#define MAX_SIZE 30
.
.
.
 do
    { 
       printf(" Enter String::: ");
       gets(str);
    }

 while (strlen(str)<MAX_SIZE);

char answer;
void main()
{
 ...
do 
  {
     clrscr();
     printf(" Enter String::: ");
     gets( str ); // same problem when i use scanf("%s",str)
     some_function( str );
     printf(" Result:: %s ", str );

   printf("\n\n\n\t\t\t Do you want to continue (y/n)");
   flushall();
   scanf(" %c ", &answer);
   } 
    
   while (answer == 'y' || answer=='Y');
}