182 lines
4.2 KiB
HTML
182 lines
4.2 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 FTOK</TITLE>
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</HEAD><BODY>
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<H1>FTOK</H1>
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Section: Linux Programmer's Manual (3)<BR>Updated: 2019-08-02<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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ftok - convert a pathname and a project identifier to a System V IPC key
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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/sys/types.h">sys/types.h</A>></B>
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<B>#include <<A HREF="file:///usr/include/sys/ipc.h">sys/ipc.h</A>></B>
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</PRE>
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<P>
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<B>key_t ftok(const char *</B><I>pathname</I><B>, int </B><I>proj_id</I><B>);</B>
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<A NAME="lbAD"> </A>
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<H2>DESCRIPTION</H2>
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The
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<B>ftok</B>()
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function uses the identity of the file named by the given
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<I>pathname</I>
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(which must refer to an existing, accessible file)
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and the least significant 8 bits of
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<I>proj_id</I>
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(which must be nonzero) to generate a
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<I>key_t</I>
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type System V IPC key, suitable for use with
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<B><A HREF="/cgi-bin/man/man2html?2+msgget">msgget</A></B>(2),
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<B><A HREF="/cgi-bin/man/man2html?2+semget">semget</A></B>(2),
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or
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<B><A HREF="/cgi-bin/man/man2html?2+shmget">shmget</A></B>(2).
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<P>
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The resulting value is the same for all pathnames that
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name the same file, when the same value of
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<I>proj_id</I>
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is used.
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The value returned should be different when the
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(simultaneously existing) files or the project IDs differ.
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<A NAME="lbAE"> </A>
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<H2>RETURN VALUE</H2>
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On success, the generated
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<I>key_t</I>
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value is returned.
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On failure -1 is returned, with
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<I>errno</I>
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indicating the error as for the
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<B><A HREF="/cgi-bin/man/man2html?2+stat">stat</A></B>(2)
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system call.
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<A NAME="lbAF"> </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>ftok</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="lbAG"> </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="lbAH"> </A>
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<H2>NOTES</H2>
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On some ancient systems, the prototype was:
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<P>
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<B>key_t ftok(char *</B><I>pathname</I><B>, char </B><I>proj_id</I><B>);</B>
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<P>
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Today,
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<I>proj_id</I>
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is an
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<I>int</I>,
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but still only 8 bits are used.
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Typical usage has an ASCII character
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<I>proj_id</I>,
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that is why the behavior is said to be undefined when
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<I>proj_id</I>
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is zero.
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<P>
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Of course, no guarantee can be given that the resulting
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<I>key_t</I>
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is unique.
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Typically, a best-effort attempt combines the given
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<I>proj_id</I>
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byte, the lower 16 bits of the inode number, and the
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lower 8 bits of the device number into a 32-bit result.
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Collisions may easily happen, for example between files on
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<I>/dev/hda1</I>
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and files on
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<I>/dev/sda1</I>.
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<A NAME="lbAI"> </A>
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<H2>SEE ALSO</H2>
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<B><A HREF="/cgi-bin/man/man2html?2+msgget">msgget</A></B>(2),
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<B><A HREF="/cgi-bin/man/man2html?2+semget">semget</A></B>(2),
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<B><A HREF="/cgi-bin/man/man2html?2+shmget">shmget</A></B>(2),
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<B><A HREF="/cgi-bin/man/man2html?2+stat">stat</A></B>(2),
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<B><A HREF="/cgi-bin/man/man2html?7+sysvipc">sysvipc</A></B>(7)
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<A NAME="lbAJ"> </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="1"><A HREF="#lbAB">NAME</A><DD>
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<DT id="2"><A HREF="#lbAC">SYNOPSIS</A><DD>
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<DT id="3"><A HREF="#lbAD">DESCRIPTION</A><DD>
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<DT id="4"><A HREF="#lbAE">RETURN VALUE</A><DD>
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<DT id="5"><A HREF="#lbAF">ATTRIBUTES</A><DD>
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<DT id="6"><A HREF="#lbAG">CONFORMING TO</A><DD>
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<DT id="7"><A HREF="#lbAH">NOTES</A><DD>
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<DT id="8"><A HREF="#lbAI">SEE ALSO</A><DD>
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<DT id="9"><A HREF="#lbAJ">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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</HTML>
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