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NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | CONFORMING TO | NOTES | EXAMPLE | SEE ALSO | COLOPHON |
SPLICE(2) Linux Programmer's Manual SPLICE(2)
splice - splice data to/from a pipe
#define _GNU_SOURCE /* See feature_test_macros(7) */
#include <fcntl.h>
ssize_t splice(int fd_in, loff_t *off_in, int fd_out,
loff_t *off_out, size_t len, unsigned int flags);
splice() moves data between two file descriptors without copying
between kernel address space and user address space. It transfers up
to len bytes of data from the file descriptor fd_in to the file
descriptor fd_out, where one of the file descriptors must refer to a
pipe.
The following semantics apply for fd_in and off_in:
* If fd_in refers to a pipe, then off_in must be NULL.
* If fd_in does not refer to a pipe and off_in is NULL, then bytes
are read from fd_in starting from the file offset, and the file
offset is adjusted appropriately.
* If fd_in does not refer to a pipe and off_in is not NULL, then
off_in must point to a buffer which specifies the starting offset
from which bytes will be read from fd_in; in this case, the file
offset of fd_in is not changed.
Analogous statements apply for fd_out and off_out.
The flags argument is a bit mask that is composed by ORing together
zero or more of the following values:
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
splice() call); in the future, a correct
implementation may be restored.
SPLICE_F_NONBLOCK Do not block on I/O. This makes the splice pipe
operations nonblocking, but splice() may
nevertheless block because the file descriptors
that are spliced to/from may block (unless they
have the O_NONBLOCK flag set).
SPLICE_F_MORE More data will be coming in a subsequent splice.
This is a helpful hint when the fd_out refers to a
socket (see also the description of MSG_MORE in
send(2), and the description of TCP_CORK in
tcp(7)).
SPLICE_F_GIFT Unused for splice(); see vmsplice(2).
Upon successful completion, splice() returns the number of bytes
spliced to or from the pipe.
A return value of 0 means end of input. If 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.
On error, splice() returns -1 and errno is set to indicate the error.
EAGAIN SPLICE_F_NONBLOCK was specified in flags, and the operation
would block.
EBADF One or both file descriptors are not valid, or do not have
proper read-write mode.
EINVAL The target filesystem doesn't support splicing.
EINVAL The target file is opened in append mode.
EINVAL Neither of the file descriptors refers to a pipe.
EINVAL An offset was given for nonseekable device (e.g., a pipe).
EINVAL fd_in and fd_out refer to the same pipe.
ENOMEM Out of memory.
ESPIPE Either off_in or off_out was not NULL, but the corresponding
file descriptor refers to a pipe.
The splice() system call first appeared in Linux 2.6.17; library
support was added to glibc in version 2.5.
This system call is Linux-specific.
The three system calls splice(), vmsplice(2), and 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:
splice() moves data from the buffer to an arbitrary file
descriptor, or vice versa, or from one buffer to another.
tee(2) "copies" the data from one buffer to another.
vmsplice(2) "copies" data from user space into the buffer.
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.
See tee(2).
copy_file_range(2), sendfile(2), tee(2), vmsplice(2), pipe(7)
This page is part of release 4.12 of the Linux 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/.
Linux 2017-07-13 SPLICE(2)
Pages that refer to this page: pv(1), copy_file_range(2), pipe(2), sendfile(2), syscalls(2), tee(2), vmsplice(2), pipe(7)