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151 lines
6.8 KiB
HTML
151 lines
6.8 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>Arrays (Debugging with GDB)</title>
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<meta name="description" content="Arrays (Debugging with GDB)">
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<meta name="keywords" content="Arrays (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="Output-Formats.html#Output-Formats" rel="next" title="Output Formats">
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<link href="Variables.html#Variables" rel="prev" title="Variables">
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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.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.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="Arrays"></a>
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<div class="header">
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<p>
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Next: <a href="Output-Formats.html#Output-Formats" accesskey="n" rel="next">Output Formats</a>, Previous: <a href="Variables.html#Variables" accesskey="p" rel="prev">Variables</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="Artificial-Arrays"></a>
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<h3 class="section">10.4 Artificial Arrays</h3>
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<a name="index-artificial-array"></a>
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<a name="index-arrays"></a>
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<a name="index-_0040_002c-referencing-memory-as-an-array"></a>
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<p>It is often useful to print out several successive objects of the
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same type in memory; a section of an array, or an array of
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dynamically determined size for which only a pointer exists in the
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program.
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</p>
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<p>You can do this by referring to a contiguous span of memory as an
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<em>artificial array</em>, using the binary operator ‘<samp>@</samp>’. The left
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operand of ‘<samp>@</samp>’ should be the first element of the desired array
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and be an individual object. The right operand should be the desired length
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of the array. The result is an array value whose elements are all of
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the type of the left argument. The first element is actually the left
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argument; the second element comes from bytes of memory immediately
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following those that hold the first element, and so on. Here is an
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example. If a program says
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</p>
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<div class="smallexample">
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<pre class="smallexample">int *array = (int *) malloc (len * sizeof (int));
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</pre></div>
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<p>you can print the contents of <code>array</code> with
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</p>
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<div class="smallexample">
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<pre class="smallexample">p *array@len
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</pre></div>
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<p>The left operand of ‘<samp>@</samp>’ must reside in memory. Array values made
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with ‘<samp>@</samp>’ in this way behave just like other arrays in terms of
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subscripting, and are coerced to pointers when used in expressions.
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Artificial arrays most often appear in expressions via the value history
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(see <a href="Value-History.html#Value-History">Value History</a>), after printing one out.
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</p>
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<p>Another way to create an artificial array is to use a cast.
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This re-interprets a value as if it were an array.
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The value need not be in memory:
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</p><div class="smallexample">
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<pre class="smallexample">(gdb) p/x (short[2])0x12345678
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$1 = {0x1234, 0x5678}
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</pre></div>
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<p>As a convenience, if you leave the array length out (as in
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‘<samp>(<var>type</var>[])<var>value</var></samp>’) <small>GDB</small> calculates the size to fill
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the value (as ‘<samp>sizeof(<var>value</var>)/sizeof(<var>type</var>)</samp>’:
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</p><div class="smallexample">
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<pre class="smallexample">(gdb) p/x (short[])0x12345678
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$2 = {0x1234, 0x5678}
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</pre></div>
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<p>Sometimes the artificial array mechanism is not quite enough; in
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moderately complex data structures, the elements of interest may not
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actually be adjacent—for example, if you are interested in the values
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of pointers in an array. One useful work-around in this situation is
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to use a convenience variable (see <a href="Convenience-Vars.html#Convenience-Vars">Convenience
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Variables</a>) as a counter in an expression that prints the first
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interesting value, and then repeat that expression via <tt class="key">RET</tt>. For
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instance, suppose you have an array <code>dtab</code> of pointers to
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structures, and you are interested in the values of a field <code>fv</code>
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in each structure. Here is an example of what you might type:
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</p>
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<div class="smallexample">
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<pre class="smallexample">set $i = 0
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p dtab[$i++]->fv
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<span class="key">RET</span>
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<span class="key">RET</span>
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…
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</pre></div>
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<hr>
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<div class="header">
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<p>
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Next: <a href="Output-Formats.html#Output-Formats" accesskey="n" rel="next">Output Formats</a>, Previous: <a href="Variables.html#Variables" accesskey="p" rel="prev">Variables</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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