mingw/info/gdb/Break-Commands.html

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  19 <h4 class="subsection">Breakpoint command lists</h4>
  20
  21    <p>You can give any breakpoint (or watchpoint or catchpoint) a series of
  22 commands to execute when your program stops due to that breakpoint.  For
  23 example, you might want to print the values of certain expressions, or
  24 enable other breakpoints.
  25
  26      <dl>
  27 <dt><code>commands </code>[<code></code><var>bnum</var><code></code>]<code></code>
  28      <dd><dt><code>... </code><var>command-list</var><code> ...</code>
  29      <dd><dt><code>end</code>
  30      <dd>Specify a list of commands for breakpoint number <var>bnum</var>.  The commands
  31 themselves appear on the following lines.  Type a line containing just
  32 <code>end</code> to terminate the commands.
  33
  34      <p>To remove all commands from a breakpoint, type <code>commands</code> and
  35 follow it immediately with <code>end</code>; that is, give no commands.
  36
  37      <p>With no <var>bnum</var> argument, <code>commands</code> refers to the last
  38 breakpoint, watchpoint, or catchpoint set (not to the breakpoint most
  39 recently encountered).
  40 </dl>
  41
  42    <p>Pressing &lt;RET&gt; as a means of repeating the last GDB command is
  43 disabled within a <var>command-list</var>.
  44
  45    <p>You can use breakpoint commands to start your program up again.  Simply
  46 use the <code>continue</code> command, or <code>step</code>, or any other command
  47 that resumes execution.
  48
  49    <p>Any other commands in the command list, after a command that resumes
  50 execution, are ignored.  This is because any time you resume execution
  51 (even with a simple <code>next</code> or <code>step</code>), you may encounter
  52 another breakpoint--which could have its own command list, leading to
  53 ambiguities about which list to execute.
  54
  55    <p>If the first command you specify in a command list is <code>silent</code>, the
  56 usual message about stopping at a breakpoint is not printed.  This may
  57 be desirable for breakpoints that are to print a specific message and
  58 then continue.  If none of the remaining commands print anything, you
  59 see no sign that the breakpoint was reached.  <code>silent</code> is
  60 meaningful only at the beginning of a breakpoint command list.
  61
  62    <p>The commands <code>echo</code>, <code>output</code>, and <code>printf</code> allow you to
  63 print precisely controlled output, and are often useful in silent
  64 breakpoints.  See <a href="Output.html#Output">Commands for controlled output</a>.
  65
  66    <p>For example, here is how you could use breakpoint commands to print the
  67 value of <code>x</code> at entry to <code>foo</code> whenever <code>x</code> is positive.
  68
  69 <pre class="example">     break foo if x&gt;0
  70      commands
  71      silent
  72      printf "x is %d\n",x
  73      cont
  74      end
  75      </pre>
  76
  77    <p>One application for breakpoint commands is to compensate for one bug so
  78 you can test for another.  Put a breakpoint just after the erroneous line
  79 of code, give it a condition to detect the case in which something
  80 erroneous has been done, and give it commands to assign correct values
  81 to any variables that need them.  End with the <code>continue</code> command
  82 so that your program does not stop, and start with the <code>silent</code>
  83 command so that no output is produced.  Here is an example:
  84
  85 <pre class="example">     break 403
  86      commands
  87      silent
  88      set x = y + 4
  89      cont
  90      end
  91      </pre>
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