splice(2)



SPLICE(2)                  Linux Programmer's Manual                 SPLICE(2)

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 be-
       tween 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 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 file offset, and the 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  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 non-
              blocking,  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 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 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 The target filesystem doesn't support splicing.

       EINVAL The target file is opened in append mode.

       EINVAL Neither of the file descriptors refers to a pipe.

       EINVAL An offset was given for nonseekable device (e.g., a pipe).

       EINVAL fd_in and fd_out refer to the same pipe.

       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 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, im-
       plemented 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_in and  fd_out  was  re-
       quired  to  be a pipe.  Since Linux 2.6.31, both arguments may refer to
       pipes.

EXAMPLE
       See tee(2).

SEE ALSO
       copy_file_range(2), sendfile(2), tee(2), vmsplice(2), pipe(7)

COLOPHON
       This page is part of release 4.16 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
       https://www.kernel.org/doc/man-pages/.

Linux                             2017-09-15                         SPLICE(2)

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