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man:splice

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 non-
            blocking, 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 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  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.
     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 4.16 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)

/data/webs/external/dokuwiki/data/pages/man/splice.txt · Last modified: 2019/05/17 09:32 by 127.0.0.1

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