Ubuntu Manpage: splice - splice data to/from a pipe

NAME

       splice - splice data to/from a pipe

SYNOPSIS

       #define _GNU_SOURCE         /* See feature_test_macros(7) */
       #include <fcntl.h>

       ssize_t 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.

The following semantics apply for fd_in and off_in:

* 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 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

       EAGAIN SPLICE_F_NONBLOCK was specified in flags, and the operation would block.

       EBADF  One or both file descriptors are not valid, or do not have proper read-write mode.

       EINVAL Target filesystem doesn't support splicing; target file is opened in  append  mode;
              neither  of  the  descriptors  refers  to  a  pipe; or offset given for nonseekable
              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; library support was added to
       glibc in version 2.5.

CONFORMING TO

       This system call is Linux-specific.

NOTES

       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.

EXAMPLE

       See tee(2).

COLOPHON

       This page is part of release 4.04 of the Linux man-pages project.  A  description  of  the
       project,  information  about  reporting  bugs, and the latest version of this page, can be
       found at http://www.kernel.org/doc/man-pages/.