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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 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:

              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.

              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).

              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)).

              Unused for splice(); see vmsplice(2).


       Upon successful completion, splice() returns the number of bytes spliced to  or  from  the

       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.


       EAGAIN SPLICE_F_NONBLOCK  was  specified  in flags or one of the file descriptors had been
              marked as nonblocking (O_NONBLOCK), 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.


       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.

       In Linux 2.6.30 and earlier, exactly one of fd_in and fd_out was required to  be  a  pipe.
       Since Linux 2.6.31, both arguments may refer to pipes.


       See tee(2).


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


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