Data structure, pointers - c

Question

I made a queue in C using pointers, my code works but I can not understand how the pointer variable rear1 is works, because every time function called, rear1 is initialized and same for front, front store the address of start for first time then after front reinitialize but it still keep start address, how is it possible.
#include <stdio.h>
#include <stdlib.h>
/* run this program using the console pauser or add your own getch, ("pause") or input loop */
struct node
{
int data;
struct node *next;
};
enqueue(struct node **start)
{
struct node *front,*rear;
if (*start==NULL)
{
*start=(struct node *)malloc(sizeof(struct node));
scanf("%d",&(*start)->data);
(*start)->next=NULL;
printf("%s","hello");
front=(*start);
rear=*start;
}
else
{
printf("%d",front->data);
struct node *temp,*curr;
curr=(struct node *)malloc(sizeof(struct node));
rear->next=curr;
rear=curr;
rear->next=NULL;
scanf("%d",&rear->data);
}
}
dequeue(struct node **front)
{
struct node *temp;
temp=(*front);
(*front)=(*front)->next;
printf("%d",(temp->data));
free(temp);
}
int main(int argc, char *argv[])
{
struct node *start=NULL;
enqueue(&start);
enqueue(&start);
enqueue(&start);
enqueue(&start);
enqueue(&start);
enqueue(&start);
dequeue(&start);
printf("\n");
dequeue(&start);
printf("\n");
dequeue(&start);
printf("\n");
dequeue(&start);
printf("\n");
dequeue(&start);
printf("\n");
dequeue(&start);
return 0;
}

Answer 1

Actually, this code shouldn't work, or has undefined behaviour.
enqueue(struct node **start)
{
struct node *front,*rear;
if (*start==NULL)
{
*start=(struct node *)malloc(sizeof(struct node));
scanf("%d",&(*start)->data);
(*start)->next=NULL;
printf("%s","hello");
front=(*start);
rear=*start;
}
else
{
printf("%d",front->data);
struct node *temp,*curr;
curr=(struct node *)malloc(sizeof(struct node));
rear->next=curr;
rear=curr;
rear->next=NULL;
scanf("%d",&rear->data);
}
}
Here, when *start is defined, you assign curr to rear->next, but rear is undefined. You have 2 solutions:
Using rear as a static variable (which is clearly not a good solution, especially if you want to use multiple queues)
Using 2 struct in your main, Start and End.
I think you use front and rear as if they were static, but by default, a variable "disappear" at the end of the function where it was declared. It's a new undefined variable at each call of the function.