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diff --git a/gdb/doc/gdb.sample-m4 b/gdb/doc/gdb.sample-m4 new file mode 100755 index 0000000..efdffab --- /dev/null +++ b/gdb/doc/gdb.sample-m4 @@ -0,0 +1,263 @@ +_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 + |