CODEWITHSUNDEEP
c
user2278710
user2278710

Reputation: 15

Crash when I send pointers to a function

I'm trying to create a traversable linked list, but my program crashes when I try to add an element to the list. I've tried many things but I can't figure out what I'm doing wrong.

typedef struct node_t node_t;
typedef struct list_t list_t;

struct node_t {
    void *data;
    node_t *prev;
    node_t *next;  
};

struct list_t {
    node_t front;
    node_t back;
};

list_t* listEmpty () {
    list_t *list = malloc (sizeof (list_t));
    list->front.prev = NULL;
    list->front.next = &(list->back);
    list->back.prev = &(list->front);
    list->back.next = NULL;
    return list;
}

void listPushFront (list_t *list, void *data) {
    node_t *newNode = malloc (sizeof (node_t));
    newNode->data = data;
    newNode->prev = &(list->front);
    newNode->next = list->front.next;
    list->front.next->prev = newNode;  //CRASHES HERE
    list->front.next = newNode;
}

int main (int argc, char *argv[]) { 
    list_t *list = malloc (sizeof (list_t));
    int a = 6; 
    void* pa = &a;
    listEmpty(list);
    listPushFront (list, pa);
    return 0;
}

Upvotes: 0

Views: 86

Answers (3)

SomeWittyUsername
SomeWittyUsername

Reputation: 18348

This is wrong in so many ways.

int main (int argc, char *argv[]) { 
    list_t *list = malloc (sizeof (list_t));

Why are you doing this in main?

Why do you call listEmpty with parameter list?

listEmpty(list);

Your listEmpty function doesn't do anything with the parameter you pass to it.

You're not doing anything with the pointer that your listEmpty returns, so how do you expect it to propagate to the following call of listPushFront?

Upvotes: 0

Dave Newman
Dave Newman

Reputation: 1016

listEmpty() returns a list, and has a void argument. In main() however you pass it a list, and don't capture its return value.

#include <stdlib.h>

typedef struct node_t node_t;
typedef struct list_t list_t;

struct node_t {
    void *data;
    node_t *prev;
    node_t *next;  
};

struct list_t {
    node_t front;
    node_t back;
};

list_t* listEmpty (void) {
    list_t *list = (list_t *)malloc (sizeof (list_t));
    list->front.prev = NULL;
    list->front.next = &(list->back);
    list->back.prev = &(list->front);
    list->back.next = NULL;
    return list;
}

void listPushFront (list_t *list, void *data) {
    node_t *newNode = (node_t *)malloc (sizeof (node_t));
    newNode->data = data;
    newNode->prev = &(list->front);
    newNode->next = list->front.next;
    list->front.next->prev = newNode;
    list->front.next = newNode;
}

int main (int argc, char *argv[]) { 
    list_t *list = 0;
    int a = 6; 
    void* pa = &a;
    list = listEmpty();
    listPushFront (list, pa);

    return 0;
}

Upvotes: 2

codaddict
codaddict

Reputation: 455000

Your list is not being initialized properly and this is leading to the crash.

listEmpty(list);

should be

list = listEmpty();

Upvotes: 0

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