Stack implementation using arrays

#include<stdio.h>
#include<stdlib.h>

void push(int S[],int item);
void pop(int S[]);
void displayStack(int S[]);
int top = -1,s_max;

int main(void)
{

        int choice,item;
        printf("\nEnter the Size of the stack : ");
        scanf("%d",&s_max);

        int S[s_max];

        while(1)
        {
        printf("\nEnter the operations choice\n1.Push\n2.Pop\n3.Display Stack\n4.Exit\n\n");
        scanf("%d",&choice);
        switch(choice)
        {
                case 1 :printf("\nEnter the element to push into the stack : ");
                        scanf("%d", &item);
                        push(S,item);
                        //displayStack(S);
                        break;
                case 2 : pop(S);
                        //displayStack(S);
                        break;
                case 3 : displayStack(S);
                         break;
                case 4 : exit(0);
                default:printf("\n\nYou have entered wrong choice !\n\n");
        }
        }
        return 0;

}

void push(int S[],int item)
{
    if(top == s_max-1)
        printf("\n Overflow Error : Stack is full you can't enter any more element.\n");
    else
    {
        top=top+1;
        S[top]=item;
    }
}

void pop(int S[])
{
    if(top == -1)
        printf("\n Underflow Error : Stack is Empty you can't delete any more element.\n");
    else
    {
       top = top-1;
        S[top+1];
        printf("\n%d is poped!\n", S[top+1]);
    }
}

void displayStack(int S[])
{
    int i;
    if(top == -1)
        printf("\nStack is Empty |__| !\n");
    else
    for(i=top; i>=0; i--)
    {
        printf("\t%d\n",S[i]);
    }
}

Output : 

Queue Implementation using arrays

#include<stdio.h>
#include<stdlib.h>
#define MAX 4       //you can increase or decrease the
                    //queue size by modifying MAX value

void Enqueue(int Q[], int item);
void Dequeue(int Q[]);
void display_Q(int Q[]);

int Front=-1,Rear=-1;


int main(void)
{
    int item,choice;
    int Q[MAX];

    while(1)
    {
        printf("\n1.Insert an item into Q\n2.Delete an item from Q"
               "\n3.Display Queue\n4.Exit\n\nEnter the choice as given : ");
        scanf("%d",&choice);

        switch(choice)
        {
            case 1:printf("\nEnter the element : ");
                    scanf("%d",&item);
                    Enqueue(Q,item);
                    break;
            case 2:Dequeue(Q);
                    break;
            case 3:display_Q(Q);
                    break;
            case 4:exit(0);
                    break;
            default: printf("\n\nYou have entered wrong choice !\n\n");
        }
    }
    return 0;
}

void Enqueue(int Q[], int item)
{

    if(Rear == MAX-1)
    {
        if(Front == 0)
            printf("\nQueue is full, you can't enter any more elemetns !\n\n");
        else
            printf("\nYou can't insert untill queue will not be empty !\n");
    }
    else
    {
        Rear++;
        Q[Rear]= item;
        if(Front==-1)
            Front = 0;
    }
}

void Dequeue(int Q[])
{
    int x;
    if(Front == -1)
        printf("\nQueue is Empty, you can't delete any more element !\n\n");
    else
    {
        x = Q[Front];
        Front++;
        if(Front == Rear+1)
        {
            Front = -1;
            Rear = -1;
        }
        printf("\n%d is deleted form Queue !\n",x);
    }
}

void display_Q(int Q[])
{
    int i;
    if(Front == -1)
        printf("\nQueue is Empty !\n\n");
    else
        for(i=Front; i<=Rear; i++)
        printf(" %d",Q[i]);
}

//Note => once you started deletion You can't insert another
//element in queue untill the queue is empty.
//cause it is array implementation and it is also not a circular queue

Output: