Implementing queue datastructure using c program.

/*queue*/
#include<stdio.h>
#include<stdlib.h>
#define queue_size 5
int c,item,f,r,q[10];
void ri()
{
    if(r==queue_size-1)
    {
        printf("queue overflow\n");
        return;
    }
    r=r+1;
    q[r]=item;
}
void fd()
{
    if(f>r)
    {
        printf("queue underflow\n");
        return;
    }
    printf("the element deleted is %d",q[(f)++]);
    if(f>r)f=0,r=-1;
}
void display()
{
    int i;
    if(f>r)
    {
        printf("queue is empty\n");
        return;
    }
    printf("contents of the queue\n");
    for(i=f;i<=r;i++)
    {
        printf("%d\n",q[i]);
    }
}
int main()
{
    f=0;
    r=-1;
    for(;;)
    {
        printf("1:insert 2:delete\n 3:display 4:quit\n");
        printf("enter your choice\n");
        scanf("%d",&c);
        switch(c)
        {
            case 1:printf("enter the item to be inserted\n");
            scanf("%d",&item);
            ri();
            break;
            case 2:fd();
            break;
            case 3:display(q,f,r);
            break;
            default:exit(0);
        }
    }
}