C / C++ / MFC

#include <stdio.h>

int IsPalindrome(const char*);

int main(void)
{
char str[21]={NULL};
int i=0;
  char c=NULL;

do
{
 if(str[0]!=NULL)
 {
  if(IsPalindrome(str))
   printf("This is a palindrome!\n");
  else
   printf("This isn't a palindrome.\n");
 }
 i = 0;
 while((c=getchar())!='\n' && i<21)
 {
  if((c>='a' && c<='z') || c=='$')
  {
   str[i] = c;
   i++;
  }
 }
 str[i] = NULL;
} while(str[0]!='$');

return 0;
}

int IsPalindrome(const char* str)
{
int len=0, i=0;
// Get string length
while(str[len++]!=NULL) {}
len--;

for(i=0;i<len;i++)
{
 if(str[i]!=str[(len-1)-i])
  return 0;
}
     
return 1;
}