View item of linked list - c

Question

I want to view all the items of a linked list.
I've created a three items list, and when i use the below "show_items" function, it just show the first element, and the other items can't be showed becuase a segmentation fault error is given by compiler.
#include <stdio.h>
#include <stdlib.h>
struct list{
int age;
struct list *next;
};
void create_item(int *total_items, struct list *where_is_first_item, struct list *where_is_last_item)
{
struct list *generic_item;
generic_item = malloc(sizeof(struct list));
printf("\nage of item %d: ", (*total_items)+1);
scanf("%d", &generic_item->age);
if(*total_items == 0){
where_is_first_item->next=generic_item;
where_is_last_item->next=generic_item;
printf("\nitem created\n");
}
else{
where_is_last_item->next=generic_item;
printf("\nitem created\n");
}
void show_items(int *total_items, struct list *where_is_first_item, struct list *temp){
temp=where_is_first_item->next;
int i;
for(i=0;i<*total_items;i++){
printf("age of element %d: %d\n", i+1, temp->age);
temp=temp->next;
}
}
int main (void){
int total_items=0;
struct list *where_is_first_item;
where_is_first_item=malloc(sizeof(struct list));
struct list *temp;
temp=malloc(sizeof(struct list));
printf("\n\n\tCREATE A NEW ITEM\n");
create_item(&total_items, where_is_first_item, where_is_last_item);
total_items++;
show_items(&total_items, where_is_first_item, temp);
return 0;
}

Answer 1

Well, I see two major issues in the code that you're showing us:
I don't see three members being created like you claim in the problem statement.
The memory returned by malloc() has unspecified contents, and you're expecting it to have useful contents.
And then for just odd things:
Why do you pass total_items as a pointer if you never modify it.
Why do you pass temp as an argument if you promptly ignore the value you passed in?

Answer 2

When iterating through your list, you should be checking the value of temp to make sure it isn't NULL before trying to access it. At the least, you should use the following:
for ( i = 0; i < *total_items && temp != NULL; i++ )
...
although it's more common to iterate through a list like
while ( temp != NULL ) // or just while ( temp )
{
...
temp = temp->next;
}
Your usage of temp seems confused; does it serve any purpose in main after you've called show_items? If not, you should make it local to show_items and not pass it as an argument:
void show_items(int *total_items, struct list *where_is_first_item)
{
struct list *temp = where_is_first_item->next;
...
}
In the code you've posted, total_items is initialized to 0; as written, this code shouldn't attempt to output anything. It would make life easier for everyone if you would either post the actual code you're having problems with, or if it's too big, reduce it to a representative sample of the problem.
Edit
Duh. Now I see the problem. You're never properly setting the next item in your generic_item struct.
So you have two pointers to keep track of the head and tail of your list, where_is_first_item and where_is_last_item. When you add the first item to the list, you create generic_item and set your head and tail pointers to point to it through their respective next members, giving you something like this (using head and tail instead of where_is_first_item and where_is_last_item for brevity):
head generic_item tail
+--+--+ +--+--+ +--+--+
| | -+------->| | -+-??? | | -+---+
+--+--+ +--+--+ +--+--+ |
^ |
| |
+------------------------+
So far so good. However, when you add a second item, you only update the list tail pointer; you don't create an explicit link from the first item to the second, so you wind up with something like the following:
head generic_item tail
+--+--+ +--+--+ +--+--+ +--+--+
| | -+------->| | -+-??? | | -+-??? | | -+--+
+--+--+ +--+--+ +--+--+ +--+--+ |
^ |
| |
+------------------------+
Hopefully you can see the problem. The next pointer in the first list element is never initialized, so it contains an indeterminate value that doesn't correspond to a valid address.
You need an extra step when you append a new element to the list:
struct list *pre = where_is_last_item->next;
pre->next = generic_item;
where_is_last_item->next = generic_item;
That will give you something like the following:
head pre generic_item tail
+--+--+ +--+--+ +--+--+ +--+--+
| | -+------->| | -+------->| | -+-??? | | -+--+
+--+--+ +--+--+ +--+--+ +--+--+ |
^ |
| |
+------------------------+
As a rule, you should always initialize the next member to NULL, rather than leave it indeterminate. It's easy to test against NULL; it's much harder to determine the validity of a non-NULL pointer value.