393 lines
7.9 KiB
HTML
393 lines
7.9 KiB
HTML
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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
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<HTML><HEAD><TITLE>Man page of SPLICE</TITLE>
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</HEAD><BODY>
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<H1>SPLICE</H1>
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Section: Linux Programmer's Manual (2)<BR>Updated: 2019-05-09<BR><A HREF="#index">Index</A>
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<A HREF="/cgi-bin/man/man2html">Return to Main Contents</A><HR>
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<A NAME="lbAB"> </A>
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<H2>NAME</H2>
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splice - splice data to/from a pipe
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<A NAME="lbAC"> </A>
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<H2>SYNOPSIS</H2>
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<PRE>
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<B>#define _GNU_SOURCE</B> /* See <A HREF="/cgi-bin/man/man2html?7+feature_test_macros">feature_test_macros</A>(7) */
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<B>#include <<A HREF="file:///usr/include/fcntl.h">fcntl.h</A>></B>
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<B>ssize_t splice(int </B><I>fd_in</I><B>, loff_t *</B><I>off_in</I><B>, int </B><I>fd_out</I><B>,</B>
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<B> loff_t *</B><I>off_out</I><B>, size_t </B><I>len</I><B>, unsigned int </B><I>flags</I><B>);</B>
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</PRE>
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<A NAME="lbAD"> </A>
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<H2>DESCRIPTION</H2>
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<B>splice</B>()
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moves data between two file descriptors
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without copying between kernel address space and user address space.
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It transfers up to
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<I>len</I>
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bytes of data from the file descriptor
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<I>fd_in</I>
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to the file descriptor
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<I>fd_out</I>,
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where one of the file descriptors must refer to a pipe.
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<P>
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The following semantics apply for
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<I>fd_in</I>
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and
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<I>off_in</I>:
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<DL COMPACT>
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<DT id="1">*<DD>
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If
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<I>fd_in</I>
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refers to a pipe, then
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<I>off_in</I>
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must be NULL.
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<DT id="2">*<DD>
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If
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<I>fd_in</I>
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does not refer to a pipe and
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<I>off_in</I>
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is NULL, then bytes are read from
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<I>fd_in</I>
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starting from the file offset,
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and the file offset is adjusted appropriately.
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<DT id="3">*<DD>
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If
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<I>fd_in</I>
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does not refer to a pipe and
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<I>off_in</I>
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is not NULL, then
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<I>off_in</I>
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must point to a buffer which specifies the starting
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offset from which bytes will be read from
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<I>fd_in</I>;
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in this case, the file offset of
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<I>fd_in</I>
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is not changed.
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</DL>
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<P>
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Analogous statements apply for
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<I>fd_out</I>
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and
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<I>off_out</I>.
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<P>
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The
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<I>flags</I>
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argument is a bit mask that is composed by ORing together
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zero or more of the following values:
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<DL COMPACT>
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<DT id="4"><B>SPLICE_F_MOVE</B>
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<DD>
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Attempt to move pages instead of copying.
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This is only a hint to the kernel:
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pages may still be copied if the kernel cannot move the
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pages from the pipe, or if
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the pipe buffers don't refer to full pages.
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The initial implementation of this flag was buggy:
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therefore starting in Linux 2.6.21 it is a no-op
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(but is still permitted in a
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<B>splice</B>()
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call);
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in the future, a correct implementation may be restored.
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<DT id="5"><B>SPLICE_F_NONBLOCK</B>
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<DD>
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Do not block on I/O.
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This makes the splice pipe operations nonblocking, but
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<B>splice</B>()
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may nevertheless block because the file descriptors that
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are spliced to/from may block (unless they have the
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<B>O_NONBLOCK</B>
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flag set).
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<DT id="6"><B>SPLICE_F_MORE</B>
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<DD>
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More data will be coming in a subsequent splice.
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This is a helpful hint when
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the
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<I>fd_out</I>
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refers to a socket (see also the description of
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<B>MSG_MORE</B>
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in
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<B><A HREF="/cgi-bin/man/man2html?2+send">send</A></B>(2),
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and the description of
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<B>TCP_CORK</B>
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in
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<B><A HREF="/cgi-bin/man/man2html?7+tcp">tcp</A></B>(7)).
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<DT id="7"><B>SPLICE_F_GIFT</B>
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<DD>
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Unused for
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<B>splice</B>();
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see
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<B><A HREF="/cgi-bin/man/man2html?2+vmsplice">vmsplice</A></B>(2).
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</DL>
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<A NAME="lbAE"> </A>
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<H2>RETURN VALUE</H2>
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Upon successful completion,
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<B>splice</B>()
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returns the number of bytes
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spliced to or from the pipe.
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<P>
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A return value of 0 means end of input.
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If
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<I>fd_in</I>
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refers to a pipe, then this means that there was no data to transfer,
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and it would not make sense to block because there are no writers
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connected to the write end of the pipe.
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<P>
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On error,
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<B>splice</B>()
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returns -1 and
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<I>errno</I>
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is set to indicate the error.
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<A NAME="lbAF"> </A>
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<H2>ERRORS</H2>
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<DL COMPACT>
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<DT id="8"><B>EAGAIN</B>
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<DD>
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<B>SPLICE_F_NONBLOCK</B>
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was specified in
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<I>flags</I>
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or one of the file descriptors had been marked as nonblocking
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(<B>O_NONBLOCK</B>)<B>,</B>
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and the operation would block.
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<DT id="9"><B>EBADF</B>
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<DD>
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One or both file descriptors are not valid,
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or do not have proper read-write mode.
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<DT id="10"><B>EINVAL</B>
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<DD>
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The target filesystem doesn't support splicing.
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<DT id="11"><B>EINVAL</B>
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<DD>
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The target file is opened in append mode.
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<DT id="12"><B>EINVAL</B>
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<DD>
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Neither of the file descriptors refers to a pipe.
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<DT id="13"><B>EINVAL</B>
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<DD>
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An offset was given for nonseekable device (e.g., a pipe).
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<DT id="14"><B>EINVAL</B>
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<DD>
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<I>fd_in</I>
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and
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<I>fd_out</I>
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refer to the same pipe.
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<DT id="15"><B>ENOMEM</B>
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<DD>
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Out of memory.
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<DT id="16"><B>ESPIPE</B>
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<DD>
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Either
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<I>off_in</I>
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or
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<I>off_out</I>
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was not NULL, but the corresponding file descriptor refers to a pipe.
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</DL>
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<A NAME="lbAG"> </A>
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<H2>VERSIONS</H2>
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The
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<B>splice</B>()
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system call first appeared in Linux 2.6.17;
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library support was added to glibc in version 2.5.
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<A NAME="lbAH"> </A>
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<H2>CONFORMING TO</H2>
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This system call is Linux-specific.
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<A NAME="lbAI"> </A>
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<H2>NOTES</H2>
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The three system calls
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<B>splice</B>(),
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<B><A HREF="/cgi-bin/man/man2html?2+vmsplice">vmsplice</A></B>(2),
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and
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<B><A HREF="/cgi-bin/man/man2html?2+tee">tee</A></B>(2),
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provide user-space programs with full control over an arbitrary
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kernel buffer, implemented within the kernel using the same type
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of buffer that is used for a pipe.
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In overview, these system calls perform the following tasks:
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<DL COMPACT>
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<DT id="17"><B>splice</B>()
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<DD>
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moves data from the buffer to an arbitrary file descriptor, or vice versa,
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or from one buffer to another.
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<DT id="18"><B><A HREF="/cgi-bin/man/man2html?2+tee">tee</A></B>(2)
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<DD>
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"copies" the data from one buffer to another.
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<DT id="19"><B><A HREF="/cgi-bin/man/man2html?2+vmsplice">vmsplice</A></B>(2)
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<DD>
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"copies" data from user space into the buffer.
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</DL>
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<P>
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Though we talk of copying, actual copies are generally avoided.
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The kernel does this by implementing a pipe buffer as a set
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of reference-counted pointers to pages of kernel memory.
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The kernel creates "copies" of pages in a buffer by creating new
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pointers (for the output buffer) referring to the pages,
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and increasing the reference counts for the pages:
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only pointers are copied, not the pages of the buffer.
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<P>
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In Linux 2.6.30 and earlier,
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exactly one of
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<I>fd_in</I>
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and
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<I>fd_out</I>
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was required to be a pipe.
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Since Linux 2.6.31,
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both arguments may refer to pipes.
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<A NAME="lbAJ"> </A>
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<H2>EXAMPLE</H2>
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See
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<B><A HREF="/cgi-bin/man/man2html?2+tee">tee</A></B>(2).
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<A NAME="lbAK"> </A>
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<H2>SEE ALSO</H2>
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<B><A HREF="/cgi-bin/man/man2html?2+copy_file_range">copy_file_range</A></B>(2),
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<B><A HREF="/cgi-bin/man/man2html?2+sendfile">sendfile</A></B>(2),
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<B><A HREF="/cgi-bin/man/man2html?2+tee">tee</A></B>(2),
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<B><A HREF="/cgi-bin/man/man2html?2+vmsplice">vmsplice</A></B>(2),
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<B><A HREF="/cgi-bin/man/man2html?7+pipe">pipe</A></B>(7)
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<A NAME="lbAL"> </A>
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<H2>COLOPHON</H2>
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This page is part of release 5.05 of the Linux
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<I>man-pages</I>
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project.
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A description of the project,
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information about reporting bugs,
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and the latest version of this page,
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can be found at
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<A HREF="https://www.kernel.org/doc/man-pages/.">https://www.kernel.org/doc/man-pages/.</A>
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<P>
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<HR>
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<A NAME="index"> </A><H2>Index</H2>
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<DL>
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<DT id="20"><A HREF="#lbAB">NAME</A><DD>
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<DT id="21"><A HREF="#lbAC">SYNOPSIS</A><DD>
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<DT id="22"><A HREF="#lbAD">DESCRIPTION</A><DD>
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<DT id="23"><A HREF="#lbAE">RETURN VALUE</A><DD>
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<DT id="24"><A HREF="#lbAF">ERRORS</A><DD>
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<DT id="25"><A HREF="#lbAG">VERSIONS</A><DD>
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<DT id="26"><A HREF="#lbAH">CONFORMING TO</A><DD>
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<DT id="27"><A HREF="#lbAI">NOTES</A><DD>
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<DT id="28"><A HREF="#lbAJ">EXAMPLE</A><DD>
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<DT id="29"><A HREF="#lbAK">SEE ALSO</A><DD>
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<DT id="30"><A HREF="#lbAL">COLOPHON</A><DD>
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</DL>
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<HR>
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This document was created by
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<A HREF="/cgi-bin/man/man2html">man2html</A>,
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using the manual pages.<BR>
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Time: 00:05:34 GMT, March 31, 2021
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