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  implementa-
	      tion 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  descrip-
	      tion 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 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.

VERSIONS

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

EXAMPLE

       See tee(2).

SEE ALSO

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

COLOPHON

       This  page is part of release 4.15 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								    2017-09-15								 SPLICE(2)