链接列表和结构

Question

I'm working on a school project and I'm trying to understand doubly linked lists and structs a bit better. Currently, I'm trying to implement a function, one that creates a new linked list. Because I think I can work from there.

typedef struct ListItem {
    struct ListItem *previousItem; //pointer to previous item, NULL if first list item
    struct ListItem *nextItem;     //pointer to next item, NULL if first list item
    void *data;                    //pointer to data

This is the struct for the doubly linked list I am trying to make. I know that a "void *" can hold a pointer to anything, also that I have to allocate any data stored in the list item.

/**
 * This function starts a new linked list. Given an allocated pointer to data it will    return a
 * pointer for a malloc()ed ListItem struct. If malloc() fails for any reason, then this function
 * returns NULL otherwise it should return a pointer to this new list item. data can be NULL.
 *
 * @param data The data to be stored in the first ListItem in this new list. Can be any valid 
 *             pointer value.
  * @return A pointer to the malloc()'d ListItem. May be NULL if an error occured.
 */

ListItem *NewList(void *data);

I know that malloc() allocates enough memory on the stack for use, so I think that in my function I have to malloc() *previousItem, *nextItem, and *data (which would be 6 bytes?) Other than that, to implement the function all I would do is copy the ListItem struct? The previous AND next item would be NULL pointers since it is the only item in the list, and the *data would be the input I think. Can anyone give me an idea of what my code would look like?

Answer 1

You're on the right track. Instead of using 6 as the argument to malloc , you could use sizeof to get the amount of memory that you need to allocate - for example:

ListItem *node = malloc(sizeof(ListItem));

After that the implementation is fairly trivial:

/* Make sure that allocation succeeded */
...
/* Assign the right values to previousItem and nextItem */
...
/* Assign the right value to data */
...
/* Return the pointer to the new list */
...

Someone else will probably submit the full function, but your english language description of what needs to happen is spot on (other than the whole heap vs. stack thing).