splice - splice data to/from a pipe
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 refers to a pipe, then
off_in must be
fd_in does not refer to a pipe and
NULL, then bytes are read from
fd_in starting from the file
offset, and the file offset is adjusted appropriately.
fd_in does not refer to a pipe and
not NULL, then
off_in must point to a buffer which specifies
the starting offset from which bytes will be read from
in this case, the file offset of
fd_in is not changed.
Analogous statements apply for
flags argument is a bit mask that is composed by ORing
together zero or more of the following values:
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.
Do 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).
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
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
set to indicate the error.
SPLICE_F_NONBLOCK was specified in
one of the file descriptors had been marked as nonblocking
(O_NONBLOCK), and the operation would
One or both file descriptors are not valid, or do not have proper read-write mode.
The target filesystem doesn't support splicing.
The target file is opened in append mode.
Neither of the file descriptors refers to a pipe.
An offset was given for nonseekable device (e.g., a pipe).
fd_out refer to the same pipe.
Out of memory.
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.
This system call is Linux-specific.
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.
In Linux 2.6.30 and earlier, exactly one of
fd_out was required to be a pipe. Since Linux 2.6.31, both
arguments may refer to pipes.
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