Program of reversing a string using stack

/* Program of reversing a string using stack */
#include<stdio.h>
#define MAX 20
#include<string.h>

int top = -1;
char stack[MAX];
char pop();
push(char);
main()
{
    char str[20];
    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()*/

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

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