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splice() - Unix, Linux System Call
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NAME
splice - splice data to/from a pipe
SYNOPSIS
#define _GNU_SOURCE
#include <fcntl.h>
long splice(int fd_in, off_t *off_in, int fd_out,
off_t *off_out, size_t len
", unsigned int " flags );
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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:
Tag | Description |
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 dont refer to full pages.
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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).
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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))
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SPLICE_F_GIFT |
Unused for
splice(); see
vmsplice(2).
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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
Tag | Description |
EBADF |
One or both file descriptors are not valid,
or do not have proper read-write mode.
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EINVAL |
Target file system doesnt support splicing;
neither of the descriptors refers to a pipe; or
offset given for non-seekable device.
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ENOMEM |
Out of memory.
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ESPIPE |
Either
off_in or
off_out was not NULL, but the corresponding file descriptor refers to a pipe.
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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:
Tag | Description |
splice() |
moves data from the buffer to an arbitrary file descriptor, or vice versa,
or from one buffer to another.
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tee(2)
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"copies" the data from one buffer to another.
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vmsplice(2)
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"copies" data from user space into the buffer.
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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.
EXAMPLE
See
tee(2).
SEE ALSO
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