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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 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  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 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.

ERRORS
       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.

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

COLOPHON
       This  page  is  part of release 3.65 of the Linux man-pages project.  A
       description of the project, and information about reporting  bugs,  can
       be found at http://www.kernel.org/doc/man-pages/.

Linux				  2012-05-04			     SPLICE(2)
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