Name
splice — splice data to/from a pipe
Synopsis
long
splice( |
int |
fd_in, |
| |
off_t * |
off_in, |
| |
int |
fd_out, |
| |
off_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 out_fd 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.
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))
SPLICE_F_GIFT-
Unused for splice();
see vmsplice(2).
RETURN VALUE
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(2) system call first
appeared in Linux 2.6.17.
CONFORMING TO
This system call is Linux specific.
NOTES
The three system calls splice(2), 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 ALSO
sendfile(2), splice(2), tee(2), feature_test_macros(7)
This manpage is Copyright (C) 2006 Jens Axboe
and Copyright (C) 2006 Michael Kerrisk <mtk-manpages@gmx.net>
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