ok. plz give an overview.
the sequence of steps i mean.
is my understanding correct ?

while(1){

if fgets == NULL{
sleep
clearerr
}
else
read & process string

}

That looks pretty close.

Remember that just because fgets() did not return NULL does NOT mean that it read a complete input line. (It is possible for fgets to return with a partial line of data if there is a partial line of data at the end of the file.)