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SOSPLICE(9)	       FreeBSD Kernel Developer's Manual	   SOSPLICE(9)

     sosplice, somove -- splice	two sockets for	zero-copy data transfer

     sosplice(struct socket *so, int fd, off_t max, struct timeval *tv);

     somove(struct socket *so, int wait);

     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 de-
     scriptor 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 userland
     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	sock-
     ets is implemented	by setting these additional fields in the struct
     sosplice *so_sp field in struct socket:

	   -   struct socket *ssp_socket links from the	source to the drain
	   -   struct socket *ssp_soback links back from the drain to the
	       source socket.
	   -   off_t ssp_len counts the	number of bytes	spliced	so far from
	       this socket.
	   -   off_t ssp_max specifies the maximum number of bytes to splice
	       from this socket	if non-zero.
	   -   struct timeval ssp_idletv specifies the maximum idle time if
	   -   struct timeout ssp_idleto provides storage for the kernel time-
	       out if idle time	is used.

     After connecting both sockets, sosplice() calls somove() to transfer the
     mbufs already in the source receive buffer	to the drain send buffer.  Fi-
     nally 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 source socket.  It may be necessary to split an mbuf to han-
     dle 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 userland	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 lim-
     ited 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.  To avoid splicing	loops created by user-
     land, the number of times an mbuf may be moved between sockets is limited
     to	128.

     For atomic	protocols, either one complete packet is taken out, or nothing
     is	taken at all if: the packet is bigger than the drain's send buffer
     size, in which case the splicing gets aborted with	an EMSGSIZE error; the
     packet does not fit into the drain's current send buffer space, in	which
     case it is	left in	the source's receive buffer for	later processing; or
     the maximum splice	length is located within a packet, in which case
     splicing gets dissolved like a short splice.  All address or control
     mbufs associated with the taken packet are	dropped.

     If	the maximum splice length has been reached, an mbuf may	get split for
     non-atomic	protocols.  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.

     For packet	oriented protocols somove() iterates over the next packet

     If	a maximum splice length	was specified and at least this	amount of data
     has been received from the	drain socket, splicing gets dissolved.	In
     this case,	an EFBIG error is set on the source socket if the maximum
     amount of data has	been transferred.  Userland can	process	this error to
     distinguish the full splice from a	short splice or	to react to the	com-
     pleted maximum splice immediately.	 If an idle timeout was	specified and
     no	data has been transferred for that period of time, the handler
     soidle() dissolves	splicing and sets an ETIMEDOUT error on	the source

     The function sounsplice() is called to dissolve the socket	splicing 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; or if the idle timeout has fired.

     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.	 Neither read nor write	wakeups	will
     be	delivered to the file descriptors.  After dissolving, a	read event or
     a socket error is signaled	to userland on the source socket.  If space is
     available,	a write	event will be signaled on the drain socket.

     sosplice()	returns	0 on success and otherwise the error number.  somove()
     returns 0 if socket splicing has been finished and	1 if it	continues.

     sosplice()	will succeed unless:

     [EBADF]		The given file descriptor fd is	not an active descrip-

     [EBUSY]		The source or the drain	socket is already spliced.

     [EINVAL]		The given maximum value	max is negative.

     [ENOTCONN]		The source socket requires a connection	and is neither
			connected nor in the process of	connecting to a	peer.

     [ENOTCONN]		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.  Only TCP and UDP socket splicing
			is supported.

     [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 buf-
			fer is already locked.

     setsockopt(2), options(4),	timeout(9)

     Socket splicing for TCP first appeared in OpenBSD 4.9; support for	UDP
     was added in OpenBSD 5.3.

     The idea for socket splicing originally came from Markus Friedl
     <>, and Alexander Bluhm <> implemented
     it.  Mike Belopuhov <> added the timeout feature.

FreeBSD	13.0			 July 4, 2019			  FreeBSD 13.0


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