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<title>Macros (Debugging with GDB)</title>
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<a name="Macros"></a>
<div class="header">
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</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>location</var></code></dt>
<dd><p>Show all macro definitions that are in effect at the location specified
by <var>location</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>
<div class="header">
<p>
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