netdev - Re: Question about SOCK

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Date:   Fri, 5 May 2017 06:09:26 -0400
From:   Sowmini Varadhan <sowmini.varadhan@...cle.com>
To:     Steven Whitehouse <swhiteho@...hat.com>
Cc:     Sam Kumar <samkumar99@...il.com>, linux-kernel@...r.kernel.org,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: Question about SOCK_SEQPACKET

On (05/05/17 10:45), Steven Whitehouse wrote:
> 
> I do wonder if the man page for recvmsg is wrong, or at least a bit
> confusing. SOCK_SEQPACKET is stream based not message based - it just
> happens to have EOR markers in the stream. There is no reason that the whole
> message needs to be returned in a single read, and in fact that would be
> impossible if the sender didn't insert any EOR markers but kept sending data
> beyond the size that the socket could buffer.
> 
> I notice that man 7 socket says SOCK_SEQPACKET is for datagrams of fixed
> maximum length which is definitely wrong, as is the statement that a
> consumer has to read an entire packet with each system call.

Which man page do you think is wrong here? The POSIX definition is here

http://pubs.opengroup.org/onlinepubs/009695399/functions/recvmsg.html

The description in

http://pubs.opengroup.org/onlinepubs/009695399/functions/xsh_chap02_10.html

says, "It is protocol-specific whether a maximum record size is imposed."
In my machine (Ubuntu 4.4.0-72, and it is in socket(2), not socket(7), btw)
doesnt have any references to max length, but I'm not sure I'd boldly assert
"definitely wrong" about the requirement of having to read entire
packet in a system call (see POSIX man page)

--Sowmini