yes you are right, so get back to the first question how could I know number of bytes read by FGETS?

>>I tried FREAD instead of FGETS and it looks like it works fine, I tried to read 8192 bytes with FREAD then write them with FWRITE without any modifications and I got same file size.
>
>With FREAD some occurences may be split between two sequential reads in which case they will not be replaced.