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342 lines
13 KiB
HTML
342 lines
13 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>Macros (Debugging with GDB)</title>
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<meta name="description" content="Macros (Debugging with GDB)">
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<meta name="keywords" content="Macros (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="index.html#Top" rel="up" title="Top">
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<link href="Tracepoints.html#Tracepoints" rel="next" title="Tracepoints">
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<link href="Tail-Call-Frames.html#Tail-Call-Frames" rel="prev" title="Tail Call Frames">
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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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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="Macros"></a>
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<div class="header">
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<p>
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Next: <a href="Tracepoints.html#Tracepoints" accesskey="n" rel="next">Tracepoints</a>, Previous: <a href="Optimized-Code.html#Optimized-Code" accesskey="p" rel="prev">Optimized Code</a>, Up: <a href="index.html#Top" accesskey="u" rel="up">Top</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="C-Preprocessor-Macros"></a>
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<h2 class="chapter">12 C Preprocessor Macros</h2>
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<p>Some languages, such as C and C<tt>++</tt>, provide a way to define and invoke
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“preprocessor macros” which expand into strings of tokens.
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<small>GDB</small> can evaluate expressions containing macro invocations, show
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the result of macro expansion, and show a macro’s definition, including
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where it was defined.
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</p>
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<p>You may need to compile your program specially to provide <small>GDB</small>
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with information about preprocessor macros. Most compilers do not
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include macros in their debugging information, even when you compile
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with the <samp>-g</samp> flag. See <a href="Compilation.html#Compilation">Compilation</a>.
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</p>
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<p>A program may define a macro at one point, remove that definition later,
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and then provide a different definition after that. Thus, at different
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points in the program, a macro may have different definitions, or have
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no definition at all. If there is a current stack frame, <small>GDB</small>
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uses the macros in scope at that frame’s source code line. Otherwise,
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<small>GDB</small> uses the macros in scope at the current listing location;
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see <a href="List.html#List">List</a>.
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</p>
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<p>Whenever <small>GDB</small> evaluates an expression, it always expands any
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macro invocations present in the expression. <small>GDB</small> also provides
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the following commands for working with macros explicitly.
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</p>
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<dl compact="compact">
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<dd>
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<a name="index-macro-expand"></a>
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<a name="index-macro-expansion_002c-showing-the-results-of-preprocessor"></a>
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<a name="index-preprocessor-macro-expansion_002c-showing-the-results-of"></a>
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<a name="index-expanding-preprocessor-macros"></a>
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</dd>
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<dt><code>macro expand <var>expression</var></code></dt>
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<dt><code>macro exp <var>expression</var></code></dt>
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<dd><p>Show the results of expanding all preprocessor macro invocations in
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<var>expression</var>. Since <small>GDB</small> simply expands macros, but does
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not parse the result, <var>expression</var> need not be a valid expression;
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it can be any string of tokens.
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</p>
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<a name="index-macro-exp1"></a>
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</dd>
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<dt><code>macro expand-once <var>expression</var></code></dt>
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<dt><code>macro exp1 <var>expression</var></code></dt>
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<dd><a name="index-expand-macro-once"></a>
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<p><i>(This command is not yet implemented.)</i> Show the results of
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expanding those preprocessor macro invocations that appear explicitly in
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<var>expression</var>. Macro invocations appearing in that expansion are
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left unchanged. This command allows you to see the effect of a
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particular macro more clearly, without being confused by further
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expansions. Since <small>GDB</small> simply expands macros, but does not
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parse the result, <var>expression</var> need not be a valid expression; it
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can be any string of tokens.
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</p>
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<a name="index-info-macro"></a>
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<a name="index-macro-definition_002c-showing"></a>
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<a name="index-definition-of-a-macro_002c-showing"></a>
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<a name="index-macros_002c-from-debug-info"></a>
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</dd>
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<dt><code>info macro [-a|-all] [--] <var>macro</var></code></dt>
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<dd><p>Show the current definition or all definitions of the named <var>macro</var>,
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and describe the source location or compiler command-line where that
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definition was established. The optional double dash is to signify the end of
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argument processing and the beginning of <var>macro</var> for non C-like macros where
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the macro may begin with a hyphen.
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</p>
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<a name="index-info-macros"></a>
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</dd>
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<dt><code>info macros <var>location</var></code></dt>
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<dd><p>Show all macro definitions that are in effect at the location specified
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by <var>location</var>, and describe the source location or compiler
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command-line where those definitions were established.
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</p>
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<a name="index-macro-define"></a>
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<a name="index-user_002ddefined-macros"></a>
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<a name="index-defining-macros-interactively"></a>
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<a name="index-macros_002c-user_002ddefined"></a>
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</dd>
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<dt><code>macro define <var>macro</var> <var>replacement-list</var></code></dt>
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<dt><code>macro define <var>macro</var>(<var>arglist</var>) <var>replacement-list</var></code></dt>
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<dd><p>Introduce a definition for a preprocessor macro named <var>macro</var>,
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invocations of which are replaced by the tokens given in
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<var>replacement-list</var>. The first form of this command defines an
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“object-like” macro, which takes no arguments; the second form
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defines a “function-like” macro, which takes the arguments given in
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<var>arglist</var>.
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</p>
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<p>A definition introduced by this command is in scope in every
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expression evaluated in <small>GDB</small>, until it is removed with the
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<code>macro undef</code> command, described below. The definition overrides
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all definitions for <var>macro</var> present in the program being debugged,
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as well as any previous user-supplied definition.
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</p>
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<a name="index-macro-undef"></a>
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</dd>
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<dt><code>macro undef <var>macro</var></code></dt>
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<dd><p>Remove any user-supplied definition for the macro named <var>macro</var>.
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This command only affects definitions provided with the <code>macro
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define</code> command, described above; it cannot remove definitions present
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in the program being debugged.
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</p>
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<a name="index-macro-list"></a>
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</dd>
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<dt><code>macro list</code></dt>
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<dd><p>List all the macros defined using the <code>macro define</code> command.
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</p></dd>
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</dl>
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<a name="index-macros_002c-example-of-debugging-with"></a>
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<p>Here is a transcript showing the above commands in action. First, we
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show our source files:
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</p>
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<div class="smallexample">
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<pre class="smallexample">$ cat sample.c
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#include <stdio.h>
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#include "sample.h"
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#define M 42
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#define ADD(x) (M + x)
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main ()
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{
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#define N 28
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printf ("Hello, world!\n");
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#undef N
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printf ("We're so creative.\n");
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#define N 1729
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printf ("Goodbye, world!\n");
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}
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$ cat sample.h
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#define Q <
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$
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</pre></div>
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<p>Now, we compile the program using the <small>GNU</small> C compiler,
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<small>GCC</small>. We pass the <samp>-gdwarf-2</samp><a name="DOCF13" href="#FOOT13"><sup>13</sup></a> <em>and</em> <samp>-g3</samp> flags to ensure the compiler
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includes information about preprocessor macros in the debugging
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information.
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</p>
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<div class="smallexample">
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<pre class="smallexample">$ gcc -gdwarf-2 -g3 sample.c -o sample
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$
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</pre></div>
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<p>Now, we start <small>GDB</small> on our sample program:
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</p>
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<div class="smallexample">
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<pre class="smallexample">$ gdb -nw sample
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GNU gdb 2002-05-06-cvs
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Copyright 2002 Free Software Foundation, Inc.
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GDB is free software, …
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(gdb)
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</pre></div>
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<p>We can expand macros and examine their definitions, even when the
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program is not running. <small>GDB</small> uses the current listing position
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to decide which macro definitions are in scope:
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</p>
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<div class="smallexample">
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<pre class="smallexample">(gdb) list main
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3
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4 #define M 42
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5 #define ADD(x) (M + x)
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6
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7 main ()
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8 {
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9 #define N 28
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10 printf ("Hello, world!\n");
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11 #undef N
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12 printf ("We're so creative.\n");
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(gdb) info macro ADD
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Defined at /home/jimb/gdb/macros/play/sample.c:5
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#define ADD(x) (M + x)
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(gdb) info macro Q
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Defined at /home/jimb/gdb/macros/play/sample.h:1
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included at /home/jimb/gdb/macros/play/sample.c:2
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#define Q <
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(gdb) macro expand ADD(1)
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expands to: (42 + 1)
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(gdb) macro expand-once ADD(1)
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expands to: once (M + 1)
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(gdb)
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</pre></div>
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<p>In the example above, note that <code>macro expand-once</code> expands only
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the macro invocation explicit in the original text — the invocation of
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<code>ADD</code> — but does not expand the invocation of the macro <code>M</code>,
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which was introduced by <code>ADD</code>.
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</p>
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<p>Once the program is running, <small>GDB</small> uses the macro definitions in
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force at the source line of the current stack frame:
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</p>
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<div class="smallexample">
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<pre class="smallexample">(gdb) break main
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Breakpoint 1 at 0x8048370: file sample.c, line 10.
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(gdb) run
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Starting program: /home/jimb/gdb/macros/play/sample
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Breakpoint 1, main () at sample.c:10
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10 printf ("Hello, world!\n");
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(gdb)
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</pre></div>
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<p>At line 10, the definition of the macro <code>N</code> at line 9 is in force:
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</p>
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<div class="smallexample">
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<pre class="smallexample">(gdb) info macro N
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Defined at /home/jimb/gdb/macros/play/sample.c:9
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#define N 28
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(gdb) macro expand N Q M
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expands to: 28 < 42
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(gdb) print N Q M
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$1 = 1
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(gdb)
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</pre></div>
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<p>As we step over directives that remove <code>N</code>’s definition, and then
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give it a new definition, <small>GDB</small> finds the definition (or lack
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thereof) in force at each point:
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</p>
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<div class="smallexample">
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<pre class="smallexample">(gdb) next
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Hello, world!
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12 printf ("We're so creative.\n");
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(gdb) info macro N
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The symbol `N' has no definition as a C/C++ preprocessor macro
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at /home/jimb/gdb/macros/play/sample.c:12
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(gdb) next
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We're so creative.
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14 printf ("Goodbye, world!\n");
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(gdb) info macro N
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Defined at /home/jimb/gdb/macros/play/sample.c:13
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#define N 1729
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(gdb) macro expand N Q M
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expands to: 1729 < 42
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(gdb) print N Q M
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$2 = 0
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(gdb)
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</pre></div>
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<p>In addition to source files, macros can be defined on the compilation command
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line using the <samp>-D<var>name</var>=<var>value</var></samp> syntax. For macros defined in
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such a way, <small>GDB</small> displays the location of their definition as line zero
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of the source file submitted to the compiler.
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||
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</p>
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<div class="smallexample">
|
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<pre class="smallexample">(gdb) info macro __STDC__
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Defined at /home/jimb/gdb/macros/play/sample.c:0
|
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|
|
-D__STDC__=1
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||
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|
(gdb)
|
||
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</pre></div>
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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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||
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<h3><a name="FOOT13" href="#DOCF13">(13)</a></h3>
|
||
|
|
<p>This is the
|
||
|
|
minimum. Recent versions of <small>GCC</small> support <samp>-gdwarf-3</samp>
|
||
|
|
and <samp>-gdwarf-4</samp>; we recommend always choosing the most recent
|
||
|
|
version of DWARF.</p>
|
||
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</div>
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||
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<hr>
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<div class="header">
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||
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<p>
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||
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Next: <a href="Tracepoints.html#Tracepoints" accesskey="n" rel="next">Tracepoints</a>, Previous: <a href="Optimized-Code.html#Optimized-Code" accesskey="p" rel="prev">Optimized Code</a>, Up: <a href="index.html#Top" accesskey="u" rel="up">Top</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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