375 lines
7.3 KiB
HTML
375 lines
7.3 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 SENDFILE</TITLE>
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</HEAD><BODY>
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<H1>SENDFILE</H1>
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Section: Linux Programmer's Manual (2)<BR>Updated: 2017-09-15<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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sendfile - transfer data between file descriptors
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<A NAME="lbAC"> </A>
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<H2>SYNOPSIS</H2>
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<B>#include <<A HREF="file:///usr/include/sys/sendfile.h">sys/sendfile.h</A>></B>
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<P>
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<B>ssize_t sendfile(int</B><I> out_fd</I><B>, int</B><I> in_fd</I><B>, off_t *</B><I></I><B>offset</B><I>, size_t</I><B> count</B><I>);</I>
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<A NAME="lbAD"> </A>
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<H2>DESCRIPTION</H2>
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<B>sendfile</B>()
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copies data between one file descriptor and another.
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Because this copying is done within the kernel,
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<B>sendfile</B>()
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is more efficient than the combination of
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<B><A HREF="/cgi-bin/man/man2html?2+read">read</A></B>(2)
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and
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<B><A HREF="/cgi-bin/man/man2html?2+write">write</A></B>(2),
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which would require transferring data to and from user space.
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<P>
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<I>in_fd</I>
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should be a file descriptor opened for reading and
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<I>out_fd</I>
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should be a descriptor opened for writing.
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<P>
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If
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<I>offset</I>
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is not NULL, then it points
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to a variable holding the file offset from which
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<B>sendfile</B>()
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will start reading data from
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<I>in_fd</I>.
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When
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<B>sendfile</B>()
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returns, this variable
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will be set to the offset of the byte following the last byte that was read.
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If
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<I>offset</I>
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is not NULL, then
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<B>sendfile</B>()
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does not modify the file offset of
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<I>in_fd</I>;
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otherwise the file offset is adjusted to reflect
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the number of bytes read from
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<I>in_fd</I>.
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<P>
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If
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<I>offset</I>
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is NULL, then data will be read from
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<I>in_fd</I>
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starting at the file offset,
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and the file offset will be updated by the call.
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<P>
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<I>count</I>
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is the number of bytes to copy between the file descriptors.
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<P>
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The
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<I>in_fd</I>
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argument must correspond to a file which supports
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<B><A HREF="/cgi-bin/man/man2html?2+mmap">mmap</A></B>(2)-like
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operations
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(i.e., it cannot be a socket).
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<P>
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In Linux kernels before 2.6.33,
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<I>out_fd</I>
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must refer to a socket.
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Since Linux 2.6.33 it can be any file.
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If it is a regular file, then
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<B>sendfile</B>()
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changes the file offset appropriately.
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<A NAME="lbAE"> </A>
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<H2>RETURN VALUE</H2>
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If the transfer was successful, the number of bytes written to
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<I>out_fd</I>
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is returned.
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Note that a successful call to
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<B>sendfile</B>()
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may write fewer bytes than requested;
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the caller should be prepared to retry the call if there were unsent bytes.
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See also NOTES.
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<P>
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On error, -1 is returned, and
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<I>errno</I>
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is set appropriately.
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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="1"><B>EAGAIN</B>
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<DD>
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Nonblocking I/O has been selected using
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<B>O_NONBLOCK</B>
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and the write would block.
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<DT id="2"><B>EBADF</B>
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<DD>
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The input file was not opened for reading or the output file
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was not opened for writing.
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<DT id="3"><B>EFAULT</B>
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<DD>
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Bad address.
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<DT id="4"><B>EINVAL</B>
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<DD>
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Descriptor is not valid or locked, or an
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<B><A HREF="/cgi-bin/man/man2html?2+mmap">mmap</A></B>(2)-like
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operation is not available for
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<I>in_fd</I>,
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or
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<I>count</I>
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is negative.
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<DT id="5"><B>EINVAL</B>
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<DD>
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<I>out_fd</I>
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has the
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<B>O_APPEND</B>
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flag set.
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This is not currently supported by
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<B>sendfile</B>().
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<DT id="6"><B>EIO</B>
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<DD>
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Unspecified error while reading from
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<I>in_fd</I>.
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<DT id="7"><B>ENOMEM</B>
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<DD>
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Insufficient memory to read from
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<I>in_fd</I>.
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<DT id="8"><B>EOVERFLOW</B>
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<DD>
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<I>count</I>
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is too large, the operation would result in exceeding the maximum size of either
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the input file or the output file.
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<DT id="9"><B>ESPIPE</B>
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<DD>
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<I>offset</I>
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is not NULL but the input file is not
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<B><A HREF="/cgi-bin/man/man2html?2+seek">seek</A></B>(2)-able.
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</DL>
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<A NAME="lbAG"> </A>
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<H2>VERSIONS</H2>
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<B>sendfile</B>()
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first appeared in Linux 2.2.
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The include file
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<I><<A HREF="file:///usr/include/sys/sendfile.h">sys/sendfile.h</A>></I>
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is present since glibc 2.1.
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<A NAME="lbAH"> </A>
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<H2>CONFORMING TO</H2>
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Not specified in POSIX.1-2001, nor in other standards.
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<P>
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Other UNIX systems implement
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<B>sendfile</B>()
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with different semantics and prototypes.
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It should not be used in portable programs.
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<A NAME="lbAI"> </A>
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<H2>NOTES</H2>
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<B>sendfile</B>()
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will transfer at most 0x7ffff000 (2,147,479,552) bytes,
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returning the number of bytes actually transferred.
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(This is true on both 32-bit and 64-bit systems.)
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<P>
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If you plan to use
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<B>sendfile</B>()
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for sending files to a TCP socket, but need
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to send some header data in front of the file contents, you will find
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it useful to employ the
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<B>TCP_CORK</B>
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option, described in
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<B><A HREF="/cgi-bin/man/man2html?7+tcp">tcp</A></B>(7),
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to minimize the number of packets and to tune performance.
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<P>
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In Linux 2.4 and earlier,
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<I>out_fd</I>
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could also refer to a regular file;
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this possibility went away in the Linux 2.6.x kernel series,
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but was restored in Linux 2.6.33.
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<P>
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The original Linux
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<B>sendfile</B>()
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system call was not designed to handle large file offsets.
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Consequently, Linux 2.4 added
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<B>sendfile64</B>(),
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with a wider type for the
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<I>offset</I>
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argument.
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The glibc
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<B>sendfile</B>()
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wrapper function transparently deals with the kernel differences.
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<P>
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Applications may wish to fall back to
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<B><A HREF="/cgi-bin/man/man2html?2+read">read</A></B>(2)/<B><A HREF="/cgi-bin/man/man2html?2+write">write</A></B>(2)
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in the case where
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<B>sendfile</B>()
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fails with
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<B>EINVAL</B>
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or
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<B>ENOSYS</B>.
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<P>
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If
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<I>out_fd</I>
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refers to a socket or pipe with zero-copy support, callers must ensure the
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transferred portions of the file referred to by
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<I>in_fd</I>
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remain unmodified until the reader on the other end of
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<I>out_fd</I>
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has consumed the transferred data.
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<P>
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The Linux-specific
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<B><A HREF="/cgi-bin/man/man2html?2+splice">splice</A></B>(2)
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call supports transferring data between arbitrary file descriptors
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provided one (or both) of them is a pipe.
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<A NAME="lbAJ"> </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+mmap">mmap</A></B>(2),
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<B><A HREF="/cgi-bin/man/man2html?2+open">open</A></B>(2),
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<B><A HREF="/cgi-bin/man/man2html?2+socket">socket</A></B>(2),
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<B><A HREF="/cgi-bin/man/man2html?2+splice">splice</A></B>(2)
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<A NAME="lbAK"> </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="10"><A HREF="#lbAB">NAME</A><DD>
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<DT id="11"><A HREF="#lbAC">SYNOPSIS</A><DD>
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<DT id="12"><A HREF="#lbAD">DESCRIPTION</A><DD>
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<DT id="13"><A HREF="#lbAE">RETURN VALUE</A><DD>
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<DT id="14"><A HREF="#lbAF">ERRORS</A><DD>
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<DT id="15"><A HREF="#lbAG">VERSIONS</A><DD>
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<DT id="16"><A HREF="#lbAH">CONFORMING TO</A><DD>
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<DT id="17"><A HREF="#lbAI">NOTES</A><DD>
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<DT id="18"><A HREF="#lbAJ">SEE ALSO</A><DD>
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<DT id="19"><A HREF="#lbAK">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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