Re: Question about SOCK_SEQPACKET

From: Steven Whitehouse
Date: Fri May 05 2017 - 06:34:12 EST


On 05/05/17 11:09, Sowmini Varadhan wrote:
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

The description in

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)


Just before the part that you've quoted, the description for SOCK_SEQPACKET says:
"The SOCK_SEQPACKET socket type is similar to the SOCK_STREAM type, and is also connection-oriented. The only difference between these types is that record boundaries are maintained using the SOCK_SEQPACKET type. A record can be sent using one or more output operations and received using one or more input operations, but a single operation never transfers parts of more than one record."

The man page for socket says SOCK_SEQPACKET "Provides a sequenced, reliable, two-way connection-based data transmission path for datagrams of fixed maximum length" which is not true, because while there may be a length restriction, it is quite possible that there is not a length restriction (as per DECnet). It also says "a consumer is required to read an entire packet with each input system call" which is also contradicted by POSIX which says that a record can be "received using one or more input operations". So both statements in the man page are wrong, I think.

I have to say that I'd not spotted the POSIX recvmsg wording before, which says "For message-based sockets, such as SOCK_DGRAM and SOCK_SEQPACKET, the entire message shall be read in a single operation" however that does contradict the earlier wording, where it explicitly says that multiple receive operations per record are ok for SOCK_SEQPACKET - at least if we assume that record == message in this case. Also, if this restriction was true (one message per recvmsg call) then MSG_EOR would never be needed on receive, since every recvmsg would be a single message/record only, and that same document does say that MSG_EOR can be set on receive for protocols which support it,