Overflow & Underflow in c pgm

Implement two Stacks using a Single Array & Check for Overflow & Underflow

#include 

#include 

#include 

#define max 10

int top1, top2, array[max];

void push(void)

{

    int x, ch;

    if (top1 == top2 - 1)

    {

        printf("stack overflow \n");

        return;

    }

    printf("enter a no \n");

    scanf("%d", &x);

    printf("\n press 1 to push the element in 1st stack or press 2 for stack 2:");

    scanf("%d", &ch);

    if (ch == 1)

        array[++top1] = x;

    else

        array[--top2] = x;

    printf("%d element is successfully pushed \n", x);

    return;

}

void pop(void)

{

    int y, ch;

    printf("\n press 1 to pop the element from 1st stack or press 2 for from stack 2");

    scanf("%d", &ch);

    if (ch == 1)

    {

        if (top1 == -1)   

        {

            printf("stack underflow \n");

            return;

        }

        y = array[top1];

        array[top1--]=0;

    }

    else

    {

        if (top2 == max)

        {

            printf("stack underflow \n");

            return;

        }

        y = array[top2];

        array[top2++] = 0;

    }

    printf("%d element is successfully poped from stack \n", y);

    return;

}

void display(void)

{

    int i;

    if (top1 == -1)

    {

        printf("stack 1 is empty \n");

    }

    else

    {

        printf("elements of Stack 1 are : \n");

        for (i = 0; i = top2; i--)

        {

            printf("%d\n", array[i]);

        }

    return ;

}

 void main(void)

{

    int ch;

    top1 = -1, top2 = max;

    do

    {

        printf("1:push\n 2:pop\n 3:display\n 4:exit\n choice:");

        scanf("%d", &ch);

        switch (ch)

        {

        case 1:push();

            break;

        case 2:pop();

            break;

        case 3:display();

            break;

        case 4:printf("quits from program \n");

            break;

        default:printf("wrong choice \n");

            break;

        }

    } while (ch != 4);

}

Output

1:push

2:pop

3:display

4:exit

choice:1

enter a no

13

press 1 to push the element in 1st stack or press 2 for stack 2:1

13 element is successfully pushed

1:push

2:pop

3:display

4:exit

choice:1

enter a no

30

press 1 to push the element in 1st stack or press 2 for stack 2:1

30 element is successfully pushed

1:push

2:pop

3:display

4:exit

choice:1

enter a no

37

press 1 to push the element in 1st stack or press 2 for stack 2:2

37 element is successfully pushed

1:push

2:pop

3:display

4:exit

choice:1

enter a no

65

press 1 to push the element in 1st stack or press 2 for stack 2:2

65 element is successfully pushed

1:push

2:pop

3:display

4:exit

choice:2

press 1 to pop the element from 1st stack or press 2 for from stack 21

30element is successfully poped from stack

1:push

2:pop

3:display

4:exit

choice:2

press 1 to pop the element from 1st stack or press 2 for from stack 22

65element is successfully poped from stack

1:push

2:pop

3:display

4:exit

choice:3

elements of Stack 1 are :

13

Elements of stack 2 are

37

1:push

2:pop

3:display

4:exit

choice:1

enter a no

10

press 1 to push the element in 1st stack or press 2 for stack 2:1

10 element is successfully pushed

1:push

2:pop

3:display

4:exit

choice:1

enter a no

98

press 1 to push the element in 1st stack or press 2 for stack 2:2

98 element is successfully pushed

1:push

2:pop

3:display

4:exit

choice:3

elements of Stack 1 are :

13

10

Elements of stack 2 are

37

98

1:push

2:pop

3:display

4:exit

choice:4

quits from program