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264 lines
7.6 KiB
Plaintext
Executable File
264 lines
7.6 KiB
Plaintext
Executable File
_dnl__ -*- Texinfo -*-
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_dnl__ Copyright (c) 1991 Free Software Foundation, Inc.
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_dnl__ This file is part of the source for the GDB manual.
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@c M4 FRAGMENT: $Id$
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@node Sample Session, Invocation, New Features, Top
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@chapter A Sample _GDBN__ Session
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You can use this manual at your leisure to read all about _GDBN__.
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However, a handful of commands are enough to get started using the
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debugger. This chapter illustrates these commands.
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@iftex
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In this sample session, we emphasize user input like this: @i{input},
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to make it easier to pick out from the surrounding output.
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@end iftex
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@c FIXME: this example may not be appropriate for some configs, where
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@c FIXME...primary interest is in remote use.
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_0__
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One of the preliminary versions of GNU @code{m4} (a generic macro
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processor) exhibits the following bug: sometimes, when we change its
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quote strings from the default, the commands used to capture one macro's
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definition in another stop working. In the following short @code{m4}
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session, we define a macro @code{foo} which expands to @code{0000}; we
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then use the @code{m4} builtin @code{defn} to define @code{bar} as the
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same thing. However, when we change the open quote string to
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@code{<QUOTE>} and the close quote string to @code{<UNQUOTE>}, the same
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procedure fails to define a new synonym @code{baz}:
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@smallexample
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$ @i{cd gnu/m4}
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$ @i{./m4}
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@i{define(foo,0000)}
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@i{foo}
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0000
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@i{define(bar,defn(`foo'))}
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@i{bar}
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0000
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@i{changequote(<QUOTE>,<UNQUOTE>)}
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@i{define(baz,defn(<QUOTE>foo<UNQUOTE>))}
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@i{baz}
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@i{C-D}
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m4: End of input: 0: fatal error: EOF in string
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@end smallexample
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@noindent
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Let's use _GDBN__ to try to see what's going on.
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@smallexample
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$ @i{_GDBP__ m4}
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Reading symbol data from m4...done.
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(_GDBP__)
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@end smallexample
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@noindent
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_GDBN__ only reads enough symbol data to know where to find the rest
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when needed; as a result, the first prompt comes up very quickly. We
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then tell _GDBN__ to use a narrower display width than usual, so
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that examples will fit in this manual.
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@smallexample
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(_GDBP__) @i{set width 70}
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@end smallexample
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@noindent
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Let's see how the @code{m4} builtin @code{changequote} works.
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Having looked at the source, we know the relevant subroutine is
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@code{m4_changequote}, so we set a breakpoint there with _GDBN__'s
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@code{break} command.
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@smallexample
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(_GDBP__) @i{break m4_changequote}
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Breakpoint 1 at 0x62f4: file builtin.c, line 879.
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@end smallexample
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@noindent
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Using the @code{run} command, we start @code{m4} running under _GDBN__
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control; as long as control does not reach the @code{m4_changequote}
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subroutine, the program runs as usual:
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@smallexample
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(_GDBP__) @i{run}
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Starting program: /work/Editorial/gdb/gnu/m4/m4
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@i{define(foo,0000)}
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@i{foo}
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0000
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@end smallexample
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@noindent
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To trigger the breakpoint, we call @code{changequote}. _GDBN__
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suspends execution of @code{m4}, displaying information about the
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context where it stops.
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@smallexample
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@i{changequote(<QUOTE>,<UNQUOTE>)}
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Breakpoint 1, m4_changequote (argc=3, argv=0x33c70) at builtin.c:879
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879 if (bad_argc(TOKEN_DATA_TEXT(argv[0]), argc, 1, 3))
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@end smallexample
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@noindent
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Now we use the command @code{n} (@code{next}) to advance execution to
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the next line of the current function.
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@smallexample
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(_GDBP__) @i{n}
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882 set_quotes((argc >= 2) ? TOKEN_DATA_TEXT(argv[1]) : nil,
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@end smallexample
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@noindent
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@code{set_quotes} looks like a promising subroutine. We can go into it
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by using the command @code{s} (@code{step}) instead of @code{next}.
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@code{step} goes to the next line to be executed in @emph{any}
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subroutine, so it steps into @code{set_quotes}.
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@smallexample
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(_GDBP__) @i{s}
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set_quotes (lq=0x34c78 "<QUOTE>", rq=0x34c88 "<UNQUOTE>")
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at input.c:530
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530 if (lquote != def_lquote)
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@end smallexample
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@noindent
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The summary display showing the subroutine where @code{m4} is now
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suspended (and its arguments) is called a stack frame display. We can
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use the @code{backtrace} command (which can also be spelled @code{bt}),
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to see where we are in the stack: it displays a stack frame for each
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active subroutine.
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@smallexample
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(_GDBP__) @i{bt}
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#0 set_quotes (lq=0x34c78 "<QUOTE>", rq=0x34c88 "<UNQUOTE>")
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at input.c:530
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#1 0x6344 in m4_changequote (argc=3, argv=0x33c70) at builtin.c:882
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#2 0x8174 in expand_macro (sym=0x33320) at macro.c:242
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#3 0x7a88 in expand_token (obs=0x0, t=209696, td=0xf7fffa30)
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at macro.c:71
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#4 0x79dc in expand_input () at macro.c:40
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#5 0x2930 in main (argc=0, argv=0xf7fffb20) at m4.c:195
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@end smallexample
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@noindent
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Let's step through a few more lines to see what happens. The first two
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times, we can use @samp{s}; the next two times we use @code{n} to avoid
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falling into the @code{xstrdup} subroutine.
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@smallexample
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(_GDBP__) @i{s}
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0x3b5c 532 if (rquote != def_rquote)
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(_GDBP__) @i{s}
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0x3b80 535 lquote = (lq == nil || *lq == '\0') ? def_lquote :\
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xstrdup(lq);
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(_GDBP__) @i{n}
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536 rquote = (rq == nil || *rq == '\0') ? def_rquote : xstrdup\
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(rq);
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(_GDBP__) @i{n}
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538 len_lquote = strlen(rquote);
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@end smallexample
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@noindent
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The last line displayed looks a little odd; let's examine the variables
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@code{lquote} and @code{rquote} to see if they are in fact the new left
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and right quotes we specified. We can use the command @code{p}
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(@code{print}) to see their values.
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@smallexample
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(_GDBP__) @i{p lquote}
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$1 = 0x35d40 "<QUOTE>"
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(_GDBP__) @i{p rquote}
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$2 = 0x35d50 "<UNQUOTE>"
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@end smallexample
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@noindent
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@code{lquote} and @code{rquote} are indeed the new left and right quotes.
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Let's look at some context; we can display ten lines of source
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surrounding the current line, with the @code{l} (@code{list}) command.
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@smallexample
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(_GDBP__) @i{l}
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533 xfree(rquote);
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534
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535 lquote = (lq == nil || *lq == '\0') ? def_lquote : xstrdup\
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(lq);
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536 rquote = (rq == nil || *rq == '\0') ? def_rquote : xstrdup\
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(rq);
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537
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538 len_lquote = strlen(rquote);
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539 len_rquote = strlen(lquote);
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540 @}
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541
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542 void
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@end smallexample
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@noindent
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Let's step past the two lines that set @code{len_lquote} and
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@code{len_rquote}, and then examine the values of those variables.
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@smallexample
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(_GDBP__) @i{n}
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539 len_rquote = strlen(lquote);
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(_GDBP__) @i{n}
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540 @}
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(_GDBP__) @i{p len_lquote}
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$3 = 9
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(_GDBP__) @i{p len_rquote}
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$4 = 7
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@end smallexample
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@noindent
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That certainly looks wrong, assuming @code{len_lquote} and
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@code{len_rquote} are meant to be the lengths of @code{lquote} and
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@code{rquote} respectively. Let's try setting them to better values.
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We can use the @code{p} command for this, since it'll print the value of
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any expression---and that expression can include subroutine calls and
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assignments.
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@smallexample
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(_GDBP__) p len_lquote=strlen(lquote)
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$5 = 7
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(_GDBP__) p len_rquote=strlen(rquote)
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$6 = 9
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@end smallexample
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@noindent
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Let's see if that fixes the problem of using the new quotes with the
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@code{m4} built-in @code{defn}. We can allow @code{m4} to continue
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executing with the @code{c} (@code{continue}) command, and then try the
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example that caused trouble initially:
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@smallexample
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(_GDBP__) @i{c}
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Continuing.
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@i{define(baz,defn(<QUOTE>foo<UNQUOTE>))}
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baz
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0000
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@end smallexample
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@noindent
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Success! The new quotes now work just as well as the default ones. The
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problem seems to have been just the two typos defining the wrong
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lengths. We'll let @code{m4} exit by giving it an EOF as input.
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@smallexample
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@i{C-D}
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Program exited normally.
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@end smallexample
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@noindent
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The message @samp{Program exited normally.} is from _GDBN__; it
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indicates @code{m4} has finished executing. We can end our _GDBN__
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session with the _GDBN__ @code{quit} command.
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@smallexample
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(_GDBP__) @i{quit}
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$
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_1__@end smallexample
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