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

To: Michael T Kerrisk <mtk-lists@xxxxxxx>
Subject: Re: Strangeness when write()-ing zero-length datagrams
From: bert hubert <ahu@xxxxxxx>
Date: Mon, 9 Aug 2004 17:40:57 +0200
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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.

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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