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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 descriptors must refer to a pipe.
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
current file offset, and the current 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 current 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 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 referred to by fd_in.
On error, splice() returns -1 and errno is set to indicate the error.
EBADF One or both file descriptors are not valid, or do not have proper read-
write mode.
EINVAL Target file system doesn't support splicing; target file is opened in
append mode; neither of the descriptors refers to a pipe; or offset
given for nonseekable device.
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.
This system call is Linux-specific.
The three system calls splice(), vmsplice(2), and tee(2), provide userspace
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).
sendfile(2), tee(2), vmsplice(2)
This page is part of release 3.32 of the Linux man-pages project. A
description of the project, and information about reporting bugs, can be found
at http://www.kernel.org/doc/man-pages/.
Linux 2009-09-15 SPLICE(2)
HTML rendering created 2010-12-03 by Michael Kerrisk, author of The Linux Programming Interface