.\" This manpage is Copyright (C) 2006 Jens Axboe

.\" and Copyright (C) 2006 Michael Kerrisk <mtk.manpages@gmail.com>

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.\"

.TH SPLICE 2 2017-09-15 "Linux" "Linux Programmer's Manual"

.SH NAME

splice \- splice data to/from a pipe

.SH SYNOPSIS

.nf

.BR "#define _GNU_SOURCE" "         /* See feature_test_macros(7) */"

.B #include <fcntl.h>

.PP

.BI "ssize_t splice(int " fd_in ", loff_t *" off_in ", int " fd_out ,

.BI "               loff_t *" off_out ", size_t " len \

", unsigned int " flags );

.\" Return type was long before glibc 2.7

.fi

.SH DESCRIPTION

.BR splice ()

moves data between two file descriptors

without copying between kernel address space and user address space.

It transfers up to

.I len

bytes of data from the file descriptor

.I fd_in

to the file descriptor

.IR fd_out ,

where one of the file descriptors must refer to a pipe.

.PP

The following semantics apply for

.I fd_in

and

.IR off_in :

.IP * 3

If

.I fd_in

refers to a pipe, then

.I off_in

must be NULL.

.IP *

If

.I fd_in

does not refer to a pipe and

.I off_in

is NULL, then bytes are read from

.I fd_in

starting from the file offset,

and the file offset is adjusted appropriately.

.IP *

If

.I fd_in

does not refer to a pipe and

.I off_in

is not NULL, then

.I off_in

must point to a buffer which specifies the starting

offset from which bytes will be read from

.IR fd_in ;

in this case, the file offset of

.I fd_in

is not changed.

.PP

Analogous statements apply for

.I fd_out

and

.IR off_out .

.PP

The

.I flags

argument is a bit mask that is composed by ORing together

zero or more of the following values:

.TP

.B SPLICE_F_MOVE

Attempt to move pages instead of copying.

This is only a hint to the kernel:

pages may still be copied if the kernel cannot move the

pages from the pipe, or if

the pipe buffers don't refer to full pages.

The initial implementation of this flag was buggy:

therefore starting in Linux 2.6.21 it is a no-op

(but is still permitted in a

.BR splice ()

call);

in the future, a correct implementation may be restored.

.TP

.B SPLICE_F_NONBLOCK

Do not block on I/O.

This makes the splice pipe operations nonblocking, but

.BR splice ()

may nevertheless block because the file descriptors that

are spliced to/from may block (unless they have the

.B O_NONBLOCK

flag set).

.TP

.B SPLICE_F_MORE

More data will be coming in a subsequent splice.

This is a helpful hint when

the

.I fd_out

refers to a socket (see also the description of

.B MSG_MORE

in

.BR send (2),

and the description of

.B TCP_CORK

in

.BR tcp (7)).

.TP

.B SPLICE_F_GIFT

Unused for

.BR splice ();

see

.BR vmsplice (2).

.SH RETURN VALUE

Upon successful completion,

.BR splice ()

returns the number of bytes

spliced to or from the pipe.

.PP

A return value of 0 means end of input.

If

.I fd_in

refers to a pipe, then this means that there was no data to transfer,

and it would not make sense to block because there are no writers

connected to the write end of the pipe.

.PP

On error,

.BR splice ()

returns \-1 and

.I errno

is set to indicate the error.

.SH ERRORS

.TP

.B EAGAIN

.B SPLICE_F_NONBLOCK

was specified in

.IR flags ,

and the operation would block.

.TP

.B EBADF

One or both file descriptors are not valid,

or do not have proper read-write mode.

.TP

.B EINVAL

The target filesystem doesn't support splicing.

.TP

.B EINVAL

The target file is opened in append mode.

.\" The append-mode error is given since 2.6.27; in earlier kernels,

.\" splice() in append mode was broken

.TP

.B EINVAL

Neither of the file descriptors refers to a pipe.

.TP

.B EINVAL

An offset was given for nonseekable device (e.g., a pipe).

.TP

.B EINVAL

.I fd_in

and

.I fd_out

refer to the same pipe.

.TP

.B ENOMEM

Out of memory.

.TP

.B ESPIPE

Either

.I off_in

or

.I off_out

was not NULL, but the corresponding file descriptor refers to a pipe.

.SH VERSIONS

The

.BR splice ()

system call first appeared in Linux 2.6.17;

library support was added to glibc in version 2.5.

.SH CONFORMING TO

This system call is Linux-specific.

.SH NOTES

The three system calls

.BR splice (),

.BR vmsplice (2),

and

.BR tee (2),

provide user-space programs with full control over an arbitrary

kernel buffer, implemented within the kernel using the same type

of buffer that is used for a pipe.

In overview, these system calls perform the following tasks:

.TP 1.2i

.BR splice ()

moves data from the buffer to an arbitrary file descriptor, or vice versa,

or from one buffer to another.

.TP

.BR tee (2)

"copies" the data from one buffer to another.

.TP

.BR vmsplice (2)

"copies" data from user space into the buffer.

.PP

Though we talk of copying, actual copies are generally avoided.

The kernel does this by implementing a pipe buffer as a set

of reference-counted pointers to pages of kernel memory.

The kernel creates "copies" of pages in a buffer by creating new

pointers (for the output buffer) referring to the pages,

and increasing the reference counts for the pages:

only pointers are copied, not the pages of the buffer.

.\"

.\" Linus: Now, imagine using the above in a media server, for example.

.\" Let's say that a year or two has passed, so that the video drivers

.\" have been updated to be able to do the splice thing, and what can

.\" you do? You can:

.\"

.\" - splice from the (mpeg or whatever - let's just assume that the video

.\"   input is either digital or does the encoding on its own - like they

.\"   pretty much all do) video input into a pipe (remember: no copies - the

.\"   video input will just DMA directly into memory, and splice will just

.\"   set up the pages in the pipe buffer)

.\" - tee that pipe to split it up

.\" - splice one end to a file (ie "save the compressed stream to disk")

.\" - splice the other end to a real-time video decoder window for your

.\"   real-time viewing pleasure.

.\"

.\" Linus: Now, the advantage of splice()/tee() is that you can

.\" do zero-copy movement of data, and unlike sendfile() you can

.\" do it on _arbitrary_ data (and, as shown by "tee()", it's more

.\" than just sending the data to somebody else: you can duplicate

.\" the data and choose to forward it to two or more different

.\" users - for things like logging etc.).

.\"

.PP

In Linux 2.6.30 and earlier,

exactly one of

.I fd_in

and

.I fd_out

was required to be a pipe.

Since Linux 2.6.31,

.\" commit 7c77f0b3f9208c339a4b40737bb2cb0f0319bb8d

both arguments may refer to pipes.

.SH EXAMPLE

See

.BR tee (2).

.SH SEE ALSO

.BR copy_file_range (2),

.BR sendfile (2),

.BR tee (2),

.BR vmsplice (2),

.BR pipe (7)

.SH COLOPHON

This page is part of release 4.15 of the Linux

.I man-pages

project.

A description of the project,

information about reporting bugs,

and the latest version of this page,

can be found at

\%https://www.kernel.org/doc/man\-pages/.