splice


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.

       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).

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.

       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  sup-
       port 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).

SEE ALSO
       sendfile(2), tee(2), vmsplice(2)

COLOPHON
       This  page  is  part of release 3.54 of the Linux man-pages project.  A
       description of the project, and information about reporting  bugs,  can
       be found at http://www.kernel.org/doc/man-pages/.



Linux                             2012-05-04                         SPLICE(2)
Man Pages Copyright Respective Owners. Site Copyright (C) 1994 - 2018 Hurricane Electric. All Rights Reserved.