1. A Sample No value for GDBN Session
You can use this manual at your leisure to read all about No value for GDBN.
However, a handful of commands are enough to get started using the
debugger. This chapter illustrates those commands.
One of the preliminary versions of GNU 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
definition within another stop working. In the following short m4
session, we define a macro foo which expands to 0000; we
then use the m4 built-in defn to define bar as the
same thing. However, when we change the open quote string to
<QUOTE> and the close quote string to <UNQUOTE>, the same
procedure fails to define a new synonym baz:
| | $ cd gnu/m4
$ ./m4
define(foo,0000)
foo
0000
define(bar,defn(`foo'))
bar
0000
changequote(<QUOTE>,<UNQUOTE>)
define(baz,defn(<QUOTE>foo<UNQUOTE>))
baz
Ctrl-d
m4: End of input: 0: fatal error: EOF in string
|
Let us use No value for GDBN to try to see what is going on.
| | $ No value for GDBP m4
No value for GDBN is free software and you are welcome to distribute copies
of it under certain conditions; type "show copying" to see
the conditions.
There is absolutely no warranty for No value for GDBN; type "show warranty"
for details.
No value for GDBN No value for GDBVN, Copyright 1999 Free Software Foundation, Inc...
(No value for GDBP)
|
No value for GDBN reads only enough symbol data to know where to find the
rest when needed; as a result, the first prompt comes up very quickly.
We now tell No value for GDBN to use a narrower display width than usual, so
that examples fit in this manual.
| | (No value for GDBP) set width 70
|
We need to see how the m4 built-in changequote works.
Having looked at the source, we know the relevant subroutine is
m4_changequote, so we set a breakpoint there with the No value for GDBN
break command.
| | (No value for GDBP) break m4_changequote
Breakpoint 1 at 0x62f4: file builtin.c, line 879.
|
Using the run command, we start m4 running under No value for GDBN
control; as long as control does not reach the m4_changequote
subroutine, the program runs as usual:
| | (No value for GDBP) run
Starting program: /work/Editorial/gdb/gnu/m4/m4
define(foo,0000)
foo
0000
|
To trigger the breakpoint, we call changequote. No value for GDBN
suspends execution of m4, displaying information about the
context where it stops.
| | 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))
|
Now we use the command n (next) to advance execution to
the next line of the current function.
| | (No value for GDBP) n
882 set_quotes((argc >= 2) ? TOKEN_DATA_TEXT(argv[1])\
: nil,
|
set_quotes looks like a promising subroutine. We can go into it
by using the command s (step) instead of next.
step goes to the next line to be executed in any
subroutine, so it steps into set_quotes.
| | (No value for GDBP) s
set_quotes (lq=0x34c78 "<QUOTE>", rq=0x34c88 "<UNQUOTE>")
at input.c:530
530 if (lquote != def_lquote)
|
The display that shows the subroutine where m4 is now
suspended (and its arguments) is called a stack frame display. It
shows a summary of the stack. We can use the backtrace
command (which can also be spelled bt), to see where we are
in the stack as a whole: the backtrace command displays a
stack frame for each active subroutine.
| | (No value for GDBP) 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
|
We step through a few more lines to see what happens. The first two
times, we can use ‘s’; the next two times we use n to avoid
falling into the xstrdup subroutine.
| | (No value for GDBP) s
0x3b5c 532 if (rquote != def_rquote)
(No value for GDBP) s
0x3b80 535 lquote = (lq == nil || *lq == '\0') ? \
def_lquote : xstrdup(lq);
(No value for GDBP) n
536 rquote = (rq == nil || *rq == '\0') ? def_rquote\
: xstrdup(rq);
(No value for GDBP) n
538 len_lquote = strlen(rquote);
|
The last line displayed looks a little odd; we can examine the variables
lquote and rquote to see if they are in fact the new left
and right quotes we specified. We use the command p
(print) to see their values.
| | (No value for GDBP) p lquote
$1 = 0x35d40 "<QUOTE>"
(No value for GDBP) p rquote
$2 = 0x35d50 "<UNQUOTE>"
|
lquote and rquote are indeed the new left and right quotes.
To look at some context, we can display ten lines of source
surrounding the current line with the l (list) command.
| | (No value for GDBP) 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
|
Let us step past the two lines that set len_lquote and
len_rquote, and then examine the values of those variables.
| | (No value for GDBP) n
539 len_rquote = strlen(lquote);
(No value for GDBP) n
540 }
(No value for GDBP) p len_lquote
$3 = 9
(No value for GDBP) p len_rquote
$4 = 7
|
That certainly looks wrong, assuming len_lquote and
len_rquote are meant to be the lengths of lquote and
rquote respectively. We can set them to better values using
the p command, since it can print the value of
any expression—and that expression can include subroutine calls and
assignments.
| | (No value for GDBP) p len_lquote=strlen(lquote)
$5 = 7
(No value for GDBP) p len_rquote=strlen(rquote)
$6 = 9
|
Is that enough to fix the problem of using the new quotes with the
m4 built-in defn? We can allow m4 to continue
executing with the c (continue) command, and then try the
example that caused trouble initially:
| | (No value for GDBP) c
Continuing.
define(baz,defn(<QUOTE>foo<UNQUOTE>))
baz
0000
|
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 allow m4 exit by giving it an EOF as input:
| | Ctrl-d
Program exited normally.
|
The message ‘Program exited normally.’ is from No value for GDBN; it
indicates m4 has finished executing. We can end our No value for GDBN
session with the No value for GDBN quit command.
