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#define _GNU_SOURCE /* See feature_test_macros(7) */
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 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 nonblocking, but splice() may neverthe-
less 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).
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
ESPIPE 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 sup-
port 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
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.
sendfile(2), tee(2), vmsplice(2)
Linux 2012-05-04 SPLICE(2)