Segmentation fault w/ pointer to pointer in different struct in C

Question

I'm trying to create a priority queue linked list, but keep running into segmentation fault.

My structure definitions are below

typedef struct node {
  char *new_element;
  struct node *next;
  int priority;
} Qnode;

typedef struct {
    Qnode *top;
    Qnode *tail;
    int size;
} Priority_queue;

int main() {
    Priority_queue q;
    init(&q);
    enqueue(&q, "hi", 1);
    return 0;
}

void init(Priority_queue *const q) {
    q->top = NULL;
    q->tail = NULL;
    q->size = 0;
    return 0;
}

And my enqueue method where the error is caused below

void enqueue(Priority_queue *const q, const char new_element[], int priority) {

    /*......*/

    Qnode *newNode = (Qnode*) malloc(sizeof(Qnode));
    q->tail->next = newNode; /*causes segmentation fault*/
    q->tail = newNode; /*doesn't cause segmentation fault*/


   /*.......*/
}

I'm guessing I'm not dynamically allocating my memory correctly, but the way my function is written, I'm pointing from one struct to the next so is there a way to fix this?

Please use a debugger. What is q->tail? What happens with the first node that you add to the queue? — Antti Haapala
– Antti Haapala, Commented Oct 19, 2018 at 4:45
q->tail->next = newNode;: Here Priority_queue q; you allocated the 1st node, here Qnode *newNode = malloc(sizeof(Qnode)); you allocated the 3rd node. Where and when is the 2nd allocated? — alk
– alk, Commented Oct 19, 2018 at 4:55
q->tail is initialised to NULL and you want to access q->tail->next..... That's the problem! You can change the value of q->tail first before looking at q->tail->next — PhoenixBlue
– PhoenixBlue, Commented Oct 19, 2018 at 6:36

restless_soul469 · Accepted Answer · 2018-10-19 08:14:38Z

5

In your code, init() initializes q->tail with NULL. And you are trying to do q->tail->next = newNode. In case of first node it will essentially means NULL->next = newNode. This is the reason of segmentation fault.

Your enqueue() should be like :

void enqueue(Priority_queue *const q, const char new_element[], int priority) {

    /*......*/

    Qnode *newNode = (Qnode*) malloc(sizeof(Qnode));
    if (q->tail) {                /*Do this, only When first node is already allocated*/
        q->tail->next = newNode; 
    }
    q->tail = newNode; 

    /*.......*/

}

edited Oct 19, 2018 at 8:14

restless_soul469

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answered Oct 19, 2018 at 4:56

conr2d

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