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<!-- Created on March, 27 2008 by texi2html 1.64 -->
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Written by: Lionel Cons <Lionel.Cons@cern.ch> (original author)
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Karl Berry <karl@freefriends.org>
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Olaf Bachmann <obachman@mathematik.uni-kl.de>
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and many others.
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Maintained by: Olaf Bachmann <obachman@mathematik.uni-kl.de>
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<HEAD>
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<TITLE>Debugging with GDB: Macros</TITLE>
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<META NAME="description" CONTENT="Debugging with GDB: Macros">
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<META NAME="keywords" CONTENT="Debugging with GDB: Macros">
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<BODY LANG="" BGCOLOR="#FFFFFF" TEXT="#000000" LINK="#0000FF" VLINK="#800080" ALINK="#FF0000">
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<A NAME="SEC83"></A>
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<TABLE CELLPADDING=1 CELLSPACING=1 BORDER=0>
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<TR><TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="gdb_9.html#SEC82"> < </A>]</TD>
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<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="gdb_11.html#SEC84"> > </A>]</TD>
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<TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT">[ << ]</TD>
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<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="gdb.html#SEC_Top"> Up </A>]</TD>
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<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="gdb_11.html#SEC84"> >> </A>]</TD>
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<TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="gdb.html#SEC_Top">Top</A>]</TD>
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<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="gdb_toc.html#SEC_Contents">Contents</A>]</TD>
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<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="gdb_38.html#SEC764">Index</A>]</TD>
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<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="gdb_abt.html#SEC_About"> ? </A>]</TD>
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</TR></TABLE>
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<H1> 9. C Preprocessor Macros </H1>
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<!--docid::SEC83::-->
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<P>
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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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GDB 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><P>
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You may need to compile your program specially to provide GDB
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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 section <A HREF="gdb_5.html#SEC19">4.1 Compiling for Debugging</A>.
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</P><P>
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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, GDB
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uses the macros in scope at that frame's source code line. Otherwise,
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GDB uses the macros in scope at the current listing location;
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see <A HREF="gdb_8.html#SEC52">7.1 Printing Source Lines</A>.
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</P><P>
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At the moment, GDB does not support the <CODE>##</CODE>
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token-splicing operator, the <CODE>#</CODE> stringification operator, or
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variable-arity macros.
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</P><P>
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Whenever GDB evaluates an expression, it always expands any
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macro invocations present in the expression. GDB also provides
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the following commands for working with macros explicitly.
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</P><P>
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<DL COMPACT>
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<A NAME="IDX507"></A>
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<A NAME="IDX508"></A>
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<A NAME="IDX509"></A>
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<A NAME="IDX510"></A>
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<DT><CODE>macro expand <VAR>expression</VAR></CODE>
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<DD><DT><CODE>macro exp <VAR>expression</VAR></CODE>
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<DD>Show the results of expanding all preprocessor macro invocations in
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<VAR>expression</VAR>. Since GDB 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="IDX511"></A>
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<DT><CODE>macro expand-once <VAR>expression</VAR></CODE>
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<DD><DT><CODE>macro exp1 <VAR>expression</VAR></CODE>
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<DD><A NAME="IDX512"></A>
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<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 GDB 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="IDX513"></A>
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<A NAME="IDX514"></A>
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<A NAME="IDX515"></A>
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<DT><CODE>info macro <VAR>macro</VAR></CODE>
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<DD>Show the definition of the macro named <VAR>macro</VAR>, and describe the
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source location where that definition was established.
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<P>
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<A NAME="IDX516"></A>
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<A NAME="IDX517"></A>
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<A NAME="IDX518"></A>
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<A NAME="IDX519"></A>
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<DT><CODE>macro define <VAR>macro</VAR> <VAR>replacement-list</VAR></CODE>
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<DD><DT><CODE>macro define <VAR>macro</VAR>(<VAR>arglist</VAR>) <VAR>replacement-list</VAR></CODE>
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<DD><I>(This command is not yet implemented.)</I> Introduce a definition for a
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preprocessor macro named <VAR>macro</VAR>, invocations of which are replaced
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by the tokens given in <VAR>replacement-list</VAR>. The first form of this
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command defines an "object-like" macro, which takes no arguments; the
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second form defines a "function-like" macro, which takes the arguments
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given in <VAR>arglist</VAR>.
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<P>
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A definition introduced by this command is in scope in every expression
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evaluated in GDB, until it is removed with the <CODE>macro
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undef</CODE> command, described below. The definition overrides all
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definitions for <VAR>macro</VAR> present in the program being debugged, as
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well as any previous user-supplied definition.
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</P><P>
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<A NAME="IDX520"></A>
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<DT><CODE>macro undef <VAR>macro</VAR></CODE>
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<DD><I>(This command is not yet implemented.)</I> Remove any user-supplied
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definition for the macro named <VAR>macro</VAR>. This command only affects
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definitions provided with the <CODE>macro define</CODE> command, described
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above; it cannot remove definitions present in the program being
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debugged.
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<P>
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<A NAME="IDX521"></A>
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<DT><CODE>macro list</CODE>
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<DD><I>(This command is not yet implemented.)</I> List all the macros
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defined using the <CODE>macro define</CODE> command.
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</DL>
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<P>
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<A NAME="IDX522"></A>
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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><P>
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<TABLE><tr><td> </td><td class=smallexample><FONT SIZE=-1><pre>$ 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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</FONT></pre></td></tr></table></P><P>
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Now, we compile the program using the GNU C compiler, GCC.
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We pass the <SAMP>`-gdwarf-2'</SAMP> and <SAMP>`-g3'</SAMP> flags to ensure the
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compiler includes information about preprocessor macros in the debugging
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information.
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</P><P>
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<TABLE><tr><td> </td><td class=smallexample><FONT SIZE=-1><pre>$ gcc -gdwarf-2 -g3 sample.c -o sample
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$
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</FONT></pre></td></tr></table></P><P>
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Now, we start GDB on our sample program:
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</P><P>
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<TABLE><tr><td> </td><td class=smallexample><FONT SIZE=-1><pre>$ 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, <small>...</small>
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(gdb)
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</FONT></pre></td></tr></table></P><P>
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We can expand macros and examine their definitions, even when the
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program is not running. GDB uses the current listing position
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to decide which macro definitions are in scope:
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</P><P>
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<TABLE><tr><td> </td><td class=smallexample><FONT SIZE=-1><pre>(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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</FONT></pre></td></tr></table></P><P>
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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><P>
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Once the program is running, GDB uses the macro definitions in
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force at the source line of the current stack frame:
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</P><P>
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<TABLE><tr><td> </td><td class=smallexample><FONT SIZE=-1><pre>(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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</FONT></pre></td></tr></table></P><P>
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At line 10, the definition of the macro <CODE>N</CODE> at line 9 is in force:
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</P><P>
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<TABLE><tr><td> </td><td class=smallexample><FONT SIZE=-1><pre>(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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</FONT></pre></td></tr></table></P><P>
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As we step over directives that remove <CODE>N</CODE>'s definition, and then
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give it a new definition, GDB finds the definition (or lack
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thereof) in force at each point:
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</P><P>
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<TABLE><tr><td> </td><td class=smallexample><FONT SIZE=-1><pre>(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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</FONT></pre></td></tr></table></P><P>
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<A NAME="Tracepoints"></A>
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<HR SIZE="6">
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<TABLE CELLPADDING=1 CELLSPACING=1 BORDER=0>
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<TR><TD VALIGN="MIDDLE" ALIGN="LEFT">[ << ]</TD>
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<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="gdb_11.html#SEC84"> >> </A>]</TD>
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<TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="gdb.html#SEC_Top">Top</A>]</TD>
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<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="gdb_toc.html#SEC_Contents">Contents</A>]</TD>
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<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="gdb_38.html#SEC764">Index</A>]</TD>
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<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="gdb_abt.html#SEC_About"> ? </A>]</TD>
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</TR></TABLE>
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<BR>
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<FONT SIZE="-1">
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<address>
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<p>Please send FSF & GNU inquiries & questions to <a
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href="mailto:gnu@gnu.org">gnu@gnu.org</a>. There are also <a
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href="http://www.gnu.org/home.html#ContactInfo">other ways to
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contact</a> the FSF.</p>
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<p>These pages are maintained by <a
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href="http://www.gnu.org/software/gdb/">the GDB developers</a>.</p>
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<p>Copyright Free Software Foundation, Inc., 59 Temple Place - Suite
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330, Boston, MA 02111, USA.</p>
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<p>Verbatim copying and distribution of this entire article is
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permitted in any medium, provided this notice is preserved.</p>
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</address>
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This document was generated
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by <I>GDB Administrator</I> on <I>March, 27 2008</I>
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using <A HREF="http://www.mathematik.uni-kl.de/~obachman/Texi2html
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"><I>texi2html</I></A>
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