reversing a string using stack

/*P4.9 Program of reversing a string using stack */
#include<stdio.h>
#include<string.h>
#include<stdlib.h>

#define MAX 20

int top = -1;
char stack[MAX];
char pop();
void push(char);

main()
{
    char str[20];
    unsigned int i;
    printf("Enter the string : " );
    gets(str);
    /*Push characters of the string str on the stack */
    for(i=0;i<strlen(str);i++)
        push(str[i]);
    /*Pop characters from the stack and store in string str */
    for(i=0;i<strlen(str);i++)
        str[i]=pop();
    printf("Reversed string is : ");
    puts(str);
}/*End of main()*/

void push(char item)
{
    if(top == (MAX-1))
    {
        printf("Stack Overflow\n");
        return;
    }
    stack[++top] =item;
}/*End of push()*/

char pop()
{
    if(top == -1)
    {
        printf("Stack Underflow\n");
        exit(1);
    }
    return stack[top--];
}/*End of pop()*/