C将元素插入升序链表

Question

I have problems with my insert method, as for some reason I end up with infinite loop. Here is my struct:

 struct List {
    char element;
    struct List *next;
 };

And here is my insert method:

void insert(struct List *first, char el){
    struct List *new=NULL;
    struct List *current = first;
    new = (struct List*) malloc (sizeof(struct List));
    new->element = el;
    new->next = NULL;
    if (first == NULL){
        first = new;    
        return;
    }
    while (1){  //this loop never ends
        if (current->next == NULL) break;
        if (current->next->element < el){
            current = current->next;        
        }else{
            break;
        }
    }
    struct List *ex_next = current->next;
    current->next = new;
    new->next = ex_next;
}

I am aware of similar question here: C - Inserting into linked list in ascending order but it didn't really help me.

Answer 1

The first argument to insert is a pointer. But you need a pointer to a pointer ( struct List **first ).

If the list is empty, you pass the VALUE NULL into the function (the variable first inside the method have the value NULL). Then you assign a new malloced value to it and return. The variable on the calling side haven't changed and your memory leaked.

When you pass a pointer of a pointer, the variable first holds the address of the variable of the calling method. This way, you can reassign it's value.

Pointers, Pointers of Pointers, Pointers of Pointers of arrays of functions returning function pointers .... thats the fun part of C ;)