mingw/info/gdb/Sample-Session.html

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  19 <h2 class="chapter">A Sample GDB Session</h2>
  20
  21    <p>You can use this manual at your leisure to read all about GDB.
  22 However, a handful of commands are enough to get started using the
  23 debugger.  This chapter illustrates those commands.
  24
  25    <p>One of the preliminary versions of <small>GNU</small> <code>m4</code> (a generic macro
  26 processor) exhibits the following bug: sometimes, when we change its
  27 quote strings from the default, the commands used to capture one macro
  28 definition within another stop working.  In the following short <code>m4</code>
  29 session, we define a macro <code>foo</code> which expands to <code>0000</code>; we
  30 then use the <code>m4</code> built-in <code>defn</code> to define <code>bar</code> as the
  31 same thing.  However, when we change the open quote string to
  32 <code>&lt;QUOTE&gt;</code> and the close quote string to <code>&lt;UNQUOTE&gt;</code>, the same
  33 procedure fails to define a new synonym <code>baz</code>:
  34
  35 <pre class="smallexample">     $ <b>cd gnu/m4</b>
  36      $ <b>./m4</b>
  37      <b>define(foo,0000)</b>
  38
  39      <b>foo</b>
  40      0000
  41      <b>define(bar,defn(`foo'))</b>
  42
  43      <b>bar</b>
  44      0000
  45      <b>changequote(&lt;QUOTE&gt;,&lt;UNQUOTE&gt;)</b>
  46
  47      <b>define(baz,defn(&lt;QUOTE&gt;foo&lt;UNQUOTE&gt;))</b>
  48      <b>baz</b>
  49      <b>C-d</b>
  50      m4: End of input: 0: fatal error: EOF in string
  51      </pre>
  52
  53 <p>Let us use GDB to try to see what is going on.
  54
  55 <pre class="smallexample">     $ <b>gdb m4</b>
  56      GDB is free software and you are welcome to distribute copies
  57       of it under certain conditions; type "show copying" to see
  58       the conditions.
  59      There is absolutely no warranty for GDB; type "show warranty"
  60       for details.
  61
  62      GDB 5.2.1, Copyright 1999 Free Software Foundation, Inc...
  63      (gdb)
  64      </pre>
  65
  66 GDB reads only enough symbol data to know where to find the
  67 rest when needed; as a result, the first prompt comes up very quickly.
  68 We now tell GDB to use a narrower display width than usual, so
  69 that examples fit in this manual.
  70
  71 <pre class="smallexample">     (gdb) <b>set width 70</b>
  72      </pre>
  73
  74 <p>We need to see how the <code>m4</code> built-in <code>changequote</code> works.
  75 Having looked at the source, we know the relevant subroutine is
  76 <code>m4_changequote</code>, so we set a breakpoint there with the GDB
  77 <code>break</code> command.
  78
  79 <pre class="smallexample">     (gdb) <b>break m4_changequote</b>
  80      Breakpoint 1 at 0x62f4: file builtin.c, line 879.
  81      </pre>
  82
  83 <p>Using the <code>run</code> command, we start <code>m4</code> running under GDB
  84 control; as long as control does not reach the <code>m4_changequote</code>
  85 subroutine, the program runs as usual:
  86
  87 <pre class="smallexample">     (gdb) <b>run</b>
  88      Starting program: /work/Editorial/gdb/gnu/m4/m4
  89      <b>define(foo,0000)</b>
  90
  91      <b>foo</b>
  92      0000
  93      </pre>
  94
  95 <p>To trigger the breakpoint, we call <code>changequote</code>.  GDB
  96 suspends execution of <code>m4</code>, displaying information about the
  97 context where it stops.
  98
  99 <pre class="smallexample">     <b>changequote(&lt;QUOTE&gt;,&lt;UNQUOTE&gt;)</b>
 100
 101      Breakpoint 1, m4_changequote (argc=3, argv=0x33c70)
 102          at builtin.c:879
 103      879         if (bad_argc(TOKEN_DATA_TEXT(argv[0]),argc,1,3))
 104      </pre>
 105
 106 <p>Now we use the command <code>n</code> (<code>next</code>) to advance execution to
 107 the next line of the current function.
 108
 109 <pre class="smallexample">     (gdb) <b>n</b>
 110      882         set_quotes((argc &gt;= 2) ? TOKEN_DATA_TEXT(argv[1])\
 111       : nil,
 112      </pre>
 113
 114 <p><code>set_quotes</code> looks like a promising subroutine.  We can go into it
 115 by using the command <code>s</code> (<code>step</code>) instead of <code>next</code>.
 116 <code>step</code> goes to the next line to be executed in <em>any</em>
 117 subroutine, so it steps into <code>set_quotes</code>.
 118
 119 <pre class="smallexample">     (gdb) <b>s</b>
 120      set_quotes (lq=0x34c78 "&lt;QUOTE&gt;", rq=0x34c88 "&lt;UNQUOTE&gt;")
 121          at input.c:530
 122      530         if (lquote != def_lquote)
 123      </pre>
 124
 125 <p>The display that shows the subroutine where <code>m4</code> is now
 126 suspended (and its arguments) is called a stack frame display.  It
 127 shows a summary of the stack.  We can use the <code>backtrace</code>
 128 command (which can also be spelled <code>bt</code>), to see where we are
 129 in the stack as a whole: the <code>backtrace</code> command displays a
 130 stack frame for each active subroutine.
 131
 132 <pre class="smallexample">     (gdb) <b>bt</b>
 133      #0  set_quotes (lq=0x34c78 "&lt;QUOTE&gt;", rq=0x34c88 "&lt;UNQUOTE&gt;")
 134          at input.c:530
 135      #1  0x6344 in m4_changequote (argc=3, argv=0x33c70)
 136          at builtin.c:882
 137      #2  0x8174 in expand_macro (sym=0x33320) at macro.c:242
 138      #3  0x7a88 in expand_token (obs=0x0, t=209696, td=0xf7fffa30)
 139          at macro.c:71
 140      #4  0x79dc in expand_input () at macro.c:40
 141      #5  0x2930 in main (argc=0, argv=0xf7fffb20) at m4.c:195
 142      </pre>
 143
 144 <p>We step through a few more lines to see what happens.  The first two
 145 times, we can use <code>s</code>; the next two times we use <code>n</code> to avoid
 146 falling into the <code>xstrdup</code> subroutine.
 147
 148 <pre class="smallexample">     (gdb) <b>s</b>
 149      0x3b5c  532         if (rquote != def_rquote)
 150      (gdb) <b>s</b>
 151      0x3b80  535         lquote = (lq == nil || *lq == '\0') ?  \
 152      def_lquote : xstrdup(lq);
 153      (gdb) <b>n</b>
 154      536         rquote = (rq == nil || *rq == '\0') ? def_rquote\
 155       : xstrdup(rq);
 156      (gdb) <b>n</b>
 157      538         len_lquote = strlen(rquote);
 158      </pre>
 159
 160 <p>The last line displayed looks a little odd; we can examine the variables
 161 <code>lquote</code> and <code>rquote</code> to see if they are in fact the new left
 162 and right quotes we specified.  We use the command <code>p</code>
 163 (<code>print</code>) to see their values.
 164
 165 <pre class="smallexample">     (gdb) <b>p lquote</b>
 166      $1 = 0x35d40 "&lt;QUOTE&gt;"
 167      (gdb) <b>p rquote</b>
 168      $2 = 0x35d50 "&lt;UNQUOTE&gt;"
 169      </pre>
 170
 171 <p><code>lquote</code> and <code>rquote</code> are indeed the new left and right quotes.
 172 To look at some context, we can display ten lines of source
 173 surrounding the current line with the <code>l</code> (<code>list</code>) command.
 174
 175 <pre class="smallexample">     (gdb) <b>l</b>
 176      533             xfree(rquote);
 177      534
 178      535         lquote = (lq == nil || *lq == '\0') ? def_lquote\
 179       : xstrdup (lq);
 180      536         rquote = (rq == nil || *rq == '\0') ? def_rquote\
 181       : xstrdup (rq);
 182      537
 183      538         len_lquote = strlen(rquote);
 184      539         len_rquote = strlen(lquote);
 185      540     }
 186      541
 187      542     void
 188      </pre>
 189
 190 <p>Let us step past the two lines that set <code>len_lquote</code> and
 191 <code>len_rquote</code>, and then examine the values of those variables.
 192
 193 <pre class="smallexample">     (gdb) <b>n</b>
 194      539         len_rquote = strlen(lquote);
 195      (gdb) <b>n</b>
 196      540     }
 197      (gdb) <b>p len_lquote</b>
 198      $3 = 9
 199      (gdb) <b>p len_rquote</b>
 200      $4 = 7
 201      </pre>
 202
 203 <p>That certainly looks wrong, assuming <code>len_lquote</code> and
 204 <code>len_rquote</code> are meant to be the lengths of <code>lquote</code> and
 205 <code>rquote</code> respectively.  We can set them to better values using
 206 the <code>p</code> command, since it can print the value of
 207 any expression--and that expression can include subroutine calls and
 208 assignments.
 209
 210 <pre class="smallexample">     (gdb) <b>p len_lquote=strlen(lquote)</b>
 211      $5 = 7
 212      (gdb) <b>p len_rquote=strlen(rquote)</b>
 213      $6 = 9
 214      </pre>
 215
 216 <p>Is that enough to fix the problem of using the new quotes with the
 217 <code>m4</code> built-in <code>defn</code>?  We can allow <code>m4</code> to continue
 218 executing with the <code>c</code> (<code>continue</code>) command, and then try the
 219 example that caused trouble initially:
 220
 221 <pre class="smallexample">     (gdb) <b>c</b>
 222      Continuing.
 223
 224      <b>define(baz,defn(&lt;QUOTE&gt;foo&lt;UNQUOTE&gt;))</b>
 225
 226      baz
 227      0000
 228      </pre>
 229
 230 <p>Success!  The new quotes now work just as well as the default ones.  The
 231 problem seems to have been just the two typos defining the wrong
 232 lengths.  We allow <code>m4</code> exit by giving it an EOF as input:
 233
 234 <pre class="smallexample">     <b>C-d</b>
 235      Program exited normally.
 236      </pre>
 237
 238 <p>The message <code>Program exited normally.</code> is from GDB; it
 239 indicates <code>m4</code> has finished executing.  We can end our GDB
 240 session with the GDB <code>quit</code> command.
 241
 242 <pre class="smallexample">     (gdb) <b>quit</b>
 243      </pre>
 244
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