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

sosplice, somovesplice 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 () 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 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 sockets is implemented by setting these additional fields in the struct sosplice *so_sp field in struct socket:

After connecting both sockets, () 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 () 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 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 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, () 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. To avoid splicing loops created by userland, 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 () iterates over the next packet queue.

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 completed maximum splice immediately. If an idle timeout was specified and no data has been transferred for that period of time, the handler () dissolves splicing and sets an ETIMEDOUT error on the source socket.

The function () 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 () and () 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:

The given file descriptor fd is not an active descriptor.
The source or the drain socket is already spliced.
The given maximum value max is negative.
The source socket requires a connection and is neither connected nor in the process of connecting to a peer.
The drain socket is neither connected nor in the process of connecting to a peer.
The given file descriptor fd is not a socket.
The source or the drain socket is a listen socket.
The source socket's protocol layer does not have the PR_SPLICE flag set. Only TCP and UDP socket splicing is supported.
The drain socket's protocol does not have the same pr_usrreq function as the source.
The source socket is non-blocking and the receive buffer 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.

July 4, 2019 OpenBSD-6.7