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263 lines
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<head>
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<title>Sample Session - Debugging with GDB</title>
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<meta name="description" content="Debugging with GDB">
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<link title="Top" rel="start" href="index.html#Top">
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<link rel="prev" href="Summary.html#Summary" title="Summary">
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<link rel="next" href="Invocation.html#Invocation" title="Invocation">
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Permission is granted to copy, distribute and/or modify this document
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under the terms of the GNU Free Documentation License, Version 1.3 or
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any later version published by the Free Software Foundation; with the
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Invariant Sections being ``Free Software'' and ``Free Software Needs
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Free Documentation'', with the Front-Cover Texts being ``A GNU Manual,''
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and with the Back-Cover Texts as in (a) below.
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(a) The FSF's Back-Cover Text is: ``You are free to copy and modify
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<body>
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<div class="node">
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<a name="Sample-Session"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="Invocation.html#Invocation">Invocation</a>,
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Previous: <a rel="previous" accesskey="p" href="Summary.html#Summary">Summary</a>,
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Up: <a rel="up" accesskey="u" href="index.html#Top">Top</a>
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<hr>
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</div>
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<h2 class="chapter">1 A Sample <span class="sc">gdb</span> Session</h2>
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<p>You can use this manual at your leisure to read all about <span class="sc">gdb</span>.
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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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<!-- FIXME: this example may not be appropriate for some configs, where -->
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<!-- FIXME...primary interest is in remote use. -->
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<p>One of the preliminary versions of <span class="sc">gnu</span> <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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<pre class="smallexample"> $ <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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</pre>
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<p class="noindent">Let us use <span class="sc">gdb</span> to try to see what is going on.
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<pre class="smallexample"> $ <b>gdb m4</b>
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<!-- FIXME: this falsifies the exact text played out, to permit smallbook -->
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<!-- FIXME... format to come out better. -->
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<span class="sc">gdb</span> 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 <span class="sc">gdb</span>; type "show warranty"
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for details.
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<span class="sc">gdb</span> 8.3.0.20190709-git, Copyright 1999 Free Software Foundation, Inc...
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(gdb)
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</pre>
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<p class="noindent"><span class="sc">gdb</span> 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 <span class="sc">gdb</span> to use a narrower display width than usual, so
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that examples fit in this manual.
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<pre class="smallexample"> (gdb) <b>set width 70</b>
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</pre>
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<p class="noindent">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 <span class="sc">gdb</span>
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<code>break</code> command.
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<pre class="smallexample"> (gdb) <b>break m4_changequote</b>
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Breakpoint 1 at 0x62f4: file builtin.c, line 879.
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</pre>
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<p class="noindent">Using the <code>run</code> command, we start <code>m4</code> running under <span class="sc">gdb</span>
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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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<pre class="smallexample"> (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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</pre>
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<p class="noindent">To trigger the breakpoint, we call <code>changequote</code>. <span class="sc">gdb</span>
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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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<pre class="smallexample"> <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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</pre>
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<p class="noindent">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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<pre class="smallexample"> (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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</pre>
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<p class="noindent"><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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<pre class="smallexample"> (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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</pre>
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<p class="noindent">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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<pre class="smallexample"> (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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</pre>
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<p class="noindent">We step through a few more lines to see what happens. The first two
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times, we can use ‘<samp><span class="samp">s</span></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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<pre class="smallexample"> (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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</pre>
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<p class="noindent">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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<pre class="smallexample"> (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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</pre>
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<p class="noindent"><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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<pre class="smallexample"> (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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</pre>
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<p class="noindent">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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<pre class="smallexample"> (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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</pre>
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<p class="noindent">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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<pre class="smallexample"> (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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</pre>
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<p class="noindent">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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<pre class="smallexample"> (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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</pre>
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<p class="noindent">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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<pre class="smallexample"> <b>Ctrl-d</b>
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Program exited normally.
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</pre>
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<p class="noindent">The message ‘<samp><span class="samp">Program exited normally.</span></samp>’ is from <span class="sc">gdb</span>; it
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indicates <code>m4</code> has finished executing. We can end our <span class="sc">gdb</span>
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session with the <span class="sc">gdb</span> <code>quit</code> command.
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<pre class="smallexample"> (gdb) <b>quit</b>
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</pre>
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</body></html>
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