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206 lines
8.9 KiB
HTML
206 lines
8.9 KiB
HTML
4 years ago
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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
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<html>
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<!-- Copyright (C) 1988-2019 Free Software Foundation, Inc.
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Permission is granted to copy, distribute and/or modify this document
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under the terms of the GNU Free Documentation License, Version 1.3 or
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any later version published by the Free Software Foundation; with the
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Invariant Sections being "Free Software" and "Free Software Needs
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Free Documentation", with the Front-Cover Texts being "A GNU Manual,"
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and with the Back-Cover Texts as in (a) below.
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(a) The FSF's Back-Cover Text is: "You are free to copy and modify
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this GNU Manual. Buying copies from GNU Press supports the FSF in
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developing GNU and promoting software freedom." -->
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<!-- Created by GNU Texinfo 6.4, http://www.gnu.org/software/texinfo/ -->
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<head>
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<title>Checkpoint/Restart (Debugging with GDB)</title>
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<meta name="description" content="Checkpoint/Restart (Debugging with GDB)">
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<meta name="keywords" content="Checkpoint/Restart (Debugging with GDB)">
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<meta name="resource-type" content="document">
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<meta name="distribution" content="global">
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<meta name="Generator" content="makeinfo">
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<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
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<link href="index.html#Top" rel="start" title="Top">
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<link href="Concept-Index.html#Concept-Index" rel="index" title="Concept Index">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="Running.html#Running" rel="up" title="Running">
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<link href="Stopping.html#Stopping" rel="next" title="Stopping">
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<link href="Forks.html#Forks" rel="prev" title="Forks">
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<style type="text/css">
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</style>
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</head>
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<body lang="en">
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<a name="Checkpoint_002fRestart"></a>
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<div class="header">
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<p>
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Previous: <a href="Forks.html#Forks" accesskey="p" rel="prev">Forks</a>, Up: <a href="Running.html#Running" accesskey="u" rel="up">Running</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
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</div>
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<hr>
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<a name="Setting-a-Bookmark-to-Return-to-Later"></a>
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<h3 class="section">4.12 Setting a <em>Bookmark</em> to Return to Later</h3>
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<a name="index-checkpoint"></a>
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<a name="index-restart"></a>
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<a name="index-bookmark"></a>
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<a name="index-snapshot-of-a-process"></a>
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<a name="index-rewind-program-state"></a>
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<p>On certain operating systems<a name="DOCF3" href="#FOOT3"><sup>3</sup></a>, <small>GDB</small> is able to save a <em>snapshot</em> of a
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program’s state, called a <em>checkpoint</em>, and come back to it
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later.
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</p>
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<p>Returning to a checkpoint effectively undoes everything that has
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happened in the program since the <code>checkpoint</code> was saved. This
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includes changes in memory, registers, and even (within some limits)
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system state. Effectively, it is like going back in time to the
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moment when the checkpoint was saved.
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</p>
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<p>Thus, if you’re stepping thru a program and you think you’re
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getting close to the point where things go wrong, you can save
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a checkpoint. Then, if you accidentally go too far and miss
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the critical statement, instead of having to restart your program
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from the beginning, you can just go back to the checkpoint and
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start again from there.
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</p>
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<p>This can be especially useful if it takes a lot of time or
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steps to reach the point where you think the bug occurs.
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</p>
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<p>To use the <code>checkpoint</code>/<code>restart</code> method of debugging:
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</p>
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<dl compact="compact">
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<dd><a name="index-checkpoint-1"></a>
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</dd>
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<dt><code>checkpoint</code></dt>
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<dd><p>Save a snapshot of the debugged program’s current execution state.
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The <code>checkpoint</code> command takes no arguments, but each checkpoint
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is assigned a small integer id, similar to a breakpoint id.
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</p>
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<a name="index-info-checkpoints"></a>
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</dd>
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<dt><code>info checkpoints</code></dt>
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<dd><p>List the checkpoints that have been saved in the current debugging
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session. For each checkpoint, the following information will be
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listed:
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</p>
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<dl compact="compact">
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<dt><code>Checkpoint ID</code></dt>
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<dt><code>Process ID</code></dt>
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<dt><code>Code Address</code></dt>
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<dt><code>Source line, or label</code></dt>
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</dl>
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<a name="index-restart-checkpoint_002did"></a>
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</dd>
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<dt><code>restart <var>checkpoint-id</var></code></dt>
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<dd><p>Restore the program state that was saved as checkpoint number
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<var>checkpoint-id</var>. All program variables, registers, stack frames
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etc. will be returned to the values that they had when the checkpoint
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was saved. In essence, gdb will “wind back the clock” to the point
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in time when the checkpoint was saved.
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</p>
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<p>Note that breakpoints, <small>GDB</small> variables, command history etc.
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are not affected by restoring a checkpoint. In general, a checkpoint
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only restores things that reside in the program being debugged, not in
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the debugger.
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</p>
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<a name="index-delete-checkpoint-checkpoint_002did"></a>
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</dd>
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<dt><code>delete checkpoint <var>checkpoint-id</var></code></dt>
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<dd><p>Delete the previously-saved checkpoint identified by <var>checkpoint-id</var>.
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</p>
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</dd>
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</dl>
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<p>Returning to a previously saved checkpoint will restore the user state
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of the program being debugged, plus a significant subset of the system
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(OS) state, including file pointers. It won’t “un-write” data from
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a file, but it will rewind the file pointer to the previous location,
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so that the previously written data can be overwritten. For files
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opened in read mode, the pointer will also be restored so that the
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previously read data can be read again.
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</p>
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<p>Of course, characters that have been sent to a printer (or other
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external device) cannot be “snatched back”, and characters received
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from eg. a serial device can be removed from internal program buffers,
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but they cannot be “pushed back” into the serial pipeline, ready to
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be received again. Similarly, the actual contents of files that have
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been changed cannot be restored (at this time).
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</p>
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<p>However, within those constraints, you actually can “rewind” your
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program to a previously saved point in time, and begin debugging it
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again — and you can change the course of events so as to debug a
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different execution path this time.
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</p>
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<a name="index-checkpoints-and-process-id"></a>
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<p>Finally, there is one bit of internal program state that will be
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different when you return to a checkpoint — the program’s process
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id. Each checkpoint will have a unique process id (or <var>pid</var>),
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and each will be different from the program’s original <var>pid</var>.
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If your program has saved a local copy of its process id, this could
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potentially pose a problem.
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</p>
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<a name="A-Non_002dobvious-Benefit-of-Using-Checkpoints"></a>
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<h4 class="subsection">4.12.1 A Non-obvious Benefit of Using Checkpoints</h4>
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<p>On some systems such as <small>GNU</small>/Linux, address space randomization
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is performed on new processes for security reasons. This makes it
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difficult or impossible to set a breakpoint, or watchpoint, on an
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absolute address if you have to restart the program, since the
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absolute location of a symbol will change from one execution to the
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next.
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</p>
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<p>A checkpoint, however, is an <em>identical</em> copy of a process.
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Therefore if you create a checkpoint at (eg.) the start of main,
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and simply return to that checkpoint instead of restarting the
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process, you can avoid the effects of address randomization and
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your symbols will all stay in the same place.
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</p>
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<div class="footnote">
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<hr>
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<h4 class="footnotes-heading">Footnotes</h4>
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<h3><a name="FOOT3" href="#DOCF3">(3)</a></h3>
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<p>Currently, only
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<small>GNU</small>/Linux.</p>
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</div>
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<hr>
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<div class="header">
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<p>
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Previous: <a href="Forks.html#Forks" accesskey="p" rel="prev">Forks</a>, Up: <a href="Running.html#Running" accesskey="u" rel="up">Running</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
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</div>
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</body>
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</html>
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