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<!-- Created on March, 27 2008 by texi2html 1.64 -->
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Written by: Lionel Cons <Lionel.Cons@cern.ch> (original author)
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Olaf Bachmann <obachman@mathematik.uni-kl.de>
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<TITLE>Debugging with GDB: Emacs</TITLE>
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<A NAME="SEC241"></A>
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<TABLE CELLPADDING=1 CELLSPACING=1 BORDER=0>
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<TR><TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="gdb_23.html#SEC240"> < </A>]</TD>
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<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="gdb_toc.html#SEC_Contents">Contents</A>]</TD>
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<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="gdb_38.html#SEC764">Index</A>]</TD>
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<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="gdb_abt.html#SEC_About"> ? </A>]</TD>
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<H1> 23. Using GDB under GNU Emacs </H1>
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<!--docid::SEC241::-->
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<P>
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<A NAME="IDX1256"></A>
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<A NAME="IDX1257"></A>
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A special interface allows you to use GNU Emacs to view (and
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edit) the source files for the program you are debugging with
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GDB.
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</P><P>
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To use this interface, use the command <KBD>M-x gdb</KBD> in Emacs. Give the
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executable file you want to debug as an argument. This command starts
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GDB as a subprocess of Emacs, with input and output through a newly
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created Emacs buffer.
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</P><P>
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Running GDB under Emacs can be just like running GDB normally except for two
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things:
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</P><P>
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<UL>
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<LI>
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All "terminal" input and output goes through an Emacs buffer, called
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the GUD buffer.
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<P>
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This applies both to GDB commands and their output, and to the input
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and output done by the program you are debugging.
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</P><P>
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This is useful because it means that you can copy the text of previous
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commands and input them again; you can even use parts of the output
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in this way.
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</P><P>
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All the facilities of Emacs' Shell mode are available for interacting
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with your program. In particular, you can send signals the usual
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way--for example, <KBD>C-c C-c</KBD> for an interrupt, <KBD>C-c C-z</KBD> for a
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stop.
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</P><P>
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<LI>
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GDB displays source code through Emacs.
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<P>
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Each time GDB displays a stack frame, Emacs automatically finds the
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source file for that frame and puts an arrow (<SAMP>`=>'</SAMP>) at the
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left margin of the current line. Emacs uses a separate buffer for
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source display, and splits the screen to show both your GDB session
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and the source.
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</P><P>
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Explicit GDB <CODE>list</CODE> or search commands still produce output as
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usual, but you probably have no reason to use them from Emacs.
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</UL>
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<P>
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We call this <EM>text command mode</EM>. Emacs 22.1, and later, also uses
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a graphical mode, enabled by default, which provides further buffers
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that can control the execution and describe the state of your program.
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See section `GDB Graphical Interface' in <CITE>The GNU Emacs Manual</CITE>.
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</P><P>
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If you specify an absolute file name when prompted for the <KBD>M-x
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gdb</KBD> argument, then Emacs sets your current working directory to where
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your program resides. If you only specify the file name, then Emacs
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sets your current working directory to to the directory associated
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with the previous buffer. In this case, GDB may find your
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program by searching your environment's <CODE>PATH</CODE> variable, but on
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some operating systems it might not find the source. So, although the
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GDB input and output session proceeds normally, the auxiliary
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buffer does not display the current source and line of execution.
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</P><P>
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The initial working directory of GDB is printed on the top
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line of the GUD buffer and this serves as a default for the commands
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that specify files for GDB to operate on. See section <A HREF="gdb_16.html#SEC155">Commands to Specify Files</A>.
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</P><P>
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By default, <KBD>M-x gdb</KBD> calls the program called <TT>`gdb'</TT>. If you
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need to call GDB by a different name (for example, if you
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keep several configurations around, with different names) you can
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customize the Emacs variable <CODE>gud-gdb-command-name</CODE> to run the
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one you want.
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</P><P>
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In the GUD buffer, you can use these special Emacs commands in
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addition to the standard Shell mode commands:
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</P><P>
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<DL COMPACT>
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<DT><KBD>C-h m</KBD>
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<DD>Describe the features of Emacs' GUD Mode.
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<P>
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<DT><KBD>C-c C-s</KBD>
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<DD>Execute to another source line, like the GDB <CODE>step</CODE> command; also
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update the display window to show the current file and location.
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<P>
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<DT><KBD>C-c C-n</KBD>
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<DD>Execute to next source line in this function, skipping all function
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calls, like the GDB <CODE>next</CODE> command. Then update the display window
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to show the current file and location.
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<P>
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<DT><KBD>C-c C-i</KBD>
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<DD>Execute one instruction, like the GDB <CODE>stepi</CODE> command; update
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display window accordingly.
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<P>
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<DT><KBD>C-c C-f</KBD>
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<DD>Execute until exit from the selected stack frame, like the GDB
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<CODE>finish</CODE> command.
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<P>
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<DT><KBD>C-c C-r</KBD>
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<DD>Continue execution of your program, like the GDB <CODE>continue</CODE>
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command.
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<P>
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<DT><KBD>C-c <</KBD>
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<DD>Go up the number of frames indicated by the numeric argument
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(see section `Numeric Arguments' in <CITE>The GNU Emacs Manual</CITE>),
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like the GDB <CODE>up</CODE> command.
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<P>
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<DT><KBD>C-c ></KBD>
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<DD>Go down the number of frames indicated by the numeric argument, like the
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GDB <CODE>down</CODE> command.
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</DL>
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<P>
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In any source file, the Emacs command <KBD>C-x <KBD>SPC</KBD></KBD> (<CODE>gud-break</CODE>)
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tells GDB to set a breakpoint on the source line point is on.
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</P><P>
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In text command mode, if you type <KBD>M-x speedbar</KBD>, Emacs displays a
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separate frame which shows a backtrace when the GUD buffer is current.
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Move point to any frame in the stack and type <KBD>RET</KBD> to make it
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become the current frame and display the associated source in the
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source buffer. Alternatively, click <KBD>Mouse-2</KBD> to make the
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selected frame become the current one. In graphical mode, the
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speedbar displays watch expressions.
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</P><P>
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If you accidentally delete the source-display buffer, an easy way to get
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it back is to type the command <CODE>f</CODE> in the GDB buffer, to
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request a frame display; when you run under Emacs, this recreates
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the source buffer if necessary to show you the context of the current
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frame.
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</P><P>
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The source files displayed in Emacs are in ordinary Emacs buffers
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which are visiting the source files in the usual way. You can edit
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the files with these buffers if you wish; but keep in mind that GDB
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communicates with Emacs in terms of line numbers. If you add or
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delete lines from the text, the line numbers that GDB knows cease
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to correspond properly with the code.
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</P><P>
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A more detailed description of Emacs' interaction with GDB is
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given in the Emacs manual (see section `Debuggers' in <CITE>The GNU Emacs Manual</CITE>).
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</P><P>
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<A NAME="GDB/MI"></A>
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<HR SIZE="6">
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<TABLE CELLPADDING=1 CELLSPACING=1 BORDER=0>
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<TR><TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="gdb_4.html#SEC14"> << </A>]</TD>
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<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="gdb_25.html#SEC242"> >> </A>]</TD>
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<TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="gdb.html#SEC_Top">Top</A>]</TD>
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<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="gdb_toc.html#SEC_Contents">Contents</A>]</TD>
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<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="gdb_38.html#SEC764">Index</A>]</TD>
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<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="gdb_abt.html#SEC_About"> ? </A>]</TD>
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</TR></TABLE>
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<BR>
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<FONT SIZE="-1">
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<address>
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<p>Please send FSF & GNU inquiries & questions to <a
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href="mailto:gnu@gnu.org">gnu@gnu.org</a>. There are also <a
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href="http://www.gnu.org/home.html#ContactInfo">other ways to
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contact</a> the FSF.</p>
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<p>These pages are maintained by <a
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href="http://www.gnu.org/software/gdb/">the GDB developers</a>.</p>
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<p>Copyright Free Software Foundation, Inc., 59 Temple Place - Suite
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330, Boston, MA 02111, USA.</p>
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<p>Verbatim copying and distribution of this entire article is
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permitted in any medium, provided this notice is preserved.</p>
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</address>
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This document was generated
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by <I>GDB Administrator</I> on <I>March, 27 2008</I>
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using <A HREF="http://www.mathematik.uni-kl.de/~obachman/Texi2html
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