nested structs/linked lists in c

Question

My question is how would I access the num variable in a node struct of a list struct? I tried two ways and both of them didnt work? Im just curious to why that is. Thank you to anyone who helps I know this is a novice question. Im fairly new to c and stack overflow, hopefully I can learn much from this website.

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

typedef struct node
{
    int num;
    struct node *next;
} node;

typedef struct list
{
    node *ptr;
    struct list *next;
} list;

int main()
{
    list *p = malloc(sizeof(list));
    //p->ptr->num = 5;

    node *x;
    x = p->ptr;
    //x->num = 5;   

    return 0;
}

A list doesn't contain any pointers to nodes until you add them. You'd need to allocate a node for x to point to, then set p to point to the node. Only then can you start accessing the value. So, p->ptr = x; is more plausible than the reverse assignment, but you still have to connect all the dots. — Jonathan Leffler
– Jonathan Leffler, Commented Apr 23, 2016 at 19:15

Gaurav Sehgal · Accepted Answer · 2016-04-23 19:17:54Z

1

What you were trying to do is correct, but the problem is that though you have allocated memory for list , no memory is allocated for the node residing inside list.

    list *p = malloc(sizeof(list));
    //p->ptr->num = 5;
    node *x;
    p->ptr = malloc(sizeof(node));
    x = p->ptr;
    x->num = 5;

answered Apr 23, 2016 at 19:17

Gaurav Sehgal

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