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| <h4 class="subsection">5.1.7 Breakpoint Command Lists</h4> |
| |
| <p><a name="index-breakpoint-commands-326"></a>You can give any breakpoint (or watchpoint or catchpoint) a series of |
| commands to execute when your program stops due to that breakpoint. For |
| example, you might want to print the values of certain expressions, or |
| enable other breakpoints. |
| |
| |
| <a name="index-commands-327"></a> |
| <a name="index-end_0040r_007b-_0028breakpoint-commands_0029_007d-328"></a> |
| <dl><dt><code>commands </code><span class="roman">[</span><var>list</var><code>...</code><span class="roman">]</span><dt><code>... </code><var>command-list</var><code> ...</code><dt><code>end</code><dd>Specify a list of commands for the given breakpoints. The commands |
| themselves appear on the following lines. Type a line containing just |
| <code>end</code> to terminate the commands. |
| |
| <p>To remove all commands from a breakpoint, type <code>commands</code> and |
| follow it immediately with <code>end</code>; that is, give no commands. |
| |
| <p>With no argument, <code>commands</code> refers to the last breakpoint, |
| watchpoint, or catchpoint set (not to the breakpoint most recently |
| encountered). If the most recent breakpoints were set with a single |
| command, then the <code>commands</code> will apply to all the breakpoints |
| set by that command. This applies to breakpoints set by |
| <code>rbreak</code>, and also applies when a single <code>break</code> command |
| creates multiple breakpoints (see <a href="Ambiguous-Expressions.html#Ambiguous-Expressions">Ambiguous Expressions</a>). |
| </dl> |
| |
| <p>Pressing <RET> as a means of repeating the last <span class="sc">gdb</span> command is |
| disabled within a <var>command-list</var>. |
| |
| <p>You can use breakpoint commands to start your program up again. Simply |
| use the <code>continue</code> command, or <code>step</code>, or any other command |
| that resumes execution. |
| |
| <p>Any other commands in the command list, after a command that resumes |
| execution, are ignored. This is because any time you resume execution |
| (even with a simple <code>next</code> or <code>step</code>), you may encounter |
| another breakpoint—which could have its own command list, leading to |
| ambiguities about which list to execute. |
| |
| <p><a name="index-silent-329"></a>If the first command you specify in a command list is <code>silent</code>, the |
| usual message about stopping at a breakpoint is not printed. This may |
| be desirable for breakpoints that are to print a specific message and |
| then continue. If none of the remaining commands print anything, you |
| see no sign that the breakpoint was reached. <code>silent</code> is |
| meaningful only at the beginning of a breakpoint command list. |
| |
| <p>The commands <code>echo</code>, <code>output</code>, and <code>printf</code> allow you to |
| print precisely controlled output, and are often useful in silent |
| breakpoints. See <a href="Output.html#Output">Commands for Controlled Output</a>. |
| |
| <p>For example, here is how you could use breakpoint commands to print the |
| value of <code>x</code> at entry to <code>foo</code> whenever <code>x</code> is positive. |
| |
| <pre class="smallexample"> break foo if x>0 |
| commands |
| silent |
| printf "x is %d\n",x |
| cont |
| end |
| </pre> |
| <p>One application for breakpoint commands is to compensate for one bug so |
| you can test for another. Put a breakpoint just after the erroneous line |
| of code, give it a condition to detect the case in which something |
| erroneous has been done, and give it commands to assign correct values |
| to any variables that need them. End with the <code>continue</code> command |
| so that your program does not stop, and start with the <code>silent</code> |
| command so that no output is produced. Here is an example: |
| |
| <pre class="smallexample"> break 403 |
| commands |
| silent |
| set x = y + 4 |
| cont |
| end |
| </pre> |
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