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       splice - splice data to/from a pipe


       #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);


       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).


       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.


       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.


       The  splice()  system  call  first  appeared  in  Linux 2.6.17; library
       support 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.


       See tee(2).


       sendfile(2), tee(2), vmsplice(2)


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