Bert,
> > On Linux 2.4.26 and 2.6.7, if we connect a datagram socket and
> > then use write() to send 0 bytes, a datagram is NOT generated
> > (this happens in both the Unix and Internet domains).
> > Datagrams are generated in this case for send() and sendto()
> > though.
>
> Some postings were made about this some time ago,
I didn't find anything with google or archive seach -- do
you have any more specific reference on this thread?
> it appears SUS has an
> opinion on zero byte writes:
>
> If nbyte is 0, write() will return 0 and have no other results if the
> file is a regular file; otherwise, the results are unspecified.
> http://www.opengroup.org/onlinepubs/007908799/xsh/write.html
>
> The send(2) manpage states:
>
> The send() function is identical to sendto() with a null pointer dest_len
> argument, and to write() if no flags are used.
>
> Which means that a zero byte send() without flags is unspecified.
Yes, but on Linux write(fd, buf, 0) != send(fd, buf, 0, 0), which
does seem inconsistent. And as I noted, all the other implementations
I tested behave differently from Linux.
(Just so I'm sure we have our scenarios clear, I'm referring to
the following:
HOST-A HOST-B
create datagram socket
bind socket to
INADDR_ANY:port-x
create datagram socket
connect socket to
HOST-B:port-x
Now the following behave
differently:
write(fd, buf, 0);
send(fd, buf, 0, 0);
)
> Also note that it is hard if impossible to usefully inform userspace of
> the reception of a zery byte packet - returning 0 from recvfrom may
> also mean an 'orderly shutdown'.
Am I missing something? There isn't the notion of a true
connection with datagram sockets, and so no notion of an orderly
shutdown (I am assuming you are meaning some scenario where the
peer calls shutdown() or close()).
It is straightforward to detect a zero-length datagram: recvfrom()
returns 0.
> Do you actually have a need for zero byte packets?
No, but I discovered this difference during some testing that I
was doing, and then observed that it seems different from
most (all?) other implementations. It seemed worth documenting
this fact.
Cheers,
Michael
--
Michael Kerrisk
mtk-lists@xxxxxxx
NEU: WLAN-Router für 0,- EUR* - auch für DSL-Wechsler!
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