Godday,
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.
As far as I can work out, this occurs because of the
following code in net/socket.c::sock_write/sock_aio_write():
if(size==0) /* Match SYS5 behaviour */
return 0;
Is the Linux behaviour intended? It seems counterintuitive,
and it does not match what I see on other systems:
Solaris 8 does not behave like Linux in this case - a zero
length datagram is sent and read by the receiver (tested for
Internet and Unix domain sockets).
Tru64 5.1 is the same as Solaris 8 (tested for Internet and
Unix domain sockets)
HP-UX 11 is the same as Solaris 8 (tested for Internet and
Unix domain sockets)
FreeBSD 4.9 and 5.1 are the same as Solaris 8 (tested for
Internet and Unix domain sockets)
Cheers,
Michael
--
Michael Kerrisk
mtk-lists@gmx.net
NEU: WLAN-Router für 0,- EUR* - auch für DSL-Wechsler!
GMX DSL = supergünstig & kabellos http://www.gmx.net/de/go/dsl
On Mon, Aug 09, 2004 at 04:38:13PM +0200, Michael T Kerrisk wrote:
Godday,
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, 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.
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'.
Do you actually have a need for zero byte packets?
--
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Bert,
quoted
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@gmx.net
NEU: WLAN-Router f�r 0,- EUR* - auch f�r DSL-Wechsler!
GMX DSL = superg�nstig & kabellos http://www.gmx.net/de/go/dsl