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196 lines
9.4 KiB
HTML
196 lines
9.4 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>Auto Display (Debugging with GDB)</title>
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<meta name="description" content="Auto Display (Debugging with GDB)">
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<meta name="keywords" content="Auto Display (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="Print-Settings.html#Print-Settings" rel="next" title="Print Settings">
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<link href="Memory.html#Memory" rel="prev" title="Memory">
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<style type="text/css">
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<!--
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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="Auto-Display"></a>
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<div class="header">
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<p>
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Next: <a href="Print-Settings.html#Print-Settings" accesskey="n" rel="next">Print Settings</a>, Previous: <a href="Memory.html#Memory" accesskey="p" rel="prev">Memory</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="Automatic-Display"></a>
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<h3 class="section">10.7 Automatic Display</h3>
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<a name="index-automatic-display"></a>
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<a name="index-display-of-expressions"></a>
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<p>If you find that you want to print the value of an expression frequently
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(to see how it changes), you might want to add it to the <em>automatic
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display list</em> so that <small>GDB</small> prints its value each time your program stops.
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Each expression added to the list is given a number to identify it;
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to remove an expression from the list, you specify that number.
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The automatic display looks like this:
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</p>
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<div class="smallexample">
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<pre class="smallexample">2: foo = 38
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3: bar[5] = (struct hack *) 0x3804
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</pre></div>
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<p>This display shows item numbers, expressions and their current values. As with
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displays you request manually using <code>x</code> or <code>print</code>, you can
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specify the output format you prefer; in fact, <code>display</code> decides
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whether to use <code>print</code> or <code>x</code> depending your format
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specification—it uses <code>x</code> if you specify either the ‘<samp>i</samp>’
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or ‘<samp>s</samp>’ format, or a unit size; otherwise it uses <code>print</code>.
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</p>
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<dl compact="compact">
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<dd><a name="index-display"></a>
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</dd>
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<dt><code>display <var>expr</var></code></dt>
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<dd><p>Add the expression <var>expr</var> to the list of expressions to display
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each time your program stops. See <a href="Expressions.html#Expressions">Expressions</a>.
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</p>
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<p><code>display</code> does not repeat if you press <tt class="key">RET</tt> again after using it.
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</p>
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</dd>
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<dt><code>display/<var>fmt</var> <var>expr</var></code></dt>
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<dd><p>For <var>fmt</var> specifying only a display format and not a size or
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count, add the expression <var>expr</var> to the auto-display list but
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arrange to display it each time in the specified format <var>fmt</var>.
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See <a href="Output-Formats.html#Output-Formats">Output Formats</a>.
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</p>
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</dd>
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<dt><code>display/<var>fmt</var> <var>addr</var></code></dt>
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<dd><p>For <var>fmt</var> ‘<samp>i</samp>’ or ‘<samp>s</samp>’, or including a unit-size or a
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number of units, add the expression <var>addr</var> as a memory address to
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be examined each time your program stops. Examining means in effect
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doing ‘<samp>x/<var>fmt</var> <var>addr</var></samp>’. See <a href="Memory.html#Memory">Examining Memory</a>.
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</p></dd>
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</dl>
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<p>For example, ‘<samp>display/i $pc</samp>’ can be helpful, to see the machine
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instruction about to be executed each time execution stops (‘<samp>$pc</samp>’
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is a common name for the program counter; see <a href="Registers.html#Registers">Registers</a>).
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</p>
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<dl compact="compact">
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<dd><a name="index-delete-display"></a>
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<a name="index-undisplay"></a>
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</dd>
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<dt><code>undisplay <var>dnums</var>…</code></dt>
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<dt><code>delete display <var>dnums</var>…</code></dt>
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<dd><p>Remove items from the list of expressions to display. Specify the
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numbers of the displays that you want affected with the command
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argument <var>dnums</var>. It can be a single display number, one of the
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numbers shown in the first field of the ‘<samp>info display</samp>’ display;
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or it could be a range of display numbers, as in <code>2-4</code>.
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</p>
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<p><code>undisplay</code> does not repeat if you press <tt class="key">RET</tt> after using it.
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(Otherwise you would just get the error ‘<samp>No display number …</samp>’.)
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</p>
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<a name="index-disable-display"></a>
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</dd>
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<dt><code>disable display <var>dnums</var>…</code></dt>
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<dd><p>Disable the display of item numbers <var>dnums</var>. A disabled display
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item is not printed automatically, but is not forgotten. It may be
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enabled again later. Specify the numbers of the displays that you
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want affected with the command argument <var>dnums</var>. It can be a
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single display number, one of the numbers shown in the first field of
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the ‘<samp>info display</samp>’ display; or it could be a range of display
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numbers, as in <code>2-4</code>.
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</p>
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<a name="index-enable-display"></a>
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</dd>
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<dt><code>enable display <var>dnums</var>…</code></dt>
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<dd><p>Enable display of item numbers <var>dnums</var>. It becomes effective once
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again in auto display of its expression, until you specify otherwise.
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Specify the numbers of the displays that you want affected with the
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command argument <var>dnums</var>. It can be a single display number, one
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of the numbers shown in the first field of the ‘<samp>info display</samp>’
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display; or it could be a range of display numbers, as in <code>2-4</code>.
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</p>
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</dd>
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<dt><code>display</code></dt>
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<dd><p>Display the current values of the expressions on the list, just as is
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done when your program stops.
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</p>
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<a name="index-info-display"></a>
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</dd>
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<dt><code>info display</code></dt>
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<dd><p>Print the list of expressions previously set up to display
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automatically, each one with its item number, but without showing the
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values. This includes disabled expressions, which are marked as such.
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It also includes expressions which would not be displayed right now
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because they refer to automatic variables not currently available.
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</p></dd>
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</dl>
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<a name="index-display-disabled-out-of-scope"></a>
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<p>If a display expression refers to local variables, then it does not make
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sense outside the lexical context for which it was set up. Such an
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expression is disabled when execution enters a context where one of its
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variables is not defined. For example, if you give the command
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<code>display last_char</code> while inside a function with an argument
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<code>last_char</code>, <small>GDB</small> displays this argument while your program
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continues to stop inside that function. When it stops elsewhere—where
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there is no variable <code>last_char</code>—the display is disabled
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automatically. The next time your program stops where <code>last_char</code>
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is meaningful, you can enable the display expression once again.
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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="Print-Settings.html#Print-Settings" accesskey="n" rel="next">Print Settings</a>, Previous: <a href="Memory.html#Memory" accesskey="p" rel="prev">Memory</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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