tail -f, does it lock the file while it scans ?

[Ssi]

    The other day I had occasion to monitor changes to a file with
`tail -f'. I have a shell script that gets input with a read, then does
a grep on a `file' with that input, and sends the match to `another_file'.
While I was tail -f'ing `another_file' I noticed that some lines did not show
up even if the pattern was matched in `file'. Does tail -f do some kind of
file lock to prevent writing by another proccess ?? This would seem to
defeat it's purpose. SunOS 3.5/BSD 4.2

                   Greg Ripp (813)628-6100 x5123
                   ...!uunet!pdn!usfvax2!system1!greg