SOSPLICE(9) OpenBSD Kernel Manual SOSPLICE(9)
NAME
sosplice, somove - splice two sockets for zero-copy data transfer
SYNOPSIS
int
sosplice(struct socket *so, int fd, off_t max, struct timeval *tv);
int
somove(struct socket *so, int wait);
DESCRIPTION
The function sosplice() is used to splice together a source and a drain
socket. The source socket is passed as the so argument; the file
descriptor of the drain is passed in fd. If fd is negative, an existing
splicing gets dissolved. If max is positive, at most that many bytes
will get transferred. If tv is not NULL, a timeout(9) is scheduled to
dissolve splicing in the case when no data can be transferred for the
specified period of time. Socket splicing can be invoked from user-land
via the setsockopt(2) system-call at the SOL_SOCKET level with the socket
option SO_SPLICE.
Before connecting both sockets, several checks are executed. See the
ERRORS section for possible failures. The connection between both
sockets is implemented by setting these additional fields in struct
socket:
- struct socket *so_splice links from the source to the drain
socket.
- struct socket *so_spliceback links back from the drain to the
source socket.
- off_t so_splicelen counts the number of bytes spliced so far
from this socket.
- off_t so_splicemax specifies the maximum number of bytes to
splice from this socket if non-zero.
After connecting both sockets, sosplice() calls somove() to transfer the
mbufs already in the source receive buffer to the drain send buffer.
Finally the socket buffer flag SB_SPLICE is set on both socket buffers,
to indicate that the protocol layer has to call somove() whenever data or
space is available.
The function somove() transfers data from the source's receive buffer to
the drain's send buffer. It must be called at splsoftnet(9) and so must
be a spliced drain socket. It may be necessary to split an mbuf to
handle out-of-band data inline or when the maximum splice length has been
reached. If wait is M_WAIT, splitting mbufs will always succeed. For
M_DONTWAIT the out-of-band property might get lost or a short splice
might happen. In the latter case, less than the given maximum number of
bytes are transferred and user-land has to cope with this. Note that a
short splice cannot happen if somove() was called by sosplice(). So a
second setsockopt(2) after a short splice pointing to the same maximum
will always succeed.
Before transferring data, somove() checks both sockets for errors and
that the drain socket is connected. If the drain cannot send anymore, an
EPIPE error is set on the source socket. The data length to move is
limited by the optional maximum splice length and the space in the
drain's send socket buffer. Up to this amount of data is taken out of
the source's receive socket buffer.
If the maximum splice length has been reached, an mbuf may get split.
Otherwise an mbuf is either moved completely to the send buffer or left
in the receive buffer for later processing. If SO_OOBINLINE is set, out-
of-band data will get moved as such although this might not be reliable.
The data is sent out to the drain socket via the protocol function. If
that fails and the drain socket cannot send anymore, an EPIPE error is
set on the source socket.
If the idle timeout was specified and no data was transferred for that
period of time, splicing gets dissolved and an ETIMEDOUT error is set on
the source socket.
Finally the socket splicing gets dissolved if the source socket cannot
receive anymore and its receive buffer is empty; or if the drain socket
cannot send anymore; or if the maximum has been reached; or if an error
occurred.
If the socket buffer flag SB_SPLICE is set, the functions sorwakeup() and
sowwakeup() will call somove() to trigger the transfer when new data or
buffer space is available. While socket splicing is active, any read(2)
from the source socket will block and the wakeup will not be delivered to
the file descriptor. A read event is signaled to user-land after
dissolving.
RETURN VALUES
sosplice() returns 0 on success and otherwise the error number. somove()
returns 0 if socket splicing has been finished and 1 if it continues.
ERRORS
sosplice() will succeed unless:
[EBADF] The given file descriptor fd is not an active
descriptor.
[EBUSY] The source or the drain socket is already spliced.
[EINVAL] The given maximum value max is negative.
[ENOTCONN] The source or the drain socket is neither connected
nor in the process of connecting to a peer.
[ENOTSOCK] The given file descriptor fd is not a socket.
[EOPNOTSUPP] The source or the drain socket is a listen socket.
[EPROTONOSUPPORT] The source socket's protocol layer does not have the
PR_SPLICE flag set. At the moment only TCP supports
socket splicing.
[EPROTONOSUPPORT] The drain socket's protocol does not have the same
pr_usrreq function as the source.
[EWOULDBLOCK] The source socket is non-blocking and the receive
buffer is already locked.
SEE ALSO
setsockopt(2), options(4), timeout(9)
HISTORY
Socket splicing first appeared in OpenBSD 4.9.
AUTHORS
The idea for socket splicing originally came from Markus Friedl
<markus@openbsd.org>, and Alexander Bluhm <bluhm@openbsd.org> implemented
it.
OpenBSD 5.0 July 4, 2011 OpenBSD 5.0