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diff --git a/share/doc/gdb/Checkpoint_002fRestart.html b/share/doc/gdb/Checkpoint_002fRestart.html new file mode 100644 index 0000000..90b9dba --- /dev/null +++ b/share/doc/gdb/Checkpoint_002fRestart.html @@ -0,0 +1,206 @@ +<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> +<html> +<!-- Copyright (C) 1988-2023 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." --> +<!-- Created by GNU Texinfo 5.1, http://www.gnu.org/software/texinfo/ --> +<head> +<title>Debugging with GDB: Checkpoint/Restart</title> + +<meta name="description" content="Debugging with GDB: Checkpoint/Restart"> +<meta name="keywords" content="Debugging with GDB: Checkpoint/Restart"> +<meta name="resource-type" content="document"> +<meta name="distribution" content="global"> +<meta name="Generator" content="makeinfo"> +<meta http-equiv="Content-Type" content="text/html; charset=utf-8"> +<link href="index.html#Top" rel="start" title="Top"> +<link href="Concept-Index.html#Concept-Index" rel="index" title="Concept Index"> +<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents"> +<link href="Running.html#Running" rel="up" title="Running"> +<link href="Stopping.html#Stopping" rel="next" title="Stopping"> +<link href="Forks.html#Forks" rel="previous" title="Forks"> +<style type="text/css"> +<!-- +a.summary-letter {text-decoration: none} +blockquote.smallquotation {font-size: smaller} +div.display {margin-left: 3.2em} +div.example {margin-left: 3.2em} +div.indentedblock {margin-left: 3.2em} +div.lisp {margin-left: 3.2em} +div.smalldisplay {margin-left: 3.2em} +div.smallexample {margin-left: 3.2em} +div.smallindentedblock {margin-left: 3.2em; font-size: smaller} +div.smalllisp {margin-left: 3.2em} +kbd {font-style:oblique} +pre.display {font-family: inherit} +pre.format {font-family: inherit} +pre.menu-comment {font-family: serif} +pre.menu-preformatted {font-family: serif} +pre.smalldisplay {font-family: inherit; font-size: smaller} +pre.smallexample {font-size: smaller} +pre.smallformat {font-family: inherit; font-size: smaller} +pre.smalllisp {font-size: smaller} +span.nocodebreak {white-space:nowrap} +span.nolinebreak {white-space:nowrap} +span.roman {font-family:serif; font-weight:normal} +span.sansserif {font-family:sans-serif; font-weight:normal} +ul.no-bullet {list-style: none} +--> +</style> + + +</head> + +<body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000"> +<a name="Checkpoint_002fRestart"></a> +<div class="header"> +<p> +Previous: <a href="Forks.html#Forks" accesskey="p" rel="previous">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> +</div> +<hr> +<a name="Setting-a-Bookmark-to-Return-to-Later"></a> +<h3 class="section">4.12 Setting a <em>Bookmark</em> to Return to Later</h3> + +<a name="index-checkpoint"></a> +<a name="index-restart"></a> +<a name="index-bookmark"></a> +<a name="index-snapshot-of-a-process"></a> +<a name="index-rewind-program-state"></a> + +<p>On certain operating systems<a name="DOCF4" href="#FOOT4"><sup>4</sup></a>, <small>GDB</small> is able to save a <em>snapshot</em> of a +program’s state, called a <em>checkpoint</em>, and come back to it +later. +</p> +<p>Returning to a checkpoint effectively undoes everything that has +happened in the program since the <code>checkpoint</code> was saved. This +includes changes in memory, registers, and even (within some limits) +system state. Effectively, it is like going back in time to the +moment when the checkpoint was saved. +</p> +<p>Thus, if you’re stepping thru a program and you think you’re +getting close to the point where things go wrong, you can save +a checkpoint. Then, if you accidentally go too far and miss +the critical statement, instead of having to restart your program +from the beginning, you can just go back to the checkpoint and +start again from there. +</p> +<p>This can be especially useful if it takes a lot of time or +steps to reach the point where you think the bug occurs. +</p> +<p>To use the <code>checkpoint</code>/<code>restart</code> method of debugging: +</p> +<dl compact="compact"> +<dd><a name="index-checkpoint-1"></a> +</dd> +<dt><code>checkpoint</code></dt> +<dd><p>Save a snapshot of the debugged program’s current execution state. +The <code>checkpoint</code> command takes no arguments, but each checkpoint +is assigned a small integer id, similar to a breakpoint id. +</p> +<a name="index-info-checkpoints"></a> +</dd> +<dt><code>info checkpoints</code></dt> +<dd><p>List the checkpoints that have been saved in the current debugging +session. For each checkpoint, the following information will be +listed: +</p> +<dl compact="compact"> +<dt><code>Checkpoint ID</code></dt> +<dt><code>Process ID</code></dt> +<dt><code>Code Address</code></dt> +<dt><code>Source line, or label</code></dt> +</dl> + +<a name="index-restart-checkpoint_002did"></a> +</dd> +<dt><code>restart <var>checkpoint-id</var></code></dt> +<dd><p>Restore the program state that was saved as checkpoint number +<var>checkpoint-id</var>. All program variables, registers, stack frames +etc. will be returned to the values that they had when the checkpoint +was saved. In essence, gdb will “wind back the clock” to the point +in time when the checkpoint was saved. +</p> +<p>Note that breakpoints, <small>GDB</small> variables, command history etc. +are not affected by restoring a checkpoint. In general, a checkpoint +only restores things that reside in the program being debugged, not in +the debugger. +</p> +<a name="index-delete-checkpoint-checkpoint_002did"></a> +</dd> +<dt><code>delete checkpoint <var>checkpoint-id</var></code></dt> +<dd><p>Delete the previously-saved checkpoint identified by <var>checkpoint-id</var>. +</p> +</dd> +</dl> + +<p>Returning to a previously saved checkpoint will restore the user state +of the program being debugged, plus a significant subset of the system +(OS) state, including file pointers. It won’t “un-write” data from +a file, but it will rewind the file pointer to the previous location, +so that the previously written data can be overwritten. For files +opened in read mode, the pointer will also be restored so that the +previously read data can be read again. +</p> +<p>Of course, characters that have been sent to a printer (or other +external device) cannot be “snatched back”, and characters received +from eg. a serial device can be removed from internal program buffers, +but they cannot be “pushed back” into the serial pipeline, ready to +be received again. Similarly, the actual contents of files that have +been changed cannot be restored (at this time). +</p> +<p>However, within those constraints, you actually can “rewind” your +program to a previously saved point in time, and begin debugging it +again — and you can change the course of events so as to debug a +different execution path this time. +</p> +<a name="index-checkpoints-and-process-id"></a> +<p>Finally, there is one bit of internal program state that will be +different when you return to a checkpoint — the program’s process +id. Each checkpoint will have a unique process id (or <var>pid</var>), +and each will be different from the program’s original <var>pid</var>. +If your program has saved a local copy of its process id, this could +potentially pose a problem. +</p> +<a name="A-Non_002dobvious-Benefit-of-Using-Checkpoints"></a> +<h4 class="subsection">4.12.1 A Non-obvious Benefit of Using Checkpoints</h4> + +<p>On some systems such as <small>GNU</small>/Linux, address space randomization +is performed on new processes for security reasons. This makes it +difficult or impossible to set a breakpoint, or watchpoint, on an +absolute address if you have to restart the program, since the +absolute location of a symbol will change from one execution to the +next. +</p> +<p>A checkpoint, however, is an <em>identical</em> copy of a process. +Therefore if you create a checkpoint at (eg.) the start of main, +and simply return to that checkpoint instead of restarting the +process, you can avoid the effects of address randomization and +your symbols will all stay in the same place. +</p> +<div class="footnote"> +<hr> +<h4 class="footnotes-heading">Footnotes</h4> + +<h3><a name="FOOT4" href="#DOCF4">(4)</a></h3> +<p>Currently, only +<small>GNU</small>/Linux.</p> +</div> +<hr> +<div class="header"> +<p> +Previous: <a href="Forks.html#Forks" accesskey="p" rel="previous">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> +</div> + + + +</body> +</html> |