C program to implement stack operations using array

#include<stdio.h>
#include<string.h>
#include<ctype.h>
#define size 100
int top=-1;
int flag=0;
int stack[size];
void push(int *,int);
int pop(int *);
void display(int *);
void push(int s[],int d)
{
    if(top==(size-1))
        flag=0;
    else
    {
        flag=1;
        ++top;
        s[top]=d;
    }
}
int pop(int s[])
{
    int popped_element;
    if(top==-1)
    {
        popped_element=0;
        flag=0;
    }
    else
    {
        flag=1;
        popped_element=s[top];
        --top;
    }
    return(popped_element);
}
void display(int s[])
{
    int i;
    if(top==-1)
    {
        printf("\n stack is empty");
    }
    else
    {
        for(i=top;i>=0;--i)
            printf("\n %d",s[i]);
    }
}
/* this is the main function */
void main()
{
    int data;
    char choice;
    int q=0;
    int top=-1;
    clrscr();
    do
    {
        printf("\n push->i pop->p quit->q:");
        printf("enter your choice");
        do
        {
            choice=getchar();
            choice=tolower(choice);
        }
        while(strchr("ipq",choice)==NULL);
        printf("your choice is %c",choice);
        switch(choice)
        {
        case'i':printf("\n input element to push");
            scanf("%d",&data);
            push(stack,data);
            if(flag)
            {
                printf("\n after inserting ");
                display(stack);
                if(top==(size-1))
                    printf("\n stack is full");
            }
            else
                printf("\n stack is overflown after pushing");
            break;
        case 'p':data=pop(stack);
            if(flag)
            {
                printf("\n data is popped:%d",data);
                printf("\n now the stack is as follows :\n");
                display(stack);
            }
            else
                printf("\n stack is underflown");
            break;
        case'q':q=1;
        }
    }
    while(!q);
}