splice — splice data to/from a pipe
#define _GNU_SOURCE #include <fcntl.h>
long
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_MOVEAttempt 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.
SPLICE_F_NONBLOCKDo not block on I/O. This makes the splice pipe
operations non-blocking, 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_MOREMore 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_GIFTUnused 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.
One or both file descriptors are not valid, or do not have proper read-write mode.
Target file system doesn't support splicing; neither of the descriptors refers to a pipe; or offset given for non-seekable device.
Out of memory.
Either off_in or off_out was not NULL, but
the corresponding file descriptor refers to a pipe.
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.
This page is part of release 2.79 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/.
|
This manpage is Copyright (C) 2006 Jens Axboe and Copyright (C) 2006 Michael Kerrisk <mtk.manpagesgmail.com> Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies. Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided that the entire resulting derived work is distributed under the terms of a permission notice identical to this one. Since the Linux kernel and libraries are constantly changing, this manual page may be incorrect or out-of-date. The author(s) assume no responsibility for errors or omissions, or for damages resulting from the use of the information contained herein. The author(s) may not have taken the same level of care in the production of this manual, which is licensed free of charge, as they might when working professionally. Formatted or processed versions of this manual, if unaccompanied by the source, must acknowledge the copyright and authors of this work. |