path: root/gdb/doc/gdb.sample-m4

_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