216 lines
5.1 KiB
HTML
216 lines
5.1 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 FLOCKFILE</TITLE>
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</HEAD><BODY>
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<H1>FLOCKFILE</H1>
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Section: Linux Programmer's Manual (3)<BR>Updated: 2017-07-13<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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flockfile, ftrylockfile, funlockfile - lock FILE for stdio
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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>#include <<A HREF="file:///usr/include/stdio.h">stdio.h</A>></B>
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<B>void flockfile(FILE *</B><I>filehandle</I><B>);</B>
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<B>int ftrylockfile(FILE *</B><I>filehandle</I><B>);</B>
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<B>void funlockfile(FILE *</B><I>filehandle</I><B>);</B>
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</PRE>
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<P>
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Feature Test Macro Requirements for glibc (see
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<B><A HREF="/cgi-bin/man/man2html?7+feature_test_macros">feature_test_macros</A></B>(7)):
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<P>
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All functions shown above:
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<DL COMPACT><DT id="1"><DD>
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/* Since glibc 2.24: */ _POSIX_C_SOURCE >= 199309L
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<BR> || /* Glibc versions <= 2.23: */ _POSIX_C_SOURCE
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<BR> || /* Glibc versions <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE
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</DL>
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<A NAME="lbAD"> </A>
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<H2>DESCRIPTION</H2>
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The stdio functions are thread-safe.
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This is achieved by assigning
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to each
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<I>FILE</I>
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object a lockcount and (if the lockcount is nonzero)
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an owning thread.
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For each library call, these functions wait until the
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<I>FILE</I>
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object
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is no longer locked by a different thread, then lock it, do the
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requested I/O, and unlock the object again.
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<P>
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(Note: this locking has nothing to do with the file locking done
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by functions like
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<B><A HREF="/cgi-bin/man/man2html?2+flock">flock</A></B>(2)
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and
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<B><A HREF="/cgi-bin/man/man2html?3+lockf">lockf</A></B>(3).)
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<P>
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All this is invisible to the C-programmer, but there may be two
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reasons to wish for more detailed control.
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On the one hand, maybe
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a series of I/O actions by one thread belongs together, and should
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not be interrupted by the I/O of some other thread.
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On the other hand, maybe the locking overhead should be avoided
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for greater efficiency.
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<P>
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To this end, a thread can explicitly lock the
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<I>FILE</I>
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object,
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then do its series of I/O actions, then unlock.
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This prevents
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other threads from coming in between.
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If the reason for doing
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this was to achieve greater efficiency, one does the I/O with
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the nonlocking versions of the stdio functions: with
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<B><A HREF="/cgi-bin/man/man2html?3+getc_unlocked">getc_unlocked</A></B>(3)
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and
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<B><A HREF="/cgi-bin/man/man2html?3+putc_unlocked">putc_unlocked</A></B>(3)
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instead of
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<B><A HREF="/cgi-bin/man/man2html?3+getc">getc</A></B>(3)
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and
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<B><A HREF="/cgi-bin/man/man2html?3+putc">putc</A></B>(3).
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<P>
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The
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<B>flockfile</B>()
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function waits for
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<I>*filehandle</I>
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to be
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no longer locked by a different thread, then makes the
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current thread owner of
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<I>*filehandle</I>,
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and increments
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the lockcount.
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<P>
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The
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<B>funlockfile</B>()
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function decrements the lock count.
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<P>
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The
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<B>ftrylockfile</B>()
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function is a nonblocking version
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of
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<B>flockfile</B>().
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It does nothing in case some other thread
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owns
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<I>*filehandle</I>,
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and it obtains ownership and increments
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the lockcount otherwise.
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<A NAME="lbAE"> </A>
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<H2>RETURN VALUE</H2>
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The
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<B>ftrylockfile</B>()
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function returns zero for success
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(the lock was obtained), and nonzero for failure.
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<A NAME="lbAF"> </A>
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<H2>ERRORS</H2>
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None.
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<A NAME="lbAG"> </A>
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<H2>ATTRIBUTES</H2>
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For an explanation of the terms used in this section, see
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<B><A HREF="/cgi-bin/man/man2html?7+attributes">attributes</A></B>(7).
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<TABLE BORDER>
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<TR VALIGN=top><TD><B>Interface</B></TD><TD><B>Attribute</B></TD><TD><B>Value</B><BR></TD></TR>
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<TR VALIGN=top><TD>
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<B>flockfile</B>(),
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<B>ftrylockfile</B>(),
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<B>funlockfile</B>()
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</TD><TD>Thread safety</TD><TD>MT-Safe<BR></TD></TR>
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</TABLE>
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<A NAME="lbAH"> </A>
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<H2>CONFORMING TO</H2>
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POSIX.1-2001, POSIX.1-2008.
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<A NAME="lbAI"> </A>
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<H2>AVAILABILITY</H2>
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These functions are available when
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<B>_POSIX_THREAD_SAFE_FUNCTIONS</B>
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is defined.
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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?3+unlocked_stdio">unlocked_stdio</A></B>(3)
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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="2"><A HREF="#lbAB">NAME</A><DD>
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<DT id="3"><A HREF="#lbAC">SYNOPSIS</A><DD>
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<DT id="4"><A HREF="#lbAD">DESCRIPTION</A><DD>
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<DT id="5"><A HREF="#lbAE">RETURN VALUE</A><DD>
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<DT id="6"><A HREF="#lbAF">ERRORS</A><DD>
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<DT id="7"><A HREF="#lbAG">ATTRIBUTES</A><DD>
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<DT id="8"><A HREF="#lbAH">CONFORMING TO</A><DD>
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<DT id="9"><A HREF="#lbAI">AVAILABILITY</A><DD>
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<DT id="10"><A HREF="#lbAJ">SEE ALSO</A><DD>
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<DT id="11"><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: 00:05:43 GMT, March 31, 2021
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</BODY>
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