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/.

Linux                                              2014-12-31                                          SPLICE(2)