#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);
DESCRIPTION
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 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_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))
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.
ERRORS
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;
neither of the descriptors refers to a pipe; or
offset given for non-seekable device.
ENOMEM
Out of memory.
ESPIPE
Either
off_in
or
off_out
was not NULL, but the corresponding file descriptor refers to a pipe.
VERSIONS
The
splice()
system call first appeared in Linux 2.6.17.
CONFORMING TO
This system call is Linux-specific.
NOTES
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.
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 3.14 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/.
