My dynamic queue in C is displaying infinite weird results

Question

I'm studyind dinamic lists in C and this is a queue that I made using structs and pointers, it is queueing the nodes normally but when I call the Dequeue and Show methods in that order, it enter in a loop and displays weird random numbers.

struct Node {
    int data;
    struct Node *next;
};

typedef struct Node node;
int size = 0;

node* front = NULL;
node* rear = NULL; 


//adds a new node at the end of the list
    void Enqueue(int n){

    node *new_node = (node*)malloc(sizeof(node));

    new_node->data = n;
    new_node->next = NULL;

    if (front == NULL && rear == NULL){
        front = new_node;
        rear = new_node;
        return;
    }

    rear->next = new_node;
    rear = new_node;
}


//when calling this method before show(), the loop issue occurs
    void Dequeue(){

    node *tmp = front;


    if (front == rear){
        front = rear = NULL;
    }

    else {
        front = front->next;
    }
    free(front);
}

//if just calling Enqueue() and Show() methods, it runs normally
void Show(){

    node *tmp = front;

    while(tmp != NULL){

        printf("%d \n", tmp->data);
        tmp = tmp->next;
    }
}


int main(void){

    Enqueue(1);
    Enqueue(2);
    Enqueue(3);


    Dequeue();
    Show();

    system("pause");
    return 0; 
}

Sidenote: C does not support methods. You have functions. — too honest for this site

cleblanc cleblanc · Accepted Answer · 2018-08-27T19:34:49

Check your Dequeue(...) function. You're freeing the node on the front after re-assigning it. Try this instead

void Dequeue(){


node *tmp = front;


if (front == rear){
    front = rear = NULL;
}

else {
    front = front->next;
}
free(tmp);
}

My dynamic queue in C is displaying infinite weird results

3 Answers