Reverse 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()*/