Re: Strangeness when write()-ing zero-length datagrams

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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