when to use to double pointers and pointers - pointers

Question

// A C program to demonstrate linked list based implementation of queue
#include <stdio.h>
#include <stdlib.h>
struct QNode {
int key;
struct QNode* next;
};
struct Queue {
struct QNode *front, *rear;
};
struct QNode* newNode(int k)
{
struct QNode* temp = (struct QNode*)malloc(sizeof(struct QNode));
temp->key = k;
temp->next = NULL;
return temp;
}
struct Queue* createQueue()
{
struct Queue* q = (struct Queue*)malloc(sizeof(struct Queue));
q->front = q->rear = NULL;
return q;
}
void enQueue(struct Queue* q, int k)
{
struct QNode* temp = newNode(k);
if (q->rear == NULL) {
q->front = q->rear = temp;
return;
}
q->rear->next = temp;
q->rear = temp;
}
void deQueue(struct Queue* q)
{
if (q->front == NULL)
return;
struct QNode* temp = q->front;
q->front = q->front->next;
if (q->front == NULL)
q->rear = NULL;
free(temp);
}
int main()
{
struct Queue* q = createQueue();
enQueue(q, 10);
enQueue(q, 20);
deQueue(q);
deQueue(q);
enQueue(q, 30);
enQueue(q, 40);
enQueue(q, 50);
deQueue(q);
printf("Queue Front : %d \n", q->front->key);
printf("Queue Rear : %d", q->rear->key);
return 0;
}
The above code is from geeksforgeeks website.
in function calls they used pointer to struct,
in function definition they passed pointer to struct.
how it works, I thought we need to use double pointers , otherwise > it is pass by value instead of pass by reference.
the above code works fine, but i have doubt about it.

Answer 1

In main there is a variable q declared which is a pointer to a struct. The variable q is used as the function argument which means the function receives a pointer to the struct. The function can dereference the pointer and modify the struct. The variable q is technically passed by value because its value is a pointer and that's what the function receives. But you have to remember that q points to a struct that could be modified by the function.
Because this situation causes some confusion some people have tried to introduce new terminology like "pass by sharing" or "object sharing" to distinguish it from passing primitive values like an `int' by value.
If you had passed a pointer to a pointer then the function could have modified the variable q declared in main and changed it so it points to a completely different struct. That would be (technically) pass by reference because you are passing a reference to the variable.