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:Just before the part that you've quoted, the description for
I do wonder if the man page for recvmsg is wrong, or at least a bit
Which man page do you think is wrong here? The POSIX definition is here
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.
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)
"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,