Function returns the oldest value in linked list

user_13968842 0
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This is a function (last) that returns the oldest value in linked list (the last node):

#include <stdio.h>
#include <stdlib.h>

typedef struct node {
  int val;
  struct node *next;
}Elem, *Pointer;

Pointer init() { return NULL; }

void last(Pointer l)
{
  while (l != NULL)
  {
    l = l->next; 
  }
  printf("%d",l->val); 
}

int main(void) {
    Pointer myl =
    insert(3, insert(7, insert(5, 
    insert(11, insert(1, init ())))));

    last(myl);
}

so the function (last) takes a pointer that points to a linked list, and in a while loop it moves the pointer to the last node and then prints its value.

Error is: exited, segmentation fault.

segmentation-fault linked-list c
2 Answers

Since checking for NULL l is your exit condition from the loop, when you reach the end of the loop

while (l != NULL)
{
    l = l->next; 
}
printf("%d",l->val);

l is NULL (otherwise you would still be looping!). So in the following printf you are dereferencing a NULL pointer and this results in a segmentation fault.

You can modify your function in the following way:

void last(Pointer l)
{
    if ( l != NULL )
    {
        while (l->next != NULL)
        {
            l = l->next; 
        }
        printf("%d",l->val); 
    }
}

Just peek in the next field of the element, after making sure that the list is not empty (NULL list pointer).

user_113367620
  1. Never hide pointers behind the typedefs. It makes programs hard to read and error prone.
typedef struct node {
  int val;
  struct node *next;
}Elem;
  1. This is something absilutelly horrible - avoid
insert(3, insert(7, insert(5, 
insert(11, insert(1, init ())))));
  1. The function. Check if the next value is NULL. IMO better return pointer and then do something with it. Also Always check if parameter is valid.
Elem *last(Elem *first)
{
    if(first)
    {
        while(first -> next != NULL) 
        {
            first = -first -> next;
        }
    }
    return first;
}
  1. Print it
printf("%d\n", last(myl) -> val);
user_61100940
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