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
       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:

       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 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 Target  filesystem  doesn't  support  splicing;  target  file is
	      opened in append mode; neither of the file 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.

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

COLOPHON
       This  page  is  part of release 4.10 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				  2016-03-15			     SPLICE(2)