177 lines
4.4 KiB
HTML
177 lines
4.4 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 READAHEAD</TITLE>
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</HEAD><BODY>
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<H1>READAHEAD</H1>
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Section: Linux Programmer's Manual (2)<BR>Updated: 2019-03-06<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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readahead - initiate file readahead into page cache
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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 readahead(int </B><I>fd</I><B>, off64_t </B><I>offset</I><B>, size_t </B><I>count</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>readahead</B>()
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initiates readahead on a file so that subsequent reads from that file will
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be satisfied from the cache, and not block on disk I/O
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(assuming the readahead was initiated early enough and that other activity
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on the system did not in the meantime flush pages from the cache).
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<P>
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The
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<I>fd</I>
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argument is a file descriptor identifying the file which is
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to be read.
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The
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<I>offset</I>
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argument specifies the starting point from which data is to be read
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and
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<I>count</I>
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specifies the number of bytes to be read.
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I/O is performed in whole pages, so that
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<I>offset</I>
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is effectively rounded down to a page boundary
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and bytes are read up to the next page boundary greater than or
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equal to
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<I>(offset+count)</I>.
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<B>readahead</B>()
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does not read beyond the end of the file.
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The file offset of the open file description referred to by the file descriptor
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<I>fd</I>
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is left unchanged.
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<A NAME="lbAE"> </A>
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<H2>RETURN VALUE</H2>
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On success,
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<B>readahead</B>()
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returns 0; on failure, -1 is returned, with
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<I>errno</I>
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set to indicate the cause of 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="1"><B>EBADF</B>
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<DD>
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<I>fd</I>
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is not a valid file descriptor or is not open for reading.
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<DT id="2"><B>EINVAL</B>
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<DD>
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<I>fd</I>
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does not refer to a file type to which
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<B>readahead</B>()
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can be applied.
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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>readahead</B>()
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system call appeared in Linux 2.4.13;
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glibc support has been provided since version 2.3.
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<A NAME="lbAH"> </A>
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<H2>CONFORMING TO</H2>
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The
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<B>readahead</B>()
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system call is Linux-specific, and its use should be avoided
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in portable applications.
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<A NAME="lbAI"> </A>
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<H2>NOTES</H2>
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On some 32-bit architectures,
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the calling signature for this system call differs,
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for the reasons described in
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<B><A HREF="/cgi-bin/man/man2html?2+syscall">syscall</A></B>(2).
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<A NAME="lbAJ"> </A>
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<H2>BUGS</H2>
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<B>readahead</B>()
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attempts to schedule the reads in the background and return immediately.
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However, it may block while it reads the filesystem metadata needed
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to locate the requested blocks.
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This occurs frequently with ext[234] on large files
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using indirect blocks instead of extents,
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giving the appearance that the call blocks until the requested data has
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been read.
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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+lseek">lseek</A></B>(2),
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<B><A HREF="/cgi-bin/man/man2html?2+madvise">madvise</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+posix_fadvise">posix_fadvise</A></B>(2),
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<B><A HREF="/cgi-bin/man/man2html?2+read">read</A></B>(2)
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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="3"><A HREF="#lbAB">NAME</A><DD>
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<DT id="4"><A HREF="#lbAC">SYNOPSIS</A><DD>
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<DT id="5"><A HREF="#lbAD">DESCRIPTION</A><DD>
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<DT id="6"><A HREF="#lbAE">RETURN VALUE</A><DD>
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<DT id="7"><A HREF="#lbAF">ERRORS</A><DD>
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<DT id="8"><A HREF="#lbAG">VERSIONS</A><DD>
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<DT id="9"><A HREF="#lbAH">CONFORMING TO</A><DD>
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<DT id="10"><A HREF="#lbAI">NOTES</A><DD>
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<DT id="11"><A HREF="#lbAJ">BUGS</A><DD>
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<DT id="12"><A HREF="#lbAK">SEE ALSO</A><DD>
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<DT id="13"><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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