Re: Question about SOCK_SEQPACKET

From: Steven Whitehouse
Date: Fri May 05 2017 - 05:45:07 EST


On 05/05/17 06:18, Sam Kumar wrote:
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.
It depends which protocol you are using as to whether that is true. SOCK_SEQPACKET is supposed to be identical to SOCK_STREAM except for the record separators. That is true for DECnet (but whether DECnet is still functional is another thing!) and not true for ax.25 which uses SOCK_SEQPACKET incorrectly. For AF_UNIX that you are using I'm not quite sure what would be expected.

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:

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:
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, commit
2a7dbc1f94bafce6950ee726bdd54da96945d083 (HEAD of master at the time
of writing). Look at server.c and client.c (don't bother with

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!

Sam Kumar
That would be my expectation of how it should work - if you ignore MSG_EOR on recvmsg then what you get is identical to a SOCK_STREAM, and that every call to recvmsg will return data from (at most) a single message, with MSG_EOR set if the end of that message has been reached. That is what POSIX says should happen anyway.

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.

Also it is probably better to ask this question on netdev where it is likely to get more attention from the net developers, so I'm copying my reply there too,