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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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Karl Berry <karl@freefriends.org>
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Olaf Bachmann <obachman@mathematik.uni-kl.de>
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and many others.
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<TITLE>Debugging with GDB: Sample Session</TITLE>
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<A NAME="SEC5"></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_1.html#SEC4"> < </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_abt.html#SEC_About"> ? </A>]</TD>
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</TR></TABLE>
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<H1> 1. A Sample GDB Session </H1>
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<!--docid::SEC5::-->
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<P>
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You can use this manual at your leisure to read all about GDB.
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However, a handful of commands are enough to get started using the
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debugger. This chapter illustrates those commands.
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</P><P>
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One of the preliminary versions of GNU <CODE>m4</CODE> (a generic macro
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processor) exhibits the following bug: sometimes, when we change its
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quote strings from the default, the commands used to capture one macro
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definition within another stop working. In the following short <CODE>m4</CODE>
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session, we define a macro <CODE>foo</CODE> which expands to <CODE>0000</CODE>; we
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then use the <CODE>m4</CODE> built-in <CODE>defn</CODE> to define <CODE>bar</CODE> as the
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same thing. However, when we change the open quote string to
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<CODE><QUOTE></CODE> and the close quote string to <CODE><UNQUOTE></CODE>, the same
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procedure fails to define a new synonym <CODE>baz</CODE>:
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</P><P>
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<TABLE><tr><td> </td><td class=smallexample><FONT SIZE=-1><pre>$ <B>cd gnu/m4</B>
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$ <B>./m4</B>
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<B>define(foo,0000)</B>
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<B>foo</B>
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0000
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<B>define(bar,defn(`foo'))</B>
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<B>bar</B>
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0000
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<B>changequote(<QUOTE>,<UNQUOTE>)</B>
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<B>define(baz,defn(<QUOTE>foo<UNQUOTE>))</B>
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<B>baz</B>
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<B>Ctrl-d</B>
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m4: End of input: 0: fatal error: EOF in string
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</FONT></pre></td></tr></table></P><P>
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Let us use GDB to try to see what is going on.
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</P><P>
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<TABLE><tr><td> </td><td class=smallexample><FONT SIZE=-1><pre>$ <B>gdb m4</B>
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GDB is free software and you are welcome to distribute copies
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of it under certain conditions; type "show copying" to see
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the conditions.
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There is absolutely no warranty for GDB; type "show warranty"
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for details.
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GDB 6.8, Copyright 1999 Free Software Foundation, Inc...
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(gdb)
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</FONT></pre></td></tr></table></P><P>
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GDB reads only enough symbol data to know where to find the
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rest when needed; as a result, the first prompt comes up very quickly.
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We now tell GDB to use a narrower display width than usual, so
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that examples fit in this manual.
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</P><P>
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<TABLE><tr><td> </td><td class=smallexample><FONT SIZE=-1><pre>(gdb) <B>set width 70</B>
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</FONT></pre></td></tr></table></P><P>
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We need to see how the <CODE>m4</CODE> built-in <CODE>changequote</CODE> works.
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Having looked at the source, we know the relevant subroutine is
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<CODE>m4_changequote</CODE>, so we set a breakpoint there with the GDB
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<CODE>break</CODE> command.
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</P><P>
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<TABLE><tr><td> </td><td class=smallexample><FONT SIZE=-1><pre>(gdb) <B>break m4_changequote</B>
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Breakpoint 1 at 0x62f4: file builtin.c, line 879.
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</FONT></pre></td></tr></table></P><P>
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Using the <CODE>run</CODE> command, we start <CODE>m4</CODE> running under GDB
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control; as long as control does not reach the <CODE>m4_changequote</CODE>
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subroutine, the program runs as usual:
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</P><P>
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<TABLE><tr><td> </td><td class=smallexample><FONT SIZE=-1><pre>(gdb) <B>run</B>
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Starting program: /work/Editorial/gdb/gnu/m4/m4
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<B>define(foo,0000)</B>
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<B>foo</B>
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0000
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</FONT></pre></td></tr></table></P><P>
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To trigger the breakpoint, we call <CODE>changequote</CODE>. GDB
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suspends execution of <CODE>m4</CODE>, displaying information about the
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context where it stops.
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</P><P>
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<TABLE><tr><td> </td><td class=smallexample><FONT SIZE=-1><pre><B>changequote(<QUOTE>,<UNQUOTE>)</B>
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Breakpoint 1, m4_changequote (argc=3, argv=0x33c70)
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at builtin.c:879
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879 if (bad_argc(TOKEN_DATA_TEXT(argv[0]),argc,1,3))
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</FONT></pre></td></tr></table></P><P>
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Now we use the command <CODE>n</CODE> (<CODE>next</CODE>) to advance execution to
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the next line of the current function.
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</P><P>
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<TABLE><tr><td> </td><td class=smallexample><FONT SIZE=-1><pre>(gdb) <B>n</B>
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882 set_quotes((argc >= 2) ? TOKEN_DATA_TEXT(argv[1])\
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: nil,
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</FONT></pre></td></tr></table></P><P>
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<CODE>set_quotes</CODE> looks like a promising subroutine. We can go into it
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by using the command <CODE>s</CODE> (<CODE>step</CODE>) instead of <CODE>next</CODE>.
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<CODE>step</CODE> goes to the next line to be executed in <EM>any</EM>
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subroutine, so it steps into <CODE>set_quotes</CODE>.
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</P><P>
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<TABLE><tr><td> </td><td class=smallexample><FONT SIZE=-1><pre>(gdb) <B>s</B>
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set_quotes (lq=0x34c78 "<QUOTE>", rq=0x34c88 "<UNQUOTE>")
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at input.c:530
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530 if (lquote != def_lquote)
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</FONT></pre></td></tr></table></P><P>
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The display that shows the subroutine where <CODE>m4</CODE> is now
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suspended (and its arguments) is called a stack frame display. It
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shows a summary of the stack. We can use the <CODE>backtrace</CODE>
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command (which can also be spelled <CODE>bt</CODE>), to see where we are
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in the stack as a whole: the <CODE>backtrace</CODE> command displays a
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stack frame for each active subroutine.
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</P><P>
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<TABLE><tr><td> </td><td class=smallexample><FONT SIZE=-1><pre>(gdb) <B>bt</B>
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#0 set_quotes (lq=0x34c78 "<QUOTE>", rq=0x34c88 "<UNQUOTE>")
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at input.c:530
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#1 0x6344 in m4_changequote (argc=3, argv=0x33c70)
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at builtin.c:882
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#2 0x8174 in expand_macro (sym=0x33320) at macro.c:242
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#3 0x7a88 in expand_token (obs=0x0, t=209696, td=0xf7fffa30)
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at macro.c:71
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#4 0x79dc in expand_input () at macro.c:40
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#5 0x2930 in main (argc=0, argv=0xf7fffb20) at m4.c:195
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</FONT></pre></td></tr></table></P><P>
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We step through a few more lines to see what happens. The first two
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times, we can use <SAMP>`s'</SAMP>; the next two times we use <CODE>n</CODE> to avoid
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falling into the <CODE>xstrdup</CODE> subroutine.
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</P><P>
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<TABLE><tr><td> </td><td class=smallexample><FONT SIZE=-1><pre>(gdb) <B>s</B>
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0x3b5c 532 if (rquote != def_rquote)
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(gdb) <B>s</B>
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0x3b80 535 lquote = (lq == nil || *lq == '\0') ? \
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def_lquote : xstrdup(lq);
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(gdb) <B>n</B>
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536 rquote = (rq == nil || *rq == '\0') ? def_rquote\
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: xstrdup(rq);
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(gdb) <B>n</B>
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538 len_lquote = strlen(rquote);
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</FONT></pre></td></tr></table></P><P>
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The last line displayed looks a little odd; we can examine the variables
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<CODE>lquote</CODE> and <CODE>rquote</CODE> to see if they are in fact the new left
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and right quotes we specified. We use the command <CODE>p</CODE>
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(<CODE>print</CODE>) to see their values.
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</P><P>
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<TABLE><tr><td> </td><td class=smallexample><FONT SIZE=-1><pre>(gdb) <B>p lquote</B>
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$1 = 0x35d40 "<QUOTE>"
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(gdb) <B>p rquote</B>
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$2 = 0x35d50 "<UNQUOTE>"
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</FONT></pre></td></tr></table></P><P>
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<CODE>lquote</CODE> and <CODE>rquote</CODE> are indeed the new left and right quotes.
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To look at some context, we can display ten lines of source
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surrounding the current line with the <CODE>l</CODE> (<CODE>list</CODE>) command.
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</P><P>
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<TABLE><tr><td> </td><td class=smallexample><FONT SIZE=-1><pre>(gdb) <B>l</B>
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533 xfree(rquote);
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534
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535 lquote = (lq == nil || *lq == '\0') ? def_lquote\
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: xstrdup (lq);
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536 rquote = (rq == nil || *rq == '\0') ? def_rquote\
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: xstrdup (rq);
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537
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538 len_lquote = strlen(rquote);
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539 len_rquote = strlen(lquote);
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540 }
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541
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542 void
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</FONT></pre></td></tr></table></P><P>
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Let us step past the two lines that set <CODE>len_lquote</CODE> and
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<CODE>len_rquote</CODE>, and then examine the values of those variables.
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</P><P>
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<TABLE><tr><td> </td><td class=smallexample><FONT SIZE=-1><pre>(gdb) <B>n</B>
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539 len_rquote = strlen(lquote);
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(gdb) <B>n</B>
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540 }
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(gdb) <B>p len_lquote</B>
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$3 = 9
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(gdb) <B>p len_rquote</B>
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$4 = 7
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</FONT></pre></td></tr></table></P><P>
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That certainly looks wrong, assuming <CODE>len_lquote</CODE> and
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<CODE>len_rquote</CODE> are meant to be the lengths of <CODE>lquote</CODE> and
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<CODE>rquote</CODE> respectively. We can set them to better values using
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the <CODE>p</CODE> command, since it can print the value of
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any expression--and that expression can include subroutine calls and
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assignments.
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</P><P>
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<TABLE><tr><td> </td><td class=smallexample><FONT SIZE=-1><pre>(gdb) <B>p len_lquote=strlen(lquote)</B>
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$5 = 7
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(gdb) <B>p len_rquote=strlen(rquote)</B>
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$6 = 9
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</FONT></pre></td></tr></table></P><P>
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Is that enough to fix the problem of using the new quotes with the
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<CODE>m4</CODE> built-in <CODE>defn</CODE>? We can allow <CODE>m4</CODE> to continue
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executing with the <CODE>c</CODE> (<CODE>continue</CODE>) command, and then try the
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example that caused trouble initially:
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</P><P>
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<TABLE><tr><td> </td><td class=smallexample><FONT SIZE=-1><pre>(gdb) <B>c</B>
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Continuing.
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<B>define(baz,defn(<QUOTE>foo<UNQUOTE>))</B>
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baz
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0000
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</FONT></pre></td></tr></table></P><P>
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Success! The new quotes now work just as well as the default ones. The
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problem seems to have been just the two typos defining the wrong
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lengths. We allow <CODE>m4</CODE> exit by giving it an EOF as input:
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</P><P>
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<TABLE><tr><td> </td><td class=smallexample><FONT SIZE=-1><pre><B>Ctrl-d</B>
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Program exited normally.
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</FONT></pre></td></tr></table></P><P>
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The message <SAMP>`Program exited normally.'</SAMP> is from GDB; it
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indicates <CODE>m4</CODE> has finished executing. We can end our GDB
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session with the GDB <CODE>quit</CODE> command.
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</P><P>
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<TABLE><tr><td> </td><td class=smallexample><FONT SIZE=-1><pre>(gdb) <B>quit</B>
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</FONT></pre></td></tr></table></P><P>
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<A NAME="Invocation"></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">[ << ]</TD>
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<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="gdb_3.html#SEC6"> >> </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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<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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