CODEWITHSUNDEEP
c++doubly-linked-list
Đạt Ngô
Đạt Ngô

Reputation: 25

Doubly linked-list stops working

I try to implement a doubly linked-list that holds task control blocks (TCBs), each of which contains a pointer to function and a void pointer. I run my program without using loop and it works properly. However, when I use a for loop, it stops working. My program is as follows:

#include <iostream>
#include <stdlib.h>

using namespace std;

class TCB {
public:
    void *data;
    TCB *next;
    TCB *prev;
public:
    void (*myTask)(void *);
};

typedef void (*task)(void *data);

class Queue {
public:
    TCB *head;
    TCB *tail;
    int numberOfElenments;
public:
    void QueueInsert(void *data, task myTask);
    void QueueRemove(TCB *task);
};

// Insert at tail
void Queue::QueueInsert(void *value, task myTask)
{
    TCB *newTask = (TCB*)calloc(1, sizeof(TCB));

    newTask->data = value;
    newTask->myTask = myTask;

    if(head == NULL) {
        head = newTask;
        tail = newTask;
    } else {
        tail->next = newTask;
        newTask->prev = tail;
        tail = newTask;
    }

    numberOfElenments++;
}

// Remove a particular node in queue
void Queue::QueueRemove(TCB *task)
{
    if(head == NULL) {
                // do nothing
    }

    if(task == head && task == tail) {
        head = NULL;
        tail = NULL;
    } else if(task == head) {
        head = task->next;
        head->prev = NULL;
    } else if(task == tail) {
        tail = task->prev;
        tail->next = NULL;
    } else {
        TCB *after = task->next;
        TCB *before = task->prev;
        after->prev = before;
        before->next = after;
    }

    numberOfElenments--;
    free(task);
}

void foo(void *data) {
    cout<<"Hello world!"<<endl;
}

void foo2(void *data) {
    cout<<"Hello, I am foo2!"<<endl;
}

int main(){
    Queue *q;
    q->QueueInsert(NULL, foo);
    q->QueueInsert(NULL, foo2);
    TCB *task;
    task = q->head;

    for(int i = 0; i < 2; i++) {
        task->myTask(task->data);
        task = task->next;
    }

    return 0;
}

What is wrong with my program?

Upvotes: 0

Views: 56

Answers (1)

David Haim
David Haim

Reputation: 26476

 Queue *q;

your Queue points to garbage memory address, you need to instantiate an object :

 Queue *q = new Queue();

or better, allocate it on the stack

 Queue q;

Upvotes: 2

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