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+_dnl__								-*- Texinfo -*-
+_dnl__ Copyright (c) 1991 Free Software Foundation, Inc.
+_dnl__ This file is part of the source for the GDB manual.
+_dnl__ $Id$
+@node Sample Session, Invocation, New Features, Top
+@chapter A Sample _GDBN__ Session
+
+You can use this manual at your leisure to read all about _GDBN__.
+However, a handful of commands are enough to get started using the
+debugger.  This chapter illustrates these commands.
+
+@iftex
+In this sample session, we emphasize user input like this: @i{input},
+to make it easier to pick out from the surrounding output.
+@end iftex
+
+@c FIXME: this example may not be appropriate for some configs, where
+@c FIXME...primary interest is in remote use.
+_0__
+One of the preliminary versions of GNU @code{m4} (a generic macro
+processor) exhibits the following bug: sometimes, when we change its
+quote strings from the default, the commands used to capture one macro's
+definition in another stop working.  In the following short @code{m4}
+session, we define a macro @code{foo} which expands to @code{0000}; we
+then use the @code{m4} builtin @code{defn} to define @code{bar} as the
+same thing.  However, when we change the open quote string to
+@code{<QUOTE>} and the close quote string to @code{<UNQUOTE>}, the same
+procedure fails to define a new synonym @code{baz}:
+
+@smallexample
+$ @i{cd gnu/m4}
+$ @i{./m4}
+@i{define(foo,0000)}
+
+@i{foo}
+0000
+@i{define(bar,defn(`foo'))}
+
+@i{bar}
+0000
+@i{changequote(<QUOTE>,<UNQUOTE>)}
+
+@i{define(baz,defn(<QUOTE>foo<UNQUOTE>))}
+@i{baz}
+@i{C-D}
+m4: End of input: 0: fatal error: EOF in string
+@end smallexample
+
+@noindent
+Let's use _GDBN__ to try to see what's going on.
+
+@smallexample
+$ @i{_GDBP__ m4}
+Reading symbol data from m4...done.
+(_GDBP__) 
+@end smallexample
+
+@noindent
+_GDBN__ only reads enough symbol data to know where to find the rest
+when needed; as a result, the first prompt comes up very quickly.  We
+then tell _GDBN__ to use a narrower display width than usual, so
+that examples will fit in this manual.
+
+@smallexample
+(_GDBP__) @i{set width 70}
+@end smallexample
+
+@noindent 
+Let's see how the @code{m4} builtin @code{changequote} works.
+Having looked at the source, we know the relevant subroutine is
+@code{m4_changequote}, so we set a breakpoint there with _GDBN__'s
+@code{break} command.
+
+@smallexample
+(_GDBP__) @i{break m4_changequote}
+Breakpoint 1 at 0x62f4: file builtin.c, line 879.
+@end smallexample
+
+@noindent
+Using the @code{run} command, we start @code{m4} running under _GDBN__
+control; as long as control does not reach the @code{m4_changequote}
+subroutine, the program runs as usual:
+
+@smallexample
+(_GDBP__) @i{run}
+Starting program: /work/Editorial/gdb/gnu/m4/m4 
+@i{define(foo,0000)}
+
+@i{foo}
+0000
+@end smallexample
+
+@noindent
+To trigger the breakpoint, we call @code{changequote}.  _GDBN__
+suspends execution of @code{m4}, displaying information about the
+context where it stops.
+
+@smallexample
+@i{changequote(<QUOTE>,<UNQUOTE>)}
+
+Breakpoint 1, m4_changequote (argc=3, argv=0x33c70) at builtin.c:879
+879         if (bad_argc(TOKEN_DATA_TEXT(argv[0]), argc, 1, 3))
+@end smallexample
+
+@noindent
+Now we use the command @code{n} (@code{next}) to advance execution to
+the next line of the current function.
+
+@smallexample
+(_GDBP__) @i{n}
+882         set_quotes((argc >= 2) ? TOKEN_DATA_TEXT(argv[1]) : nil,
+@end smallexample
+
+@noindent
+@code{set_quotes} looks like a promising subroutine.  We can go into it
+by using the command @code{s} (@code{step}) instead of @code{next}.
+@code{step} goes to the next line to be executed in @emph{any}
+subroutine, so it steps into @code{set_quotes}.  
+
+@smallexample
+(_GDBP__) @i{s}
+set_quotes (lq=0x34c78 "<QUOTE>", rq=0x34c88 "<UNQUOTE>")
+    at input.c:530
+530         if (lquote != def_lquote)
+@end smallexample
+
+@noindent
+The summary display showing the subroutine where @code{m4} is now
+suspended (and its arguments) is called a stack frame display.  We can
+use the @code{backtrace} command (which can also be spelled @code{bt}),
+to see where we are in the stack: it displays a stack frame for each
+active subroutine.
+
+@smallexample
+(_GDBP__) @i{bt}
+#0  set_quotes (lq=0x34c78 "<QUOTE>", rq=0x34c88 "<UNQUOTE>")
+    at input.c:530
+#1  0x6344 in m4_changequote (argc=3, argv=0x33c70) at builtin.c:882
+#2  0x8174 in expand_macro (sym=0x33320) at macro.c:242
+#3  0x7a88 in expand_token (obs=0x0, t=209696, td=0xf7fffa30)
+    at macro.c:71
+#4  0x79dc in expand_input () at macro.c:40
+#5  0x2930 in main (argc=0, argv=0xf7fffb20) at m4.c:195
+@end smallexample
+
+@noindent
+Let's step through a few more lines to see what happens.  The first two
+times, we can use @samp{s}; the next two times we use @code{n} to avoid
+falling into the @code{xstrdup} subroutine.
+@smallexample
+(_GDBP__) @i{s}
+0x3b5c  532         if (rquote != def_rquote)
+(_GDBP__) @i{s}
+0x3b80  535         lquote = (lq == nil || *lq == '\0') ? def_lquote :\
+ xstrdup(lq);
+(_GDBP__) @i{n}
+536         rquote = (rq == nil || *rq == '\0') ? def_rquote : xstrdup\
+(rq);
+(_GDBP__) @i{n}
+538         len_lquote = strlen(rquote);
+@end smallexample
+
+@noindent
+The last line displayed looks a little odd; let's examine the variables
+@code{lquote} and @code{rquote} to see if they are in fact the new left
+and right quotes we specified.  We can use the command @code{p}
+(@code{print}) to see their values.
+
+@smallexample
+(_GDBP__) @i{p lquote}
+$1 = 0x35d40 "<QUOTE>"
+(_GDBP__) @i{p rquote}
+$2 = 0x35d50 "<UNQUOTE>"
+@end smallexample
+
+@noindent
+@code{lquote} and @code{rquote} are indeed the new left and right quotes.
+Let's look at some context; we can display ten lines of source
+surrounding the current line, with the @code{l} (@code{list}) command.
+
+@smallexample
+(_GDBP__) @i{l}
+533             xfree(rquote);
+534     
+535         lquote = (lq == nil || *lq == '\0') ? def_lquote : xstrdup\
+(lq);
+536         rquote = (rq == nil || *rq == '\0') ? def_rquote : xstrdup\
+(rq);
+537     
+538         len_lquote = strlen(rquote);
+539         len_rquote = strlen(lquote);
+540     @}
+541     
+542     void
+@end smallexample
+
+@noindent
+Let's step past the two lines that set @code{len_lquote} and
+@code{len_rquote}, and then examine the values of those variables.
+
+@smallexample
+(_GDBP__) @i{n}
+539         len_rquote = strlen(lquote);
+(_GDBP__) @i{n}
+540     @}
+(_GDBP__) @i{p len_lquote}
+$3 = 9
+(_GDBP__) @i{p len_rquote}
+$4 = 7
+@end smallexample
+
+@noindent
+That certainly looks wrong, assuming @code{len_lquote} and
+@code{len_rquote} are meant to be the lengths of @code{lquote} and
+@code{rquote} respectively.  Let's try setting them to better values.
+We can use the @code{p} command for this, since it'll print the value of
+any expression---and that expression can include subroutine calls and
+assignments. 
+
+@smallexample
+(_GDBP__) p len_lquote=strlen(lquote)
+$5 = 7
+(_GDBP__) p len_rquote=strlen(rquote)
+$6 = 9
+@end smallexample
+
+@noindent
+Let's see if that fixes the problem of using the new quotes with the
+@code{m4} built-in @code{defn}.  We can allow @code{m4} to continue
+executing with the @code{c} (@code{continue}) command, and then try the
+example that caused trouble initially:
+
+@smallexample
+(_GDBP__) @i{c}
+Continuing.
+
+@i{define(baz,defn(<QUOTE>foo<UNQUOTE>))}
+
+baz
+0000
+@end smallexample
+
+@noindent
+Success!  The new quotes now work just as well as the default ones.  The
+problem seems to have been just the two typos defining the wrong
+lengths.  We'll let @code{m4} exit by giving it an EOF as input.
+
+@smallexample
+@i{C-D}
+Program exited normally.
+@end smallexample
+
+@noindent 
+The message @samp{Program exited normally.} is from _GDBN__; it
+indicates @code{m4} has finished executing.  We can end our _GDBN__
+session with the _GDBN__ @code{quit} command.
+
+@smallexample
+(_GDBP__) @i{quit}
+
+$ 
+_1__@end smallexample
+