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261 lines
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<title>Macros - Debugging with GDB</title>
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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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<body>
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<div class="node">
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<a name="Macros"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="Tracepoints.html#Tracepoints">Tracepoints</a>,
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Previous: <a rel="previous" accesskey="p" href="Optimized-Code.html#Optimized-Code">Optimized Code</a>,
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Up: <a rel="up" accesskey="u" href="index.html#Top">Top</a>
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<hr>
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</div>
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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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<span class="sc">gdb</span> 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>You may need to compile your program specially to provide <span class="sc">gdb</span>
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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><span class="option">-g</span></samp> flag. See <a href="Compilation.html#Compilation">Compilation</a>.
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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, <span class="sc">gdb</span>
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uses the macros in scope at that frame's source code line. Otherwise,
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<span class="sc">gdb</span> 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>Whenever <span class="sc">gdb</span> evaluates an expression, it always expands any
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macro invocations present in the expression. <span class="sc">gdb</span> also provides
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the following commands for working with macros explicitly.
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<a name="index-macro-expand-840"></a>
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<a name="index-macro-expansion_002c-showing-the-results-of-preprocessor-841"></a>
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<a name="index-preprocessor-macro-expansion_002c-showing-the-results-of-842"></a>
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<a name="index-expanding-preprocessor-macros-843"></a>
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<dl><dt><code>macro expand </code><var>expression</var><dt><code>macro exp </code><var>expression</var><dd>Show the results of expanding all preprocessor macro invocations in
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<var>expression</var>. Since <span class="sc">gdb</span> 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><a name="index-macro-exp1-844"></a><br><dt><code>macro expand-once </code><var>expression</var><dt><code>macro exp1 </code><var>expression</var><dd><a name="index-expand-macro-once-845"></a><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 <span class="sc">gdb</span> 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><a name="index-info-macro-846"></a><a name="index-macro-definition_002c-showing-847"></a><a name="index-definition-of-a-macro_002c-showing-848"></a><a name="index-macros_002c-from-debug-info-849"></a><br><dt><code>info macro [-a|-all] [--] </code><var>macro</var><dd>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><a name="index-info-macros-850"></a><br><dt><code>info macros </code><var>location</var><dd>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><a name="index-macro-define-851"></a><a name="index-user_002ddefined-macros-852"></a><a name="index-defining-macros-interactively-853"></a><a name="index-macros_002c-user_002ddefined-854"></a><br><dt><code>macro define </code><var>macro</var> <var>replacement-list</var><dt><code>macro define </code><var>macro</var><code>(</code><var>arglist</var><code>) </code><var>replacement-list</var><dd>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>A definition introduced by this command is in scope in every
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expression evaluated in <span class="sc">gdb</span>, 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><a name="index-macro-undef-855"></a><br><dt><code>macro undef </code><var>macro</var><dd>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><a name="index-macro-list-856"></a><br><dt><code>macro list</code><dd>List all the macros defined using the <code>macro define</code> command.
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</dl>
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<p><a name="index-macros_002c-example-of-debugging-with-857"></a>Here is a transcript showing the above commands in action. First, we
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show our source files:
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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>
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<p>Now, we compile the program using the <span class="sc">gnu</span> C compiler,
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<span class="sc">gcc</span>. We pass the <samp><span class="option">-gdwarf-2</span></samp><a rel="footnote" href="#fn-1" name="fnd-1"><sup>1</sup></a> <em>and</em> <samp><span class="option">-g3</span></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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<pre class="smallexample"> $ gcc -gdwarf-2 -g3 sample.c -o sample
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$
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</pre>
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<p>Now, we start <span class="sc">gdb</span> on our sample program:
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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>
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<p>We can expand macros and examine their definitions, even when the
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program is not running. <span class="sc">gdb</span> uses the current listing position
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to decide which macro definitions are in scope:
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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>
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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>Once the program is running, <span class="sc">gdb</span> uses the macro definitions in
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force at the source line of the current stack frame:
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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>
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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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<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>
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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, <span class="sc">gdb</span> finds the definition (or lack
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thereof) in force at each point:
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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>
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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><span class="option">-D</span><var>name</var><span class="option">=</span><var>value</var></samp> syntax. For macros defined in
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such a way, <span class="sc">gdb</span> 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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<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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(gdb)
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</pre>
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<div class="footnote">
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<hr>
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<h4>Footnotes</h4><p class="footnote"><small>[<a name="fn-1" href="#fnd-1">1</a>]</small> This is the
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minimum. Recent versions of <span class="sc">gcc</span> support <samp><span class="option">-gdwarf-3</span></samp>
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and <samp><span class="option">-gdwarf-4</span></samp>; we recommend always choosing the most recent
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version of DWARF.</p>
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<hr></div>
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