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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
                      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).

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
   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
   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 4.09 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/.



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