diff options
Diffstat (limited to 'share/doc/gdb/Sample-Session.html')
| -rw-r--r-- | share/doc/gdb/Sample-Session.html | 325 |
1 files changed, 325 insertions, 0 deletions
diff --git a/share/doc/gdb/Sample-Session.html b/share/doc/gdb/Sample-Session.html new file mode 100644 index 0000000..25e9d21 --- /dev/null +++ b/share/doc/gdb/Sample-Session.html @@ -0,0 +1,325 @@ +<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> +<html> +<!-- Copyright (C) 1988-2023 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." --> +<!-- Created by GNU Texinfo 5.1, http://www.gnu.org/software/texinfo/ --> +<head> +<title>Debugging with GDB: Sample Session</title> + +<meta name="description" content="Debugging with GDB: Sample Session"> +<meta name="keywords" content="Debugging with GDB: Sample Session"> +<meta name="resource-type" content="document"> +<meta name="distribution" content="global"> +<meta name="Generator" content="makeinfo"> +<meta http-equiv="Content-Type" content="text/html; charset=utf-8"> +<link href="index.html#Top" rel="start" title="Top"> +<link href="Concept-Index.html#Concept-Index" rel="index" title="Concept Index"> +<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents"> +<link href="index.html#Top" rel="up" title="Top"> +<link href="Invocation.html#Invocation" rel="next" title="Invocation"> +<link href="Contributors.html#Contributors" rel="previous" title="Contributors"> +<style type="text/css"> +<!-- +a.summary-letter {text-decoration: none} +blockquote.smallquotation {font-size: smaller} +div.display {margin-left: 3.2em} +div.example {margin-left: 3.2em} +div.indentedblock {margin-left: 3.2em} +div.lisp {margin-left: 3.2em} +div.smalldisplay {margin-left: 3.2em} +div.smallexample {margin-left: 3.2em} +div.smallindentedblock {margin-left: 3.2em; font-size: smaller} +div.smalllisp {margin-left: 3.2em} +kbd {font-style:oblique} +pre.display {font-family: inherit} +pre.format {font-family: inherit} +pre.menu-comment {font-family: serif} +pre.menu-preformatted {font-family: serif} +pre.smalldisplay {font-family: inherit; font-size: smaller} +pre.smallexample {font-size: smaller} +pre.smallformat {font-family: inherit; font-size: smaller} +pre.smalllisp {font-size: smaller} +span.nocodebreak {white-space:nowrap} +span.nolinebreak {white-space:nowrap} +span.roman {font-family:serif; font-weight:normal} +span.sansserif {font-family:sans-serif; font-weight:normal} +ul.no-bullet {list-style: none} +--> +</style> + + +</head> + +<body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000"> +<a name="Sample-Session"></a> +<div class="header"> +<p> +Next: <a href="Invocation.html#Invocation" accesskey="n" rel="next">Invocation</a>, Previous: <a href="Summary.html#Summary" accesskey="p" rel="previous">Summary</a>, Up: <a href="index.html#Top" accesskey="u" rel="up">Top</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p> +</div> +<hr> +<a name="A-Sample-GDB-Session"></a> +<h2 class="chapter">1 A Sample <small>GDB</small> Session</h2> + +<p>You can use this manual at your leisure to read all about <small>GDB</small>. +However, a handful of commands are enough to get started using the +debugger. This chapter illustrates those commands. +</p> + + +<p>One of the preliminary versions of <small>GNU</small> <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>: +</p> +<div class="smallexample"> +<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></div> + +<p>Let us use <small>GDB</small> to try to see what is going on. +</p> +<div class="smallexample"> +<pre class="smallexample">$ <b>gdb m4</b> +GDB 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 GDB; type "show warranty" + for details. + +GDB 13.2.90.20231008-git, Copyright 1999 Free Software Foundation, Inc... +(gdb) +</pre></div> + +<p><small>GDB</small> 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 <small>GDB</small> to use a narrower display width than usual, so +that examples fit in this manual. +</p> +<div class="smallexample"> +<pre class="smallexample">(gdb) <b>set width 70</b> +</pre></div> + +<p>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 <small>GDB</small> +<code>break</code> command. +</p> +<div class="smallexample"> +<pre class="smallexample">(gdb) <b>break m4_changequote</b> +Breakpoint 1 at 0x62f4: file builtin.c, line 879. +</pre></div> + +<p>Using the <code>run</code> command, we start <code>m4</code> running under <small>GDB</small> +control; as long as control does not reach the <code>m4_changequote</code> +subroutine, the program runs as usual: +</p> +<div class="smallexample"> +<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></div> + +<p>To trigger the breakpoint, we call <code>changequote</code>. <small>GDB</small> +suspends execution of <code>m4</code>, displaying information about the +context where it stops. +</p> +<div class="smallexample"> +<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></div> + +<p>Now we use the command <code>n</code> (<code>next</code>) to advance execution to +the next line of the current function. +</p> +<div class="smallexample"> +<pre class="smallexample">(gdb) <b>n</b> +882 set_quotes((argc >= 2) ? TOKEN_DATA_TEXT(argv[1])\ + : nil, +</pre></div> + +<p><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>. +</p> +<div class="smallexample"> +<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></div> + +<p>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. +</p> +<div class="smallexample"> +<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></div> + +<p>We step through a few more lines to see what happens. The first two +times, we can use ‘<samp>s</samp>’; the next two times we use <code>n</code> to avoid +falling into the <code>xstrdup</code> subroutine. +</p> +<div class="smallexample"> +<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></div> + +<p>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. +</p> +<div class="smallexample"> +<pre class="smallexample">(gdb) <b>p lquote</b> +$1 = 0x35d40 "<QUOTE>" +(gdb) <b>p rquote</b> +$2 = 0x35d50 "<UNQUOTE>" +</pre></div> + +<p><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. +</p> +<div class="smallexample"> +<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></div> + +<p>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. +</p> +<div class="smallexample"> +<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></div> + +<p>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. +</p> +<div class="smallexample"> +<pre class="smallexample">(gdb) <b>p len_lquote=strlen(lquote)</b> +$5 = 7 +(gdb) <b>p len_rquote=strlen(rquote)</b> +$6 = 9 +</pre></div> + +<p>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: +</p> +<div class="smallexample"> +<pre class="smallexample">(gdb) <b>c</b> +Continuing. + +<b>define(baz,defn(<QUOTE>foo<UNQUOTE>))</b> + +baz +0000 +</pre></div> + +<p>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: +</p> +<div class="smallexample"> +<pre class="smallexample"><b>Ctrl-d</b> +Program exited normally. +</pre></div> + +<p>The message ‘<samp>Program exited normally.</samp>’ is from <small>GDB</small>; it +indicates <code>m4</code> has finished executing. We can end our <small>GDB</small> +session with the <small>GDB</small> <code>quit</code> command. +</p> +<div class="smallexample"> +<pre class="smallexample">(gdb) <b>quit</b> +</pre></div> + +<hr> +<div class="header"> +<p> +Next: <a href="Invocation.html#Invocation" accesskey="n" rel="next">Invocation</a>, Previous: <a href="Summary.html#Summary" accesskey="p" rel="previous">Summary</a>, Up: <a href="index.html#Top" accesskey="u" rel="up">Top</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p> +</div> + + + +</body> +</html> |