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159 lines
7.0 KiB
HTML
159 lines
7.0 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>Value History (Debugging with GDB)</title>
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<meta name="description" content="Value History (Debugging with GDB)">
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<meta name="keywords" content="Value History (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="Data.html#Data" rel="up" title="Data">
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<link href="Convenience-Vars.html#Convenience-Vars" rel="next" title="Convenience Vars">
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<link href="Pretty_002dPrinter-Commands.html#Pretty_002dPrinter-Commands" rel="prev" title="Pretty-Printer Commands">
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<style type="text/css">
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a.summary-letter {text-decoration: none}
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blockquote.indentedblock {margin-right: 0em}
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blockquote.smallindentedblock {margin-right: 0em; font-size: smaller}
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blockquote.smallquotation {font-size: smaller}
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div.display {margin-left: 3.2em}
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div.example {margin-left: 3.2em}
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div.lisp {margin-left: 3.2em}
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div.smalldisplay {margin-left: 3.2em}
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div.smallexample {margin-left: 3.2em}
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div.smalllisp {margin-left: 3.2em}
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kbd {font-style: oblique}
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pre.display {font-family: inherit}
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pre.format {font-family: inherit}
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pre.menu-comment {font-family: serif}
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pre.menu-preformatted {font-family: serif}
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pre.smalldisplay {font-family: inherit; font-size: smaller}
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pre.smallexample {font-size: smaller}
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pre.smallformat {font-family: inherit; font-size: smaller}
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pre.smalllisp {font-size: smaller}
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span.nolinebreak {white-space: nowrap}
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span.roman {font-family: initial; font-weight: normal}
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span.sansserif {font-family: sans-serif; font-weight: normal}
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ul.no-bullet {list-style: none}
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-->
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</style>
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</head>
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<body lang="en">
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<a name="Value-History"></a>
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<div class="header">
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<p>
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Next: <a href="Convenience-Vars.html#Convenience-Vars" accesskey="n" rel="next">Convenience Vars</a>, Previous: <a href="Pretty-Printing.html#Pretty-Printing" accesskey="p" rel="prev">Pretty Printing</a>, Up: <a href="Data.html#Data" accesskey="u" rel="up">Data</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="Value-History-1"></a>
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<h3 class="section">10.10 Value History</h3>
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<a name="index-value-history"></a>
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<a name="index-history-of-values-printed-by-GDB"></a>
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<p>Values printed by the <code>print</code> command are saved in the <small>GDB</small>
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<em>value history</em>. This allows you to refer to them in other expressions.
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Values are kept until the symbol table is re-read or discarded
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(for example with the <code>file</code> or <code>symbol-file</code> commands).
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When the symbol table changes, the value history is discarded,
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since the values may contain pointers back to the types defined in the
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symbol table.
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</p>
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<a name="index-_0024"></a>
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<a name="index-_0024_0024"></a>
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<a name="index-history-number"></a>
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<p>The values printed are given <em>history numbers</em> by which you can
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refer to them. These are successive integers starting with one.
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<code>print</code> shows you the history number assigned to a value by
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printing ‘<samp>$<var>num</var> = </samp>’ before the value; here <var>num</var> is the
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history number.
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</p>
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<p>To refer to any previous value, use ‘<samp>$</samp>’ followed by the value’s
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history number. The way <code>print</code> labels its output is designed to
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remind you of this. Just <code>$</code> refers to the most recent value in
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the history, and <code>$$</code> refers to the value before that.
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<code>$$<var>n</var></code> refers to the <var>n</var>th value from the end; <code>$$2</code>
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is the value just prior to <code>$$</code>, <code>$$1</code> is equivalent to
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<code>$$</code>, and <code>$$0</code> is equivalent to <code>$</code>.
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</p>
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<p>For example, suppose you have just printed a pointer to a structure and
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want to see the contents of the structure. It suffices to type
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</p>
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<div class="smallexample">
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<pre class="smallexample">p *$
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</pre></div>
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<p>If you have a chain of structures where the component <code>next</code> points
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to the next one, you can print the contents of the next one with this:
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</p>
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<div class="smallexample">
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<pre class="smallexample">p *$.next
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</pre></div>
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<p>You can print successive links in the chain by repeating this
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command—which you can do by just typing <tt class="key">RET</tt>.
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</p>
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<p>Note that the history records values, not expressions. If the value of
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<code>x</code> is 4 and you type these commands:
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</p>
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<div class="smallexample">
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<pre class="smallexample">print x
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set x=5
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</pre></div>
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<p>then the value recorded in the value history by the <code>print</code> command
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remains 4 even though the value of <code>x</code> has changed.
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</p>
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<dl compact="compact">
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<dd><a name="index-show-values"></a>
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</dd>
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<dt><code>show values</code></dt>
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<dd><p>Print the last ten values in the value history, with their item numbers.
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This is like ‘<samp>p $$9</samp>’ repeated ten times, except that <code>show
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values</code> does not change the history.
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</p>
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</dd>
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<dt><code>show values <var>n</var></code></dt>
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<dd><p>Print ten history values centered on history item number <var>n</var>.
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</p>
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</dd>
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<dt><code>show values +</code></dt>
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<dd><p>Print ten history values just after the values last printed. If no more
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values are available, <code>show values +</code> produces no display.
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</p></dd>
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</dl>
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<p>Pressing <tt class="key">RET</tt> to repeat <code>show values <var>n</var></code> has exactly the
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same effect as ‘<samp>show values +</samp>’.
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</p>
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<hr>
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<div class="header">
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<p>
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Next: <a href="Convenience-Vars.html#Convenience-Vars" accesskey="n" rel="next">Convenience Vars</a>, Previous: <a href="Pretty-Printing.html#Pretty-Printing" accesskey="p" rel="prev">Pretty Printing</a>, Up: <a href="Data.html#Data" accesskey="u" rel="up">Data</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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