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 <body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000">
 <a name="Macros"></a>
 <div class="header">
 <p>
 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> &nbsp; [<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="C-Preprocessor-Macros"></a>
 <h2 class="chapter">12 C Preprocessor Macros</h2>

 <p>Some languages, such as C and C<tt>++</tt>, provide a way to define and invoke
 &ldquo;preprocessor macros&rdquo; which expand into strings of tokens.
 <small>GDB</small> can evaluate expressions containing macro invocations, show
 the result of macro expansion, and show a macro&rsquo;s definition, including
 where it was defined.
 </p>
 <p>You may need to compile your program specially to provide <small>GDB</small>
 with information about preprocessor macros.  Most compilers do not
 include macros in their debugging information, even when you compile
 with the <samp>-g</samp> flag.  See <a href="Compilation.html#Compilation">Compilation</a>.
 </p>
 <p>A program may define a macro at one point, remove that definition later,
 and then provide a different definition after that.  Thus, at different
 points in the program, a macro may have different definitions, or have
 no definition at all.  If there is a current stack frame, <small>GDB</small>
 uses the macros in scope at that frame&rsquo;s source code line.  Otherwise,
 <small>GDB</small> uses the macros in scope at the current listing location;
 see <a href="List.html#List">List</a>.
 </p>
 <p>Whenever <small>GDB</small> evaluates an expression, it always expands any
 macro invocations present in the expression.  <small>GDB</small> also provides
 the following commands for working with macros explicitly.
 </p>
 <dl compact="compact">
 <dd>
 <a name="index-macro-expand"></a>
 <a name="index-macro-expansion_002c-showing-the-results-of-preprocessor"></a>
 <a name="index-preprocessor-macro-expansion_002c-showing-the-results-of"></a>
 <a name="index-expanding-preprocessor-macros"></a>
 </dd>
 <dt><code>macro expand <var>expression</var></code></dt>
 <dt><code>macro exp <var>expression</var></code></dt>
 <dd><p>Show the results of expanding all preprocessor macro invocations in
 <var>expression</var>.  Since <small>GDB</small> simply expands macros, but does
 not parse the result, <var>expression</var> need not be a valid expression;
 it can be any string of tokens.
 </p>
 <a name="index-macro-exp1"></a>
 </dd>
 <dt><code>macro expand-once <var>expression</var></code></dt>
 <dt><code>macro exp1 <var>expression</var></code></dt>
 <dd><a name="index-expand-macro-once"></a>
 <p><i>(This command is not yet implemented.)</i>  Show the results of
 expanding those preprocessor macro invocations that appear explicitly in
 <var>expression</var>.  Macro invocations appearing in that expansion are
 left unchanged.  This command allows you to see the effect of a
 particular macro more clearly, without being confused by further
 expansions.  Since <small>GDB</small> simply expands macros, but does not
 parse the result, <var>expression</var> need not be a valid expression; it
 can be any string of tokens.
 </p>
 <a name="index-info-macro"></a>
 <a name="index-macro-definition_002c-showing"></a>
 <a name="index-definition-of-a-macro_002c-showing"></a>
 <a name="index-macros_002c-from-debug-info"></a>
 </dd>
 <dt><code>info macro [-a|-all] [--] <var>macro</var></code></dt>
 <dd><p>Show the current definition or all definitions of the named <var>macro</var>,
 and describe the source location or compiler command-line where that
 definition was established.  The optional double dash is to signify the end of
 argument processing and the beginning of <var>macro</var> for non C-like macros where
 the macro may begin with a hyphen.
 </p>
 <a name="index-info-macros"></a>
 </dd>
 <dt><code>info macros <var>linespec</var></code></dt>
 <dd><p>Show all macro definitions that are in effect at the location specified
 by <var>linespec</var>,  and describe the source location or compiler
 command-line where those definitions were established.
 </p>
 <a name="index-macro-define"></a>
 <a name="index-user_002ddefined-macros"></a>
 <a name="index-defining-macros-interactively"></a>
 <a name="index-macros_002c-user_002ddefined"></a>
 </dd>
 <dt><code>macro define <var>macro</var> <var>replacement-list</var></code></dt>
 <dt><code>macro define <var>macro</var>(<var>arglist</var>) <var>replacement-list</var></code></dt>
 <dd><p>Introduce a definition for a preprocessor macro named <var>macro</var>,
 invocations of which are replaced by the tokens given in
 <var>replacement-list</var>.  The first form of this command defines an
 &ldquo;object-like&rdquo; macro, which takes no arguments; the second form
 defines a &ldquo;function-like&rdquo; macro, which takes the arguments given in
 <var>arglist</var>.
 </p>
 <p>A definition introduced by this command is in scope in every
 expression evaluated in <small>GDB</small>, until it is removed with the
 <code>macro undef</code> command, described below.  The definition overrides
 all definitions for <var>macro</var> present in the program being debugged,
 as well as any previous user-supplied definition.
 </p>
 <a name="index-macro-undef"></a>
 </dd>
 <dt><code>macro undef <var>macro</var></code></dt>
 <dd><p>Remove any user-supplied definition for the macro named <var>macro</var>.
 This command only affects definitions provided with the <code>macro
 define</code> command, described above; it cannot remove definitions present
 in the program being debugged.
 </p>
 <a name="index-macro-list"></a>
 </dd>
 <dt><code>macro list</code></dt>
 <dd><p>List all the macros defined using the <code>macro define</code> command.
 </p></dd>
 </dl>

 <a name="index-macros_002c-example-of-debugging-with"></a>
 <p>Here is a transcript showing the above commands in action.  First, we
 show our source files:
 </p>
 <div class="smallexample">
 <pre class="smallexample">$ cat sample.c
 #include &lt;stdio.h&gt;
 #include &quot;sample.h&quot;

 #define M 42
 #define ADD(x) (M + x)

 main ()
 {
 #define N 28
   printf (&quot;Hello, world!\n&quot;);
 #undef N
   printf (&quot;We're so creative.\n&quot;);
 #define N 1729
   printf (&quot;Goodbye, world!\n&quot;);
 }
 $ cat sample.h
 #define Q &lt;
 $
 </pre></div>

 <p>Now, we compile the program using the <small>GNU</small> C compiler,
 <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
 includes information about preprocessor macros in the debugging
 information.
 </p>
 <div class="smallexample">
 <pre class="smallexample">$ gcc -gdwarf-2 -g3 sample.c -o sample
 $
 </pre></div>

 <p>Now, we start <small>GDB</small> on our sample program:
 </p>
 <div class="smallexample">
 <pre class="smallexample">$ gdb -nw sample
 GNU gdb 2002-05-06-cvs
 Copyright 2002 Free Software Foundation, Inc.
 GDB is free software, &hellip;
 (gdb)
 </pre></div>

 <p>We can expand macros and examine their definitions, even when the
 program is not running.  <small>GDB</small> uses the current listing position
 to decide which macro definitions are in scope:
 </p>
 <div class="smallexample">
 <pre class="smallexample">(gdb) list main
 3
 4       #define M 42
 5       #define ADD(x) (M + x)
 6
 7       main ()
 8       {
 9       #define N 28
 10        printf (&quot;Hello, world!\n&quot;);
 11      #undef N
 12        printf (&quot;We're so creative.\n&quot;);
 (gdb) info macro ADD
 Defined at /home/jimb/gdb/macros/play/sample.c:5
 #define ADD(x) (M + x)
 (gdb) info macro Q
 Defined at /home/jimb/gdb/macros/play/sample.h:1
   included at /home/jimb/gdb/macros/play/sample.c:2
 #define Q &lt;
 (gdb) macro expand ADD(1)
 expands to: (42 + 1)
 (gdb) macro expand-once ADD(1)
 expands to: once (M + 1)
 (gdb)
 </pre></div>

 <p>In the example above, note that <code>macro expand-once</code> expands only
 the macro invocation explicit in the original text &mdash; the invocation of
 <code>ADD</code> &mdash; but does not expand the invocation of the macro <code>M</code>,
 which was introduced by <code>ADD</code>.
 </p>
 <p>Once the program is running, <small>GDB</small> uses the macro definitions in
 force at the source line of the current stack frame:
 </p>
 <div class="smallexample">
 <pre class="smallexample">(gdb) break main
 Breakpoint 1 at 0x8048370: file sample.c, line 10.
 (gdb) run
 Starting program: /home/jimb/gdb/macros/play/sample

 Breakpoint 1, main () at sample.c:10
 10        printf (&quot;Hello, world!\n&quot;);
 (gdb)
 </pre></div>

 <p>At line 10, the definition of the macro <code>N</code> at line 9 is in force:
 </p>
 <div class="smallexample">
 <pre class="smallexample">(gdb) info macro N
 Defined at /home/jimb/gdb/macros/play/sample.c:9
 #define N 28
 (gdb) macro expand N Q M
 expands to: 28 &lt; 42
 (gdb) print N Q M
 $1 = 1
 (gdb)
 </pre></div>

 <p>As we step over directives that remove <code>N</code>&rsquo;s definition, and then
 give it a new definition, <small>GDB</small> finds the definition (or lack
 thereof) in force at each point:
 </p>
 <div class="smallexample">
 <pre class="smallexample">(gdb) next
 Hello, world!
 12        printf (&quot;We're so creative.\n&quot;);
 (gdb) info macro N
 The symbol `N' has no definition as a C/C++ preprocessor macro
 at /home/jimb/gdb/macros/play/sample.c:12
 (gdb) next
 We're so creative.
 14        printf (&quot;Goodbye, world!\n&quot;);
 (gdb) info macro N
 Defined at /home/jimb/gdb/macros/play/sample.c:13
 #define N 1729
 (gdb) macro expand N Q M
 expands to: 1729 &lt; 42
 (gdb) print N Q M
 $2 = 0
 (gdb)
 </pre></div>

 <p>In addition to source files, macros can be defined on the compilation command
 line using the <samp>-D<var>name</var>=<var>value</var></samp> syntax.  For macros defined in
 such a way, <small>GDB</small> displays the location of their definition as line zero
 of the source file submitted to the compiler.
 </p>
 <div class="smallexample">
 <pre class="smallexample">(gdb) info macro __STDC__
 Defined at /home/jimb/gdb/macros/play/sample.c:0
 -D__STDC__=1
 (gdb)
 </pre></div>


 <div class="footnote">
 <hr>
 <h4 class="footnotes-heading">Footnotes</h4>

 <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>
 </div>
 <hr>
 <div class="header">
 <p>
 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> &nbsp; [<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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	<body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000">
	<a name="Macros"></a>
	<div class="header">
	<p>
	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>
	</div>
	<hr>
	<a name="C-Preprocessor-Macros"></a>
	<h2 class="chapter">12 C Preprocessor Macros</h2>

	<p>Some languages, such as C and C<tt>++</tt>, provide a way to define and invoke
	“preprocessor macros” which expand into strings of tokens.
	<small>GDB</small> can evaluate expressions containing macro invocations, show
	the result of macro expansion, and show a macro’s definition, including
	where it was defined.
	</p>
	<p>You may need to compile your program specially to provide <small>GDB</small>
	with information about preprocessor macros. Most compilers do not
	include macros in their debugging information, even when you compile
	with the <samp>-g</samp> flag. See <a href="Compilation.html#Compilation">Compilation</a>.
	</p>
	<p>A program may define a macro at one point, remove that definition later,
	and then provide a different definition after that. Thus, at different
	points in the program, a macro may have different definitions, or have
	no definition at all. If there is a current stack frame, <small>GDB</small>
	uses the macros in scope at that frame’s source code line. Otherwise,
	<small>GDB</small> uses the macros in scope at the current listing location;
	see <a href="List.html#List">List</a>.
	</p>
	<p>Whenever <small>GDB</small> evaluates an expression, it always expands any
	macro invocations present in the expression. <small>GDB</small> also provides
	the following commands for working with macros explicitly.
	</p>
	<dl compact="compact">
	<dd>
	<a name="index-macro-expand"></a>
	<a name="index-macro-expansion_002c-showing-the-results-of-preprocessor"></a>
	<a name="index-preprocessor-macro-expansion_002c-showing-the-results-of"></a>
	<a name="index-expanding-preprocessor-macros"></a>
	</dd>
	<dt><code>macro expand <var>expression</var></code></dt>
	<dt><code>macro exp <var>expression</var></code></dt>
	<dd><p>Show the results of expanding all preprocessor macro invocations in
	<var>expression</var>. Since <small>GDB</small> simply expands macros, but does
	not parse the result, <var>expression</var> need not be a valid expression;
	it can be any string of tokens.
	</p>
	<a name="index-macro-exp1"></a>
	</dd>
	<dt><code>macro expand-once <var>expression</var></code></dt>
	<dt><code>macro exp1 <var>expression</var></code></dt>
	<dd><a name="index-expand-macro-once"></a>
	<p><i>(This command is not yet implemented.)</i> Show the results of
	expanding those preprocessor macro invocations that appear explicitly in
	<var>expression</var>. Macro invocations appearing in that expansion are
	left unchanged. This command allows you to see the effect of a
	particular macro more clearly, without being confused by further
	expansions. Since <small>GDB</small> simply expands macros, but does not
	parse the result, <var>expression</var> need not be a valid expression; it
	can be any string of tokens.
	</p>
	<a name="index-info-macro"></a>
	<a name="index-macro-definition_002c-showing"></a>
	<a name="index-definition-of-a-macro_002c-showing"></a>
	<a name="index-macros_002c-from-debug-info"></a>
	</dd>
	<dt><code>info macro [-a\|-all] [--] <var>macro</var></code></dt>
	<dd><p>Show the current definition or all definitions of the named <var>macro</var>,
	and describe the source location or compiler command-line where that
	definition was established. The optional double dash is to signify the end of
	argument processing and the beginning of <var>macro</var> for non C-like macros where
	the macro may begin with a hyphen.
	</p>
	<a name="index-info-macros"></a>
	</dd>
	<dt><code>info macros <var>linespec</var></code></dt>
	<dd><p>Show all macro definitions that are in effect at the location specified
	by <var>linespec</var>, and describe the source location or compiler
	command-line where those definitions were established.
	</p>
	<a name="index-macro-define"></a>
	<a name="index-user_002ddefined-macros"></a>
	<a name="index-defining-macros-interactively"></a>
	<a name="index-macros_002c-user_002ddefined"></a>
	</dd>
	<dt><code>macro define <var>macro</var> <var>replacement-list</var></code></dt>
	<dt><code>macro define <var>macro</var>(<var>arglist</var>) <var>replacement-list</var></code></dt>
	<dd><p>Introduce a definition for a preprocessor macro named <var>macro</var>,
	invocations of which are replaced by the tokens given in
	<var>replacement-list</var>. The first form of this command defines an
	“object-like” macro, which takes no arguments; the second form
	defines a “function-like” macro, which takes the arguments given in
	<var>arglist</var>.
	</p>
	<p>A definition introduced by this command is in scope in every
	expression evaluated in <small>GDB</small>, until it is removed with the
	<code>macro undef</code> command, described below. The definition overrides
	all definitions for <var>macro</var> present in the program being debugged,
	as well as any previous user-supplied definition.
	</p>
	<a name="index-macro-undef"></a>
	</dd>
	<dt><code>macro undef <var>macro</var></code></dt>
	<dd><p>Remove any user-supplied definition for the macro named <var>macro</var>.
	This command only affects definitions provided with the <code>macro
	define</code> command, described above; it cannot remove definitions present
	in the program being debugged.
	</p>
	<a name="index-macro-list"></a>
	</dd>
	<dt><code>macro list</code></dt>
	<dd><p>List all the macros defined using the <code>macro define</code> command.
	</p></dd>
	</dl>

	<a name="index-macros_002c-example-of-debugging-with"></a>
	<p>Here is a transcript showing the above commands in action. First, we
	show our source files:
	</p>
	<div class="smallexample">
	<pre class="smallexample">$ cat sample.c
	#include <stdio.h>
	#include "sample.h"

	#define M 42
	#define ADD(x) (M + x)

	main ()
	{
	#define N 28
	printf ("Hello, world!\n");
	#undef N
	printf ("We're so creative.\n");
	#define N 1729
	printf ("Goodbye, world!\n");
	}
	$ cat sample.h
	#define Q <
	$
	</pre></div>

	<p>Now, we compile the program using the <small>GNU</small> C compiler,
	<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
	includes information about preprocessor macros in the debugging
	information.
	</p>
	<div class="smallexample">
	<pre class="smallexample">$ gcc -gdwarf-2 -g3 sample.c -o sample
	$
	</pre></div>

	<p>Now, we start <small>GDB</small> on our sample program:
	</p>
	<div class="smallexample">
	<pre class="smallexample">$ gdb -nw sample
	GNU gdb 2002-05-06-cvs
	Copyright 2002 Free Software Foundation, Inc.
	GDB is free software, …
	(gdb)
	</pre></div>

	<p>We can expand macros and examine their definitions, even when the
	program is not running. <small>GDB</small> uses the current listing position
	to decide which macro definitions are in scope:
	</p>
	<div class="smallexample">
	<pre class="smallexample">(gdb) list main
	3
	4 #define M 42
	5 #define ADD(x) (M + x)
	6
	7 main ()
	8 {
	9 #define N 28
	10 printf ("Hello, world!\n");
	11 #undef N
	12 printf ("We're so creative.\n");
	(gdb) info macro ADD
	Defined at /home/jimb/gdb/macros/play/sample.c:5
	#define ADD(x) (M + x)
	(gdb) info macro Q
	Defined at /home/jimb/gdb/macros/play/sample.h:1
	included at /home/jimb/gdb/macros/play/sample.c:2
	#define Q <
	(gdb) macro expand ADD(1)
	expands to: (42 + 1)
	(gdb) macro expand-once ADD(1)
	expands to: once (M + 1)
	(gdb)
	</pre></div>

	<p>In the example above, note that <code>macro expand-once</code> expands only
	the macro invocation explicit in the original text — the invocation of
	<code>ADD</code> — but does not expand the invocation of the macro <code>M</code>,
	which was introduced by <code>ADD</code>.
	</p>
	<p>Once the program is running, <small>GDB</small> uses the macro definitions in
	force at the source line of the current stack frame:
	</p>
	<div class="smallexample">
	<pre class="smallexample">(gdb) break main
	Breakpoint 1 at 0x8048370: file sample.c, line 10.
	(gdb) run
	Starting program: /home/jimb/gdb/macros/play/sample

	Breakpoint 1, main () at sample.c:10
	10 printf ("Hello, world!\n");
	(gdb)
	</pre></div>

	<p>At line 10, the definition of the macro <code>N</code> at line 9 is in force:
	</p>
	<div class="smallexample">
	<pre class="smallexample">(gdb) info macro N
	Defined at /home/jimb/gdb/macros/play/sample.c:9
	#define N 28
	(gdb) macro expand N Q M
	expands to: 28 < 42
	(gdb) print N Q M
	$1 = 1
	(gdb)
	</pre></div>

	<p>As we step over directives that remove <code>N</code>’s definition, and then
	give it a new definition, <small>GDB</small> finds the definition (or lack
	thereof) in force at each point:
	</p>
	<div class="smallexample">
	<pre class="smallexample">(gdb) next
	Hello, world!
	12 printf ("We're so creative.\n");
	(gdb) info macro N
	The symbol `N' has no definition as a C/C++ preprocessor macro
	at /home/jimb/gdb/macros/play/sample.c:12
	(gdb) next
	We're so creative.
	14 printf ("Goodbye, world!\n");
	(gdb) info macro N
	Defined at /home/jimb/gdb/macros/play/sample.c:13
	#define N 1729
	(gdb) macro expand N Q M
	expands to: 1729 < 42
	(gdb) print N Q M
	$2 = 0
	(gdb)
	</pre></div>

	<p>In addition to source files, macros can be defined on the compilation command
	line using the <samp>-D<var>name</var>=<var>value</var></samp> syntax. For macros defined in
	such a way, <small>GDB</small> displays the location of their definition as line zero
	of the source file submitted to the compiler.
	</p>
	<div class="smallexample">
	<pre class="smallexample">(gdb) info macro __STDC__
	Defined at /home/jimb/gdb/macros/play/sample.c:0
	-D__STDC__=1
	(gdb)
	</pre></div>


	<div class="footnote">
	<hr>
	<h4 class="footnotes-heading">Footnotes</h4>

	<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>
	</div>
	<hr>
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