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_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. 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_NONBLOCKDo 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_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 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.
SPLICE_F_NONBLOCK was
specified in flags, and the operation
would block.
One or both file descriptors are not valid, or do not have proper read-write mode.
Target filesystem doesn't support splicing; target file is opened in append mode; neither of the file descriptors refers to a pipe; or offset given for nonseekable device.
Out of memory.
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.
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.
This page is part of release 4.07 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/.
|
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