263 lines
11 KiB
HTML
263 lines
11 KiB
HTML
<html lang="en">
|
|
<head>
|
|
<title>Sample Session - Debugging with GDB</title>
|
|
<meta http-equiv="Content-Type" content="text/html">
|
|
<meta name="description" content="Debugging with GDB">
|
|
<meta name="generator" content="makeinfo 4.13">
|
|
<link title="Top" rel="start" href="index.html#Top">
|
|
<link rel="prev" href="Summary.html#Summary" title="Summary">
|
|
<link rel="next" href="Invocation.html#Invocation" title="Invocation">
|
|
<link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage">
|
|
<!--
|
|
Copyright (C) 1988-2019 Free Software Foundation, Inc.
|
|
|
|
Permission is granted to copy, distribute and/or modify this document
|
|
under the terms of the GNU Free Documentation License, Version 1.3 or
|
|
any later version published by the Free Software Foundation; with the
|
|
Invariant Sections being ``Free Software'' and ``Free Software Needs
|
|
Free Documentation'', with the Front-Cover Texts being ``A GNU Manual,''
|
|
and with the Back-Cover Texts as in (a) below.
|
|
|
|
(a) The FSF's Back-Cover Text is: ``You are free to copy and modify
|
|
this GNU Manual. Buying copies from GNU Press supports the FSF in
|
|
developing GNU and promoting software freedom.''
|
|
-->
|
|
<meta http-equiv="Content-Style-Type" content="text/css">
|
|
<style type="text/css"><!--
|
|
pre.display { font-family:inherit }
|
|
pre.format { font-family:inherit }
|
|
pre.smalldisplay { font-family:inherit; font-size:smaller }
|
|
pre.smallformat { font-family:inherit; font-size:smaller }
|
|
pre.smallexample { font-size:smaller }
|
|
pre.smalllisp { font-size:smaller }
|
|
span.sc { font-variant:small-caps }
|
|
span.roman { font-family:serif; font-weight:normal; }
|
|
span.sansserif { font-family:sans-serif; font-weight:normal; }
|
|
--></style>
|
|
</head>
|
|
<body>
|
|
<div class="node">
|
|
<a name="Sample-Session"></a>
|
|
<p>
|
|
Next: <a rel="next" accesskey="n" href="Invocation.html#Invocation">Invocation</a>,
|
|
Previous: <a rel="previous" accesskey="p" href="Summary.html#Summary">Summary</a>,
|
|
Up: <a rel="up" accesskey="u" href="index.html#Top">Top</a>
|
|
<hr>
|
|
</div>
|
|
|
|
<h2 class="chapter">1 A Sample <span class="sc">gdb</span> Session</h2>
|
|
|
|
<p>You can use this manual at your leisure to read all about <span class="sc">gdb</span>.
|
|
However, a handful of commands are enough to get started using the
|
|
debugger. This chapter illustrates those commands.
|
|
|
|
<!-- FIXME: this example may not be appropriate for some configs, where -->
|
|
<!-- FIXME...primary interest is in remote use. -->
|
|
<p>One of the preliminary versions of <span class="sc">gnu</span> <code>m4</code> (a generic macro
|
|
processor) exhibits the following bug: sometimes, when we change its
|
|
quote strings from the default, the commands used to capture one macro
|
|
definition within another stop working. In the following short <code>m4</code>
|
|
session, we define a macro <code>foo</code> which expands to <code>0000</code>; we
|
|
then use the <code>m4</code> built-in <code>defn</code> to define <code>bar</code> as the
|
|
same thing. However, when we change the open quote string to
|
|
<code><QUOTE></code> and the close quote string to <code><UNQUOTE></code>, the same
|
|
procedure fails to define a new synonym <code>baz</code>:
|
|
|
|
<pre class="smallexample"> $ <b>cd gnu/m4</b>
|
|
$ <b>./m4</b>
|
|
<b>define(foo,0000)</b>
|
|
|
|
<b>foo</b>
|
|
0000
|
|
<b>define(bar,defn(`foo'))</b>
|
|
|
|
<b>bar</b>
|
|
0000
|
|
<b>changequote(<QUOTE>,<UNQUOTE>)</b>
|
|
|
|
<b>define(baz,defn(<QUOTE>foo<UNQUOTE>))</b>
|
|
<b>baz</b>
|
|
<b>Ctrl-d</b>
|
|
m4: End of input: 0: fatal error: EOF in string
|
|
</pre>
|
|
<p class="noindent">Let us use <span class="sc">gdb</span> to try to see what is going on.
|
|
|
|
<pre class="smallexample"> $ <b>gdb m4</b>
|
|
<!-- FIXME: this falsifies the exact text played out, to permit smallbook -->
|
|
<!-- FIXME... format to come out better. -->
|
|
<span class="sc">gdb</span> is free software and you are welcome to distribute copies
|
|
of it under certain conditions; type "show copying" to see
|
|
the conditions.
|
|
There is absolutely no warranty for <span class="sc">gdb</span>; type "show warranty"
|
|
for details.
|
|
|
|
<span class="sc">gdb</span> 8.3.0.20190709-git, Copyright 1999 Free Software Foundation, Inc...
|
|
(gdb)
|
|
</pre>
|
|
<p class="noindent"><span class="sc">gdb</span> reads only enough symbol data to know where to find the
|
|
rest when needed; as a result, the first prompt comes up very quickly.
|
|
We now tell <span class="sc">gdb</span> to use a narrower display width than usual, so
|
|
that examples fit in this manual.
|
|
|
|
<pre class="smallexample"> (gdb) <b>set width 70</b>
|
|
</pre>
|
|
<p class="noindent">We need to see how the <code>m4</code> built-in <code>changequote</code> works.
|
|
Having looked at the source, we know the relevant subroutine is
|
|
<code>m4_changequote</code>, so we set a breakpoint there with the <span class="sc">gdb</span>
|
|
<code>break</code> command.
|
|
|
|
<pre class="smallexample"> (gdb) <b>break m4_changequote</b>
|
|
Breakpoint 1 at 0x62f4: file builtin.c, line 879.
|
|
</pre>
|
|
<p class="noindent">Using the <code>run</code> command, we start <code>m4</code> running under <span class="sc">gdb</span>
|
|
control; as long as control does not reach the <code>m4_changequote</code>
|
|
subroutine, the program runs as usual:
|
|
|
|
<pre class="smallexample"> (gdb) <b>run</b>
|
|
Starting program: /work/Editorial/gdb/gnu/m4/m4
|
|
<b>define(foo,0000)</b>
|
|
|
|
<b>foo</b>
|
|
0000
|
|
</pre>
|
|
<p class="noindent">To trigger the breakpoint, we call <code>changequote</code>. <span class="sc">gdb</span>
|
|
suspends execution of <code>m4</code>, displaying information about the
|
|
context where it stops.
|
|
|
|
<pre class="smallexample"> <b>changequote(<QUOTE>,<UNQUOTE>)</b>
|
|
|
|
Breakpoint 1, m4_changequote (argc=3, argv=0x33c70)
|
|
at builtin.c:879
|
|
879 if (bad_argc(TOKEN_DATA_TEXT(argv[0]),argc,1,3))
|
|
</pre>
|
|
<p class="noindent">Now we use the command <code>n</code> (<code>next</code>) to advance execution to
|
|
the next line of the current function.
|
|
|
|
<pre class="smallexample"> (gdb) <b>n</b>
|
|
882 set_quotes((argc >= 2) ? TOKEN_DATA_TEXT(argv[1])\
|
|
: nil,
|
|
</pre>
|
|
<p class="noindent"><code>set_quotes</code> looks like a promising subroutine. We can go into it
|
|
by using the command <code>s</code> (<code>step</code>) instead of <code>next</code>.
|
|
<code>step</code> goes to the next line to be executed in <em>any</em>
|
|
subroutine, so it steps into <code>set_quotes</code>.
|
|
|
|
<pre class="smallexample"> (gdb) <b>s</b>
|
|
set_quotes (lq=0x34c78 "<QUOTE>", rq=0x34c88 "<UNQUOTE>")
|
|
at input.c:530
|
|
530 if (lquote != def_lquote)
|
|
</pre>
|
|
<p class="noindent">The display that shows the subroutine where <code>m4</code> is now
|
|
suspended (and its arguments) is called a stack frame display. It
|
|
shows a summary of the stack. We can use the <code>backtrace</code>
|
|
command (which can also be spelled <code>bt</code>), to see where we are
|
|
in the stack as a whole: the <code>backtrace</code> command displays a
|
|
stack frame for each active subroutine.
|
|
|
|
<pre class="smallexample"> (gdb) <b>bt</b>
|
|
#0 set_quotes (lq=0x34c78 "<QUOTE>", rq=0x34c88 "<UNQUOTE>")
|
|
at input.c:530
|
|
#1 0x6344 in m4_changequote (argc=3, argv=0x33c70)
|
|
at builtin.c:882
|
|
#2 0x8174 in expand_macro (sym=0x33320) at macro.c:242
|
|
#3 0x7a88 in expand_token (obs=0x0, t=209696, td=0xf7fffa30)
|
|
at macro.c:71
|
|
#4 0x79dc in expand_input () at macro.c:40
|
|
#5 0x2930 in main (argc=0, argv=0xf7fffb20) at m4.c:195
|
|
</pre>
|
|
<p class="noindent">We step through a few more lines to see what happens. The first two
|
|
times, we can use ‘<samp><span class="samp">s</span></samp>’; the next two times we use <code>n</code> to avoid
|
|
falling into the <code>xstrdup</code> subroutine.
|
|
|
|
<pre class="smallexample"> (gdb) <b>s</b>
|
|
0x3b5c 532 if (rquote != def_rquote)
|
|
(gdb) <b>s</b>
|
|
0x3b80 535 lquote = (lq == nil || *lq == '\0') ? \
|
|
def_lquote : xstrdup(lq);
|
|
(gdb) <b>n</b>
|
|
536 rquote = (rq == nil || *rq == '\0') ? def_rquote\
|
|
: xstrdup(rq);
|
|
(gdb) <b>n</b>
|
|
538 len_lquote = strlen(rquote);
|
|
</pre>
|
|
<p class="noindent">The last line displayed looks a little odd; we can examine the variables
|
|
<code>lquote</code> and <code>rquote</code> to see if they are in fact the new left
|
|
and right quotes we specified. We use the command <code>p</code>
|
|
(<code>print</code>) to see their values.
|
|
|
|
<pre class="smallexample"> (gdb) <b>p lquote</b>
|
|
$1 = 0x35d40 "<QUOTE>"
|
|
(gdb) <b>p rquote</b>
|
|
$2 = 0x35d50 "<UNQUOTE>"
|
|
</pre>
|
|
<p class="noindent"><code>lquote</code> and <code>rquote</code> are indeed the new left and right quotes.
|
|
To look at some context, we can display ten lines of source
|
|
surrounding the current line with the <code>l</code> (<code>list</code>) command.
|
|
|
|
<pre class="smallexample"> (gdb) <b>l</b>
|
|
533 xfree(rquote);
|
|
534
|
|
535 lquote = (lq == nil || *lq == '\0') ? def_lquote\
|
|
: xstrdup (lq);
|
|
536 rquote = (rq == nil || *rq == '\0') ? def_rquote\
|
|
: xstrdup (rq);
|
|
537
|
|
538 len_lquote = strlen(rquote);
|
|
539 len_rquote = strlen(lquote);
|
|
540 }
|
|
541
|
|
542 void
|
|
</pre>
|
|
<p class="noindent">Let us step past the two lines that set <code>len_lquote</code> and
|
|
<code>len_rquote</code>, and then examine the values of those variables.
|
|
|
|
<pre class="smallexample"> (gdb) <b>n</b>
|
|
539 len_rquote = strlen(lquote);
|
|
(gdb) <b>n</b>
|
|
540 }
|
|
(gdb) <b>p len_lquote</b>
|
|
$3 = 9
|
|
(gdb) <b>p len_rquote</b>
|
|
$4 = 7
|
|
</pre>
|
|
<p class="noindent">That certainly looks wrong, assuming <code>len_lquote</code> and
|
|
<code>len_rquote</code> are meant to be the lengths of <code>lquote</code> and
|
|
<code>rquote</code> respectively. We can set them to better values using
|
|
the <code>p</code> command, since it can print the value of
|
|
any expression—and that expression can include subroutine calls and
|
|
assignments.
|
|
|
|
<pre class="smallexample"> (gdb) <b>p len_lquote=strlen(lquote)</b>
|
|
$5 = 7
|
|
(gdb) <b>p len_rquote=strlen(rquote)</b>
|
|
$6 = 9
|
|
</pre>
|
|
<p class="noindent">Is that enough to fix the problem of using the new quotes with the
|
|
<code>m4</code> built-in <code>defn</code>? We can allow <code>m4</code> to continue
|
|
executing with the <code>c</code> (<code>continue</code>) command, and then try the
|
|
example that caused trouble initially:
|
|
|
|
<pre class="smallexample"> (gdb) <b>c</b>
|
|
Continuing.
|
|
|
|
<b>define(baz,defn(<QUOTE>foo<UNQUOTE>))</b>
|
|
|
|
baz
|
|
0000
|
|
</pre>
|
|
<p class="noindent">Success! The new quotes now work just as well as the default ones. The
|
|
problem seems to have been just the two typos defining the wrong
|
|
lengths. We allow <code>m4</code> exit by giving it an EOF as input:
|
|
|
|
<pre class="smallexample"> <b>Ctrl-d</b>
|
|
Program exited normally.
|
|
</pre>
|
|
<p class="noindent">The message ‘<samp><span class="samp">Program exited normally.</span></samp>’ is from <span class="sc">gdb</span>; it
|
|
indicates <code>m4</code> has finished executing. We can end our <span class="sc">gdb</span>
|
|
session with the <span class="sc">gdb</span> <code>quit</code> command.
|
|
|
|
<pre class="smallexample"> (gdb) <b>quit</b>
|
|
</pre>
|
|
</body></html>
|
|
|