C - Segmentation fault when checking if a pointer is NULL

Question

so got this segmentation fault every time I tried to check if a pointer variable is a NULL. The error is from these lines of code in add function:

if (it->head == NULL){
        printf("worksfine");
    }

This is the whole code that I have:

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

typedef struct Node{
    int val;
    struct Node *prev;
    struct Node *next;
} node;

typedef struct IteratorInt{
    node *head;
    node *tail;
    node *last;
} IteratorInt;

IteratorInt IteratorIntNew(){
    IteratorInt *listint;
    listint = malloc(sizeof(IteratorInt));
    listint->head = NULL;
    listint->tail = NULL;
    listint->last = NULL;
    printf("address made %u\n", listint);
    return *listint;
}

int add(IteratorInt *it, int v){
    node *n;
    n->val = v;
    n->next = NULL;
    n->prev = NULL;
    printf("func works\n");
    printf("n %d\n", n->val);
    printf("address %u", it);
    it->head = n;
    printf("result %d", it->head->val);
    if (it->head == NULL){
        printf("worksfine");
    }
   /* if (it->head == 0){
       it->head = n;
    }
    if (it->tail == 0){
        it->tail = n;
    }
    if (it->last == 0){
        it->last = n;
    }*/

    return 1;
}

int main() {
    IteratorInt lit = IteratorIntNew();
    printf("works %u", &lit);
    add(&lit, 10);

    /*printf("Node value %d\n", lit.head.val);
    add(&lit, 15);
    printf("Node value %d", lit.tail.val);*/
    return 0;
}

Can you tell me what's wrong with it? and how to solve that? Thanks a lot.

Most likely: it is NULL, and you can't get to NULL->head. Thumbs up for debugging enough to find the offending line! — John3136
– John3136, Commented Dec 11, 2017 at 0:45
@John3136, but if I delete the if statement. The printf statement above itwhich lead to it->head works fine. — jted95
– jted95, Commented Dec 11, 2017 at 0:52
@little_birdie, but in the IteratorIntNew function above, I allocate a memory to it. Is it different? — jted95
– jted95, Commented Dec 11, 2017 at 0:54
@John3136, yeah, I just realize that yesterday. I need to return a pointer, right? and it's either I change it into a function pointer or change the data type into a pointer data type. Thanks for explaining anyway. — jted95
– jted95, Commented Dec 11, 2017 at 21:36

Roun · Accepted Answer · 2017-12-11 01:04:26Z

2

In your add function, the variable n is an uninitialized pointer. So it is not the problem of checking it->head.

answered Dec 11, 2017 at 1:04

Roun

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Comments

Employed Russian · Accepted Answer · 2017-12-11 01:08:08Z

2

if (it->head == NULL)

This like will crash if it itself is not a valid pointer (such as NULL).

int add(IteratorInt *it, int v){ node *n; n->val = v;

This takes an un-initialized pointer n, and dereferences it. The most likely outcome would be a crash.

if I delete the if statement. The printf statement above it which lead to it->head works fine

It is hard to believe you, because the n->val is above the printf, and it would most likely crash before you got to printf.

answered Dec 11, 2017 at 1:08

Employed Russian

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Comments

sg7 · Accepted Answer · 2017-12-12 04:57:55Z

If you want to print the address use %p and (void *) cast.

printf("address made %p\n", (void *) listint);
printf("address %p",(void *)  it);
printf("works %p",(void *)  &lit);

Also

node *n; // - is not initialized 
it->head = n;
printf("result %d", it->head->val); // will print garbage

Allocate memory properly in IteratorIntNew(). This is one way:

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

typedef struct Node{
    int val;
    struct Node *prev;
    struct Node *next;
} node;

typedef struct IteratorInt{
    node *head;
    node *tail;
    node *last;
} IteratorInt;

IteratorInt *IteratorIntNew(){
    IteratorInt *listint;
    listint = malloc(sizeof(IteratorInt));
    listint->head = NULL;
    listint->tail = NULL;
    listint->last = NULL;
    printf("address made %p\n", (void *) listint);
    return listint;
}

int add(IteratorInt *it, int v){
    node *n;
    n->val = v;
    n->next = NULL;
    n->prev = NULL;

    printf("func works\n");
    printf("n %d\n", n->val);
    printf("address %p",(void *)  it);

    it->head = n;
    printf("result %d", it->head->val);

    if (it->head == NULL){
        printf("worksfine");
    }

   /* if (it->head == 0){
       it->head = n;
    }
    if (it->tail == 0){
        it->tail = n;
    }
    if (it->last == 0){
        it->last = n;
    }*/

    return 1;
}

int main() {
    IteratorInt *lit = IteratorIntNew();
    printf("works %p",(void *)  lit);
    add(lit, 10);

    /*printf("Node value %d\n", lit.head.val);
    add(&lit, 15);
    printf("Node value %d", lit.tail.val);*/
    return 0;
}

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