Implementation of Queue using pointers : segmentation error

Question

I am trying to implement a FIFO. The code compiles without errors but i get segmentation fault when running the program. What is the problem?

#include <stdio.h>    
#include <stdlib.h>    
struct cell            
{
 int element;
 struct cell *next;
};
struct queue           
{
 struct cell *front;   
 struct cell *rear;    
};

void enqueue(int x, struct queue *Q);
void dequeue(struct queue *Q);

main()  
/* Manipulation of a linked queue of cells. */
{
 struct queue *Q;
 struct cell *q;
 int i;

 Q->front=Q->rear=NULL;
 for(i=0; i<8; i++) {enqueue(i+1, Q);} 
 printf("Q->front = %p,  Q->rear = %p\n", Q->front, Q->rear);
 q=Q->front;
 while(q!=NULL)
   {
    printf("cell = %d, %p\n", q->element, q->next);
    q=q->next;
   }  
 for(i=0; i<10; i++) dequeue(Q);
 printf("Q->front = %p,  Q->rear = %p\n", Q->front, Q->rear);
 q=Q->front;
 while(q!=NULL)
   {
    printf("cell = %d, %p\n", q->element, q->next);
    q=q->next;
   } 
 return(0);
}

void enqueue(int x, struct queue *Q)

{
 struct cell *p;

 p=(struct cell *)malloc(sizeof(struct cell)); 
 if(Q->rear != NULL) Q->rear->next = p;        
 Q->rear = p;
 if(Q->front == NULL) Q->front = p;            
 Q->rear->element = x; Q->rear->next = NULL;
 return;
}

void dequeue(struct queue *Q)
{
 struct cell *q;

 if(Q->front == NULL) {printf("Error: Queue is empty.\n"); exit(1);} 

 else {q=Q->front; Q->front = Q->front->next; free(q);} 
 if(Q->front == NULL) Q->rear = NULL;
 return;
}

The problem is that your use of single-letter variable names, some distinguished only by case, has made the code almost impossible to work on. Crashing bugs are the first and most obvious fall-out from that original problem. — Will Dean
– Will Dean, Commented Jul 5, 2011 at 7:56
-1: It would be nice if you'd tell us WHERE in the program you get the seg-fault? — Binary Worrier
– Binary Worrier, Commented Jul 5, 2011 at 8:09

cnicutar · Accepted Answer · 2011-07-05 07:54:47Z

3

You are not allocating memory for Q and using it right away:

struct queue *Q; /* Where does Q point ? */

Q->front=Q->rear=NULL; /* Q not initialized, undefined behavior */

So Q points to some random value on the stack. To solve it, use malloc:

struct queue *Q;
Q = malloc(sizeof(*Q));
if (NULL == Q) {
    /* Tinfoil hat. */
}

answered Jul 5, 2011 at 7:54

cnicutar

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2 Comments

Donotalo Over a year ago

Q = malloc(sizeof(*Q)); ?? :)

cnicutar Over a year ago

@Donotalo Yup :-) I always do it like that (perhaps I want to change the type of Q later?) + less to type.

nos · Accepted Answer · 2011-07-05 07:58:03Z

0

You have no space allocated to your struct queue *Q;

 struct queue *Q;
 struct cell *q;
 int i;

 Q->front=Q->rear=NULL;

Q is just an uninitialized pointer. It doesn't point to anything valid, so you can't dereference it as you do in Q->front=Q->rear=NULL;

Change it to allocate Q on the stack, and pass its address to your functions.

 struct queue Q;
 struct cell *q;
 int i;

 Q.front=Q.rear=NULL;
 for(i=0; i<8; i++) {enqueue(i+1, &Q);}

answered Jul 5, 2011 at 7:58

nos

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