Program to implement a circular queue as a linked list

/* Program to implement a circular queue as a linked list. */

#include <stdio.h>
#include <conio.h>
#include <alloc.h>

/* structure containing a data part and link part */
struct node
{
int data ;
struct node * link ;
} ;

void addcirq ( struct node **, struct node **, int ) ;
int delcirq ( struct node **, struct node ** ) ;
void cirq_display ( struct node * ) ;

void main( )
{
struct node *front, *rear ;

front = rear = NULL ;

addcirq ( &front, &rear, 10 ) ;
addcirq ( &front, &rear, 17 ) ;
addcirq ( &front, &rear, 18 ) ;
addcirq ( &front, &rear, 5 ) ;
addcirq ( &front, &rear, 30 ) ;
addcirq ( &front, &rear, 15 ) ;

clrscr( ) ;

printf ( "Before deletion:\n" ) ;
cirq_display ( front ) ;

delcirq ( &front, &rear ) ;
delcirq ( &front, &rear ) ;
delcirq ( &front, &rear ) ;

printf ( "\n\nAfter deletion:\n" ) ;
cirq_display ( front ) ;
}

/* adds a new element at the end of queue */
void addcirq ( struct node **f, struct node **r, int item )
{
struct node *q ;

/* create new node */
q = malloc ( sizeof ( struct node ) ) ;
q -> data = item ;

/* if the queue is empty */
if ( *f == NULL )
*f = q ;
else
( *r ) -> link = q ;

*r = q ;
( *r ) -> link = *f ;
}

/* removes an element from front of queue */
int delcirq ( struct node **f, struct node **r )
{
struct node *q ;
int item ;

/* if queue is empty */
if ( *f == NULL )
printf ( "queue is empty" ) ;
else
{
if ( *f == *r )
{
item = ( *f ) -> data ;
free ( *f ) ;
*f = NULL ;
*r = NULL ;
}
else
{
/* delete the node */
q = *f ;
item = q -> data ;
*f = ( *f ) -> link ;
( *r ) -> link = *f ;
free ( q ) ;
}
return ( item ) ;
}
return NULL ;
}

/* displays whole of the queue */
void cirq_display ( struct node *f )
{
struct node *q = f, *p = NULL ;

/* traverse the entire linked list */
while ( q != p )
{
printf ( "%d\t", q -> data ) ;

q = q -> link ;
p = f ;
}
}