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155 lines
9.4 KiB
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<title>Reverse Execution - Debugging with GDB</title>
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<link rel="prev" href="Stopping.html#Stopping" title="Stopping">
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<link rel="next" href="Process-Record-and-Replay.html#Process-Record-and-Replay" title="Process Record and Replay">
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Free Documentation'', with the Front-Cover Texts being ``A GNU Manual,''
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<body>
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<div class="node">
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<a name="Reverse-Execution"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="Process-Record-and-Replay.html#Process-Record-and-Replay">Process Record and Replay</a>,
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Previous: <a rel="previous" accesskey="p" href="Stopping.html#Stopping">Stopping</a>,
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Up: <a rel="up" accesskey="u" href="index.html#Top">Top</a>
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<hr>
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</div>
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<h2 class="chapter">6 Running programs backward</h2>
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<p><a name="index-reverse-execution-441"></a><a name="index-running-programs-backward-442"></a>
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When you are debugging a program, it is not unusual to realize that
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you have gone too far, and some event of interest has already happened.
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If the target environment supports it, <span class="sc">gdb</span> can allow you to
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“rewind” the program by running it backward.
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<p>A target environment that supports reverse execution should be able
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to “undo” the changes in machine state that have taken place as the
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program was executing normally. Variables, registers etc. should
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revert to their previous values. Obviously this requires a great
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deal of sophistication on the part of the target environment; not
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all target environments can support reverse execution.
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<p>When a program is executed in reverse, the instructions that
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have most recently been executed are “un-executed”, in reverse
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order. The program counter runs backward, following the previous
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thread of execution in reverse. As each instruction is “un-executed”,
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the values of memory and/or registers that were changed by that
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instruction are reverted to their previous states. After executing
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a piece of source code in reverse, all side effects of that code
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should be “undone”, and all variables should be returned to their
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prior values<a rel="footnote" href="#fn-1" name="fnd-1"><sup>1</sup></a>.
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<p>If you are debugging in a target environment that supports
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reverse execution, <span class="sc">gdb</span> provides the following commands.
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<a name="index-reverse_002dcontinue-443"></a>
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<a name="index-rc-_0040r_007b_0028_0040code_007breverse_002dcontinue_007d_0029_007d-444"></a>
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<dl><dt><code>reverse-continue </code><span class="roman">[</span><var>ignore-count</var><span class="roman">]</span><dt><code>rc </code><span class="roman">[</span><var>ignore-count</var><span class="roman">]</span><dd>Beginning at the point where your program last stopped, start executing
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in reverse. Reverse execution will stop for breakpoints and synchronous
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exceptions (signals), just like normal execution. Behavior of
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asynchronous signals depends on the target environment.
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<p><a name="index-reverse_002dstep-445"></a><a name="index-rs-_0040r_007b_0028_0040code_007bstep_007d_0029_007d-446"></a><br><dt><code>reverse-step </code><span class="roman">[</span><var>count</var><span class="roman">]</span><dd>Run the program backward until control reaches the start of a
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different source line; then stop it, and return control to <span class="sc">gdb</span>.
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<p>Like the <code>step</code> command, <code>reverse-step</code> will only stop
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at the beginning of a source line. It “un-executes” the previously
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executed source line. If the previous source line included calls to
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debuggable functions, <code>reverse-step</code> will step (backward) into
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the called function, stopping at the beginning of the <em>last</em>
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statement in the called function (typically a return statement).
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<p>Also, as with the <code>step</code> command, if non-debuggable functions are
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called, <code>reverse-step</code> will run thru them backward without stopping.
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<p><a name="index-reverse_002dstepi-447"></a><a name="index-rsi-_0040r_007b_0028_0040code_007breverse_002dstepi_007d_0029_007d-448"></a><br><dt><code>reverse-stepi </code><span class="roman">[</span><var>count</var><span class="roman">]</span><dd>Reverse-execute one machine instruction. Note that the instruction
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to be reverse-executed is <em>not</em> the one pointed to by the program
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counter, but the instruction executed prior to that one. For instance,
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if the last instruction was a jump, <code>reverse-stepi</code> will take you
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back from the destination of the jump to the jump instruction itself.
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<p><a name="index-reverse_002dnext-449"></a><a name="index-rn-_0040r_007b_0028_0040code_007breverse_002dnext_007d_0029_007d-450"></a><br><dt><code>reverse-next </code><span class="roman">[</span><var>count</var><span class="roman">]</span><dd>Run backward to the beginning of the previous line executed in
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the current (innermost) stack frame. If the line contains function
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calls, they will be “un-executed” without stopping. Starting from
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the first line of a function, <code>reverse-next</code> will take you back
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to the caller of that function, <em>before</em> the function was called,
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just as the normal <code>next</code> command would take you from the last
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line of a function back to its return to its caller
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<a rel="footnote" href="#fn-2" name="fnd-2"><sup>2</sup></a>.
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<p><a name="index-reverse_002dnexti-451"></a><a name="index-rni-_0040r_007b_0028_0040code_007breverse_002dnexti_007d_0029_007d-452"></a><br><dt><code>reverse-nexti </code><span class="roman">[</span><var>count</var><span class="roman">]</span><dd>Like <code>nexti</code>, <code>reverse-nexti</code> executes a single instruction
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in reverse, except that called functions are “un-executed” atomically.
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That is, if the previously executed instruction was a return from
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another function, <code>reverse-nexti</code> will continue to execute
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in reverse until the call to that function (from the current stack
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frame) is reached.
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<p><a name="index-reverse_002dfinish-453"></a><br><dt><code>reverse-finish</code><dd>Just as the <code>finish</code> command takes you to the point where the
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current function returns, <code>reverse-finish</code> takes you to the point
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where it was called. Instead of ending up at the end of the current
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function invocation, you end up at the beginning.
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<p><a name="index-set-exec_002ddirection-454"></a><br><dt><code>set exec-direction</code><dd>Set the direction of target execution.
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<br><dt><code>set exec-direction reverse</code><dd><a name="index-execute-forward-or-backward-in-time-455"></a><span class="sc">gdb</span> will perform all execution commands in reverse, until the
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exec-direction mode is changed to “forward”. Affected commands include
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<code>step, stepi, next, nexti, continue, and finish</code>. The <code>return</code>
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command cannot be used in reverse mode.
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<br><dt><code>set exec-direction forward</code><dd><span class="sc">gdb</span> will perform all execution commands in the normal fashion.
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This is the default.
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</dl>
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<div class="footnote">
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<hr>
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<h4>Footnotes</h4><p class="footnote"><small>[<a name="fn-1" href="#fnd-1">1</a>]</small>
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Note that some side effects are easier to undo than others. For instance,
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memory and registers are relatively easy, but device I/O is hard. Some
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targets may be able undo things like device I/O, and some may not.
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<p>The contract between <span class="sc">gdb</span> and the reverse executing target
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requires only that the target do something reasonable when
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<span class="sc">gdb</span> tells it to execute backwards, and then report the
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results back to <span class="sc">gdb</span>. Whatever the target reports back to
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<span class="sc">gdb</span>, <span class="sc">gdb</span> will report back to the user. <span class="sc">gdb</span>
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assumes that the memory and registers that the target reports are in a
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consistant state, but <span class="sc">gdb</span> accepts whatever it is given.
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</p>
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<p class="footnote"><small>[<a name="fn-2" href="#fnd-2">2</a>]</small> Unless the code is too heavily optimized.</p>
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<hr></div>
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</body></html>
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