Displaying the elements of a singly linked list

Question

I have written a simple program to traverse through the nodes of a linked list.

struct node
{
    int data;
    struct node *next;
}*start;
start=(struct node *)malloc(sizeof(struct node));
start->next=NULL;

int main(void)
{
    if(start==NULL)
    {
        printf("There are no elements in the list");
    }

    else
    {
        struct node *tmp;
        printf("\nThe elemnets are ");
        for(tmp=start;tmp!=NULL;tmp=tmp->next)
        {
            printf("%d\t",tmp->data);
        }
    }
    return 0;
}

Whenever i am trying to print the elements of the linked list, however even though the list is empty, it gives the output

The elements are 5640144

What am i doing wrong ? Am i declaring the start pointer correctly ?

Why do i need to do this (actually i was not doing this initially, but was asked to by one of my friends)

start=(struct node *)malloc(sizeof(struct node));
start->next=NULL;

Your list contains one node, an empty list would be `start=NULL;` — Musa, Jul 13 '12 at 14:34
By the way, my compiler (and most others) will choke when they see `start` being initialized in that way outside a function. Which compiler are you using? The code you pasted shouldn't even compile. — ArjunShankar, Jul 13 '12 at 14:47

Aasmund Eldhuset · Accepted Answer · 2012-07-13T15:38:34.690

4

Declaring a pointer to a node (start) does not actually create the node itself - you need malloc() to do this. However, malloc() doesn't bother to fill the node with reasonable values, which is why you get a seemingly random value in data. Try start->data = 42; after start->next = NULL;.

Edit: See ArjunShankar's comment about not calling malloc() from outside a function.

edited Jul 13 '12 at 15:38

answered Jul 13 '12 at 14:35

Aasmund Eldhuset

37,289
4
68
81

Do i always need to create at least one node ? – OneMoreError Jul 13 '12 at 14:37
1

@CSSS: No, it is perfectly legal to have a linked list without any nodes at all. However, this is indicated by having the `start` pointer point to nothing at all - namely, `NULL`: `start = NULL;` instead of `start = (struct node*)...; start->next = NULL; start->data = 42;`. – Aasmund Eldhuset Jul 13 '12 at 14:45
1

@AasmundEldhuset - I don't know if you tried compiling the example, but doing a `malloc` (or executing functions) to initialize a global *outside a function* [is not allowed](http://stackoverflow.com/q/6742820/274261). You might want to point this out. – ArjunShankar Jul 13 '12 at 14:51
1

@ArjunShankar: I actually wasn't aware of that; thank you for the information. I have edited my post to refer to your comment. – Aasmund Eldhuset Jul 13 '12 at 15:39
static initializers must be constants, and of course a call to `malloc` isn't. How could that possibly work, even if it were allowed? – Jim Balter Jul 14 '12 at 11:12

Ashutosh · Answer 2 · 2012-07-13T14:51:30.070

presently its may be giving you the garbage value for sure , present in the allocated memory block, as you have on assigned start->next = NULL; not assigned value to the data,

kindly use memset with 0, it will intialize memory block with zero for whole structure or intialize some value to the structure.

if you would have used gcc , it might not have raised this issue.I guess for this you wouldn't have.

struct is global, and start is an pointer to the structure, at the time of malloc, you have been alloted a structure size memory, initialized. if you have taken a global structure variable, you would not have faced such an issue, as its get initialized in bss.

#include <stdio.h>
#include <stdlib.h>
struct node
{
int data;
struct node *next;
}*start;

int main(void)
{
start=(struct node *)malloc(sizeof(struct node));
start->next = NULL;
start->data = 100;
//memset(start,0,sizeof(struct node));
if(start==NULL)
{
    printf("There are no elements in the list");
}
else
{
    struct node *tmp;
    printf("\nThe elemnets are ");
    for(tmp=start;tmp!=NULL;tmp=tmp->next)
    {
        printf("%d\n",tmp->data);
    }
}
return 0;
}

and as you are trying to declare an pointer to struct,globally it would raise a warning i think telling pointer not constant some thing. i dont knw.. ! it should.. — Ashutosh, Jul 13 '12 at 14:56

Displaying the elements of a singly linked list

2 Answers2