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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 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:

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 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, () 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 () 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 and the wakeup will not be delivered to the file descriptor. A read event is signaled to user-land after dissolving.

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 or 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. At the moment only TCP supports socket splicing.
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 first appeared in OpenBSD 4.9.

The idea for socket splicing originally came from Markus Friedl ⟨⟩, and Alexander Bluhm ⟨⟩ implemented it.

July 4, 2011 OpenBSD-5.1