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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.

       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.


       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

       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 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.


       See tee(2).


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


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