273 lines
6.5 KiB
HTML
273 lines
6.5 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 DLSYM</TITLE>
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</HEAD><BODY>
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<H1>DLSYM</H1>
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Section: Linux Programmer's Manual (3)<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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dlsym, dlvsym - obtain address of a symbol in a shared object or executable
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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/dlfcn.h">dlfcn.h</A>></B>
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<P>
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<B>void *dlsym(void *</B><I>handle</I><B>, const char *</B><I>symbol</I><B>);</B>
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<P>
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<B>#define _GNU_SOURCE</B>
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<BR>
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<B>#include <<A HREF="file:///usr/include/dlfcn.h">dlfcn.h</A>></B>
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<P>
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<B>void *dlvsym(void *</B><I>handle</I><B>, char *</B><I>symbol</I><B>, char *</B><I>version</I><B>);</B>
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<P>
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Link with <I>-ldl</I>.
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<A NAME="lbAD"> </A>
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<H2>DESCRIPTION</H2>
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The function
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<B>dlsym</B>()
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takes a "handle" of a dynamic loaded shared object returned by
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<B><A HREF="/cgi-bin/man/man2html?3+dlopen">dlopen</A></B>(3)
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along with a null-terminated symbol name,
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and returns the address where that symbol is
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loaded into memory.
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If the symbol is not found, in the specified
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object or any of the shared objects that were automatically loaded by
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<B><A HREF="/cgi-bin/man/man2html?3+dlopen">dlopen</A></B>(3)
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when that object was loaded,
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<B>dlsym</B>()
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returns NULL.
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(The search performed by
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<B>dlsym</B>()
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is breadth first through the dependency tree of these shared objects.)
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<P>
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In unusual cases (see NOTES) the value of the symbol could actually be NULL.
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Therefore, a NULL return from
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<B>dlsym</B>()
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need not indicate an error.
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The correct way to distinguish an error from a symbol whose value is NULL
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is to call
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<B><A HREF="/cgi-bin/man/man2html?3+dlerror">dlerror</A></B>(3)
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to clear any old error conditions, then call
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<B>dlsym</B>(),
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and then call
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<B><A HREF="/cgi-bin/man/man2html?3+dlerror">dlerror</A></B>(3)
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again, saving its return value into a variable, and check whether
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this saved value is not NULL.
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<P>
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There are two special pseudo-handles that may be specified in
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<I>handle</I>:
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<DL COMPACT>
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<DT id="1"><B>RTLD_DEFAULT</B>
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<DD>
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Find the first occurrence of the desired symbol
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using the default shared object search order.
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The search will include global symbols in the executable
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and its dependencies,
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as well as symbols in shared objects that were dynamically loaded with the
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<B>RTLD_GLOBAL</B>
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flag.
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<DT id="2"><B>RTLD_NEXT</B>
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<DD>
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Find the next occurrence of the desired symbol in the search order
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after the current object.
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This allows one to provide a wrapper
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around a function in another shared object, so that, for example,
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the definition of a function in a preloaded shared object
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(see
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<B>LD_PRELOAD</B>
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in
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<B><A HREF="/cgi-bin/man/man2html?8+ld.so">ld.so</A></B>(8))
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can find and invoke the "real" function provided in another shared object
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(or for that matter, the "next" definition of the function in cases
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where there are multiple layers of preloading).
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</DL>
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<P>
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The
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<B>_GNU_SOURCE</B>
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feature test macro must be defined in order to obtain the
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definitions of
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<B>RTLD_DEFAULT</B>
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and
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<B>RTLD_NEXT</B>
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from
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<I><<A HREF="file:///usr/include/dlfcn.h">dlfcn.h</A>></I>.
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<P>
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<P>
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The function
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<B>dlvsym</B>()
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does the same as
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<B>dlsym</B>()
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but takes a version string as an additional argument.
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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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these functions return the address associated with
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<I>symbol</I>.
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On failure, they return NULL;
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the cause of the error can be diagnosed using
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<B><A HREF="/cgi-bin/man/man2html?3+dlerror">dlerror</A></B>(3).
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<A NAME="lbAF"> </A>
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<H2>VERSIONS</H2>
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<B>dlsym</B>()
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is present in glibc 2.0 and later.
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<B>dlvsym</B>()
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first appeared in glibc 2.1.
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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>dlsym</B>(),
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<B>dlvsym</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 describes
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<B>dlsym</B>().
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The
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<B>dlvsym</B>()
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function is a GNU extension.
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<A NAME="lbAI"> </A>
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<H2>NOTES</H2>
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The value of a symbol returned by
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<B>dlsym</B>()
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will never be NULL if the shared object is the result of normal compilation,
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since a global symbol is never placed at the NULL address.
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There are nevertheless cases where a lookup using
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<B>dlsym</B>()
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may return NULL as the value of a symbol.
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For example, the symbol value may be the result of
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a GNU indirect function (IFUNC) resolver function that returns
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NULL as the resolved value.
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<A NAME="lbAJ"> </A>
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<H3>History</H3>
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The
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<B>dlsym</B>()
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function is part of the dlopen API, derived from SunOS.
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That system does not have
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<B>dlvsym</B>().
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<A NAME="lbAK"> </A>
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<H2>EXAMPLE</H2>
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See
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<B><A HREF="/cgi-bin/man/man2html?3+dlopen">dlopen</A></B>(3).
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<A NAME="lbAL"> </A>
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<H2>SEE ALSO</H2>
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<B><A HREF="/cgi-bin/man/man2html?3+dl_iterate_phdr">dl_iterate_phdr</A></B>(3),
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<B><A HREF="/cgi-bin/man/man2html?3+dladdr">dladdr</A></B>(3),
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<B><A HREF="/cgi-bin/man/man2html?3+dlerror">dlerror</A></B>(3),
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<B><A HREF="/cgi-bin/man/man2html?3+dlinfo">dlinfo</A></B>(3),
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<B><A HREF="/cgi-bin/man/man2html?3+dlopen">dlopen</A></B>(3),
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<B><A HREF="/cgi-bin/man/man2html?8+ld.so">ld.so</A></B>(8)
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<A NAME="lbAM"> </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">VERSIONS</A><DD>
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<DT id="8"><A HREF="#lbAG">ATTRIBUTES</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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<DL>
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<DT id="11"><A HREF="#lbAJ">History</A><DD>
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</DL>
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<DT id="12"><A HREF="#lbAK">EXAMPLE</A><DD>
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<DT id="13"><A HREF="#lbAL">SEE ALSO</A><DD>
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<DT id="14"><A HREF="#lbAM">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: 00:05:38 GMT, March 31, 2021
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