linux-kernel - Question about SOCK

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Date:   Thu, 4 May 2017 22:18:32 -0700
From:   Sam Kumar <samkumar99@...il.com>
To:     linux-kernel@...r.kernel.org
Subject: Question about SOCK_SEQPACKET

Hello,
I have recently had occasion to use SOCK_SEQPACKET sockets on Linux,
and noticed some odd behavior. When using sendmsg and recvmsg with
these sockets, it seems that the "end-of-record" flag (MSG_EOR) is not
being propagated correctly.

The man page for recvmsg(2) states:
> The  msg_flags  field  in the msghdr is set on return of recvmsg().  It
>        can contain several flags:
>
>        MSG_EOR
>               indicates end-of-record; the data returned  completed  a  record
>               (generally used with sockets of type SOCK_SEQPACKET).
>

The man page for recvmsg(3) states:
> For
>       message-based  sockets,  such as SOCK_DGRAM and SOCK_SEQPACKET, the entire
>       message shall be read in a single operation.

This leads me to believe that MSG_EOR should be set in the msghdr
struct whenever recvmsg() returns data. However, I am not observing
this flag ever being set, whether or not I set the MSG_EOR when
sending the messages.

If it helps you can take a look at the code I'm using. It is at
https://github.com/samkumar/seqpacket-test/, commit
2a7dbc1f94bafce6950ee726bdd54da96945d083 (HEAD of master at the time
of writing). Look at server.c and client.c (don't bother with
goclient.go).

The reason that I need to check MSG_EOR is that I need to distinguish
between EOF and messages of length 0. For SOCK_STREAM sockets, a
return value of 0 unambiguously means EOF, and for SOCK_DGRAM sockets
a return value of 0 unambiguously means that a datagram of length 0
was received.

Because SOCK_SEQPACKET is both connection-based and message-oriented,
a return value of 0 is ambiguous. Based on my reading of the man
pages, reading the MSG_EOR bit would let me disambiguate between EOF
and a zero-length datagram, because MSG_EOR would be set for a
zero-length datagram, but would not be set for EOF.

If someone could please help me understand MSG_EOR, and how to
distinguish between EOF and zero-length messages in a SOCK_SEQPACKET
connection, I would definitely appreciate it!

Thanks,
Sam Kumar