a global pointer or, as it seems in your case, passing the address of the head pointer.