Palindrome ( without built-in functions )

Aim:
To check given string is Palindrome or not.

Program:
#include<stdio.h>
#include<conio.h>
void main ( )
  {
     char str[80],revstr[80];
     int i,length=0,flag=0;
     clrscr ();
printf("\n Enter a string:");
gets (str);
for( i=0;string[i] != '\0';i++)
   {
     length++;
   }
printf("\nLength of the string \'%s'\ is : %d",str,length);
for(i=length-1;i>=0;i--)
   {
     revstr[length-i-1] = str[i];
   }
for(i=0;i<length;i++)
  {
     if(revstr[i] == str[i])
           flag = 1;
     else
          flag = 0;
    }
   if ( flag == 1)
printf("\n%s is a palindrome",str);
   else
      printf("\n%s is not a palindrome",str);
}