| <html lang="en"> |
| <head> |
| <title>Conditions - 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="up" href="Breakpoints.html#Breakpoints" title="Breakpoints"> |
| <link rel="prev" href="Disabling.html#Disabling" title="Disabling"> |
| <link rel="next" href="Break-Commands.html#Break-Commands" title="Break Commands"> |
| <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="Conditions"></a> |
| <p> |
| Next: <a rel="next" accesskey="n" href="Break-Commands.html#Break-Commands">Break Commands</a>, |
| Previous: <a rel="previous" accesskey="p" href="Disabling.html#Disabling">Disabling</a>, |
| Up: <a rel="up" accesskey="u" href="Breakpoints.html#Breakpoints">Breakpoints</a> |
| <hr> |
| </div> |
| |
| <h4 class="subsection">5.1.6 Break Conditions</h4> |
| |
| <p><a name="index-conditional-breakpoints-321"></a><a name="index-breakpoint-conditions-322"></a> |
| <!-- FIXME what is scope of break condition expr? Context where wanted? --> |
| <!-- in particular for a watchpoint? --> |
| The simplest sort of breakpoint breaks every time your program reaches a |
| specified place. You can also specify a <dfn>condition</dfn> for a |
| breakpoint. A condition is just a Boolean expression in your |
| programming language (see <a href="Expressions.html#Expressions">Expressions</a>). A breakpoint with |
| a condition evaluates the expression each time your program reaches it, |
| and your program stops only if the condition is <em>true</em>. |
| |
| <p>This is the converse of using assertions for program validation; in that |
| situation, you want to stop when the assertion is violated—that is, |
| when the condition is false. In C, if you want to test an assertion expressed |
| by the condition <var>assert</var>, you should set the condition |
| ‘<samp><span class="samp">! </span><var>assert</var></samp>’ on the appropriate breakpoint. |
| |
| <p>Conditions are also accepted for watchpoints; you may not need them, |
| since a watchpoint is inspecting the value of an expression anyhow—but |
| it might be simpler, say, to just set a watchpoint on a variable name, |
| and specify a condition that tests whether the new value is an interesting |
| one. |
| |
| <p>Break conditions can have side effects, and may even call functions in |
| your program. This can be useful, for example, to activate functions |
| that log program progress, or to use your own print functions to |
| format special data structures. The effects are completely predictable |
| unless there is another enabled breakpoint at the same address. (In |
| that case, <span class="sc">gdb</span> might see the other breakpoint first and stop your |
| program without checking the condition of this one.) Note that |
| breakpoint commands are usually more convenient and flexible than break |
| conditions for the |
| purpose of performing side effects when a breakpoint is reached |
| (see <a href="Break-Commands.html#Break-Commands">Breakpoint Command Lists</a>). |
| |
| <p>Breakpoint conditions can also be evaluated on the target's side if |
| the target supports it. Instead of evaluating the conditions locally, |
| <span class="sc">gdb</span> encodes the expression into an agent expression |
| (see <a href="Agent-Expressions.html#Agent-Expressions">Agent Expressions</a>) suitable for execution on the target, |
| independently of <span class="sc">gdb</span>. Global variables become raw memory |
| locations, locals become stack accesses, and so forth. |
| |
| <p>In this case, <span class="sc">gdb</span> will only be notified of a breakpoint trigger |
| when its condition evaluates to true. This mechanism may provide faster |
| response times depending on the performance characteristics of the target |
| since it does not need to keep <span class="sc">gdb</span> informed about |
| every breakpoint trigger, even those with false conditions. |
| |
| <p>Break conditions can be specified when a breakpoint is set, by using |
| ‘<samp><span class="samp">if</span></samp>’ in the arguments to the <code>break</code> command. See <a href="Set-Breaks.html#Set-Breaks">Setting Breakpoints</a>. They can also be changed at any time |
| with the <code>condition</code> command. |
| |
| <p>You can also use the <code>if</code> keyword with the <code>watch</code> command. |
| The <code>catch</code> command does not recognize the <code>if</code> keyword; |
| <code>condition</code> is the only way to impose a further condition on a |
| catchpoint. |
| |
| |
| <a name="index-condition-323"></a> |
| <dl><dt><code>condition </code><var>bnum</var> <var>expression</var><dd>Specify <var>expression</var> as the break condition for breakpoint, |
| watchpoint, or catchpoint number <var>bnum</var>. After you set a condition, |
| breakpoint <var>bnum</var> stops your program only if the value of |
| <var>expression</var> is true (nonzero, in C). When you use |
| <code>condition</code>, <span class="sc">gdb</span> checks <var>expression</var> immediately for |
| syntactic correctness, and to determine whether symbols in it have |
| referents in the context of your breakpoint. If <var>expression</var> uses |
| symbols not referenced in the context of the breakpoint, <span class="sc">gdb</span> |
| prints an error message: |
| |
| <pre class="smallexample"> No symbol "foo" in current context. |
| </pre> |
| <p class="noindent"><span class="sc">gdb</span> does |
| not actually evaluate <var>expression</var> at the time the <code>condition</code> |
| command (or a command that sets a breakpoint with a condition, like |
| <code>break if ...</code>) is given, however. See <a href="Expressions.html#Expressions">Expressions</a>. |
| |
| <br><dt><code>condition </code><var>bnum</var><dd>Remove the condition from breakpoint number <var>bnum</var>. It becomes |
| an ordinary unconditional breakpoint. |
| </dl> |
| |
| <p><a name="index-ignore-count-_0028of-breakpoint_0029-324"></a>A special case of a breakpoint condition is to stop only when the |
| breakpoint has been reached a certain number of times. This is so |
| useful that there is a special way to do it, using the <dfn>ignore |
| count</dfn> of the breakpoint. Every breakpoint has an ignore count, which |
| is an integer. Most of the time, the ignore count is zero, and |
| therefore has no effect. But if your program reaches a breakpoint whose |
| ignore count is positive, then instead of stopping, it just decrements |
| the ignore count by one and continues. As a result, if the ignore count |
| value is <var>n</var>, the breakpoint does not stop the next <var>n</var> times |
| your program reaches it. |
| |
| |
| <a name="index-ignore-325"></a> |
| <dl><dt><code>ignore </code><var>bnum</var> <var>count</var><dd>Set the ignore count of breakpoint number <var>bnum</var> to <var>count</var>. |
| The next <var>count</var> times the breakpoint is reached, your program's |
| execution does not stop; other than to decrement the ignore count, <span class="sc">gdb</span> |
| takes no action. |
| |
| <p>To make the breakpoint stop the next time it is reached, specify |
| a count of zero. |
| |
| <p>When you use <code>continue</code> to resume execution of your program from a |
| breakpoint, you can specify an ignore count directly as an argument to |
| <code>continue</code>, rather than using <code>ignore</code>. See <a href="Continuing-and-Stepping.html#Continuing-and-Stepping">Continuing and Stepping</a>. |
| |
| <p>If a breakpoint has a positive ignore count and a condition, the |
| condition is not checked. Once the ignore count reaches zero, |
| <span class="sc">gdb</span> resumes checking the condition. |
| |
| <p>You could achieve the effect of the ignore count with a condition such |
| as ‘<samp><span class="samp">$foo-- <= 0</span></samp>’<!-- /@w --> using a debugger convenience variable that |
| is decremented each time. See <a href="Convenience-Vars.html#Convenience-Vars">Convenience Variables</a>. |
| </dl> |
| |
| <p>Ignore counts apply to breakpoints, watchpoints, and catchpoints. |
| |
| </body></html> |
| |
