C issue - Can't figure how to assign pointer to beginning of list

Question

I have a simple assignment that the professor wants us to do. Basically to pull in some numbers from a text file and load into a linked list. I don\'t want to get to much in

Answer 1

In C, everything is passed by value. If you want a function to change something, you need to pass its address to the function. Since in int insert_intlist( INTLIST *lst, int n ), you want to change the list head, you need to pass a pointer to it, i.e., the first parameter should be INTLIST **lst (see below too, though). But the function prototype is given and cannot be changed.

What that means is that you can't add a number to the beginning of the list—the caller can't know that you did so. So, you have to traverse the list pointed to by lst, and then add the new node anywhere down the chain. The professor probably wants you to add the node at the end, but he might have asked for some other condition.

With that information, let's look at the comments for the prototypes:

/* Inserts an int (n) into an intlist from the beginning*/
int insert_intlist( INTLIST *lst, int n );

The comment or the prototype is wrong. If your professor has given you this file, insert_intlist() cannot be written to satisfy the comment, since it can't return to the caller the new head. The prototype should be either:

/* Inserts an int (n) into an intlist from the beginning
   and returns the new head */
INTLIST *insert_intlist( INTLIST *lst, int n );

Or:

/* Inserts an int (n) into an intlist from the beginning */
int insert_intlist( INTLIST **lst, int n );

(Note the **.)

The header also has:

/*return the element at the front of the list, and remove it from the list*/
INTLIST* list_front(INTLIST *list);

This is correct. Note that you need to modify the list's head in list_front(), so you're returning the new head.

Finally, you don't want to free() anything in insert_intlist(). You want to keep the new node in the list, don't you? Once the caller is done with the list, he will have to call list_delete(), which will traverse the linked list, and free each node.