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<H1> 1. A Sample GDB Session </H1>
<!--docid::SEC5::-->
<P>
You can use this manual at your leisure to read all about GDB.
However, a handful of commands are enough to get started using the
debugger. This chapter illustrates those commands.
</P><P>
One of the preliminary versions of GNU <CODE>m4</CODE> (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 <CODE>m4</CODE>
session, we define a macro <CODE>foo</CODE> which expands to <CODE>0000</CODE>; we
then use the <CODE>m4</CODE> built-in <CODE>defn</CODE> to define <CODE>bar</CODE> as the
same thing. However, when we change the open quote string to
<CODE><QUOTE></CODE> and the close quote string to <CODE><UNQUOTE></CODE>, the same
procedure fails to define a new synonym <CODE>baz</CODE>:
</P><P>
<TABLE><tr><td> </td><td class=smallexample><FONT SIZE=-1><pre>$ <B>cd gnu/m4</B>
$ <B>./m4</B>
<B>define(foo,0000)</B>
<B>foo</B>
0000
<B>define(bar,defn(`foo'))</B>
<B>bar</B>
0000
<B>changequote(<QUOTE>,<UNQUOTE>)</B>
<B>define(baz,defn(<QUOTE>foo<UNQUOTE>))</B>
<B>baz</B>
<B>Ctrl-d</B>
m4: End of input: 0: fatal error: EOF in string
</FONT></pre></td></tr></table></P><P>
Let us use GDB to try to see what is going on.
</P><P>
<TABLE><tr><td> </td><td class=smallexample><FONT SIZE=-1><pre>$ <B>gdb m4</B>
GDB 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 GDB; type "show warranty"
for details.
GDB 6.8, Copyright 1999 Free Software Foundation, Inc...
(gdb)
</FONT></pre></td></tr></table></P><P>
GDB 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 GDB to use a narrower display width than usual, so
that examples fit in this manual.
</P><P>
<TABLE><tr><td> </td><td class=smallexample><FONT SIZE=-1><pre>(gdb) <B>set width 70</B>
</FONT></pre></td></tr></table></P><P>
We need to see how the <CODE>m4</CODE> built-in <CODE>changequote</CODE> works.
Having looked at the source, we know the relevant subroutine is
<CODE>m4_changequote</CODE>, so we set a breakpoint there with the GDB
<CODE>break</CODE> command.
</P><P>
<TABLE><tr><td> </td><td class=smallexample><FONT SIZE=-1><pre>(gdb) <B>break m4_changequote</B>
Breakpoint 1 at 0x62f4: file builtin.c, line 879.
</FONT></pre></td></tr></table></P><P>
Using the <CODE>run</CODE> command, we start <CODE>m4</CODE> running under GDB
control; as long as control does not reach the <CODE>m4_changequote</CODE>
subroutine, the program runs as usual:
</P><P>
<TABLE><tr><td> </td><td class=smallexample><FONT SIZE=-1><pre>(gdb) <B>run</B>
Starting program: /work/Editorial/gdb/gnu/m4/m4
<B>define(foo,0000)</B>
<B>foo</B>
0000
</FONT></pre></td></tr></table></P><P>
To trigger the breakpoint, we call <CODE>changequote</CODE>. GDB
suspends execution of <CODE>m4</CODE>, displaying information about the
context where it stops.
</P><P>
<TABLE><tr><td> </td><td class=smallexample><FONT SIZE=-1><pre><B>changequote(<QUOTE>,<UNQUOTE>)</B>
Breakpoint 1, m4_changequote (argc=3, argv=0x33c70)
at builtin.c:879
879 if (bad_argc(TOKEN_DATA_TEXT(argv[0]),argc,1,3))
</FONT></pre></td></tr></table></P><P>
Now we use the command <CODE>n</CODE> (<CODE>next</CODE>) to advance execution to
the next line of the current function.
</P><P>
<TABLE><tr><td> </td><td class=smallexample><FONT SIZE=-1><pre>(gdb) <B>n</B>
882 set_quotes((argc >= 2) ? TOKEN_DATA_TEXT(argv[1])\
: nil,
</FONT></pre></td></tr></table></P><P>
<CODE>set_quotes</CODE> looks like a promising subroutine. We can go into it
by using the command <CODE>s</CODE> (<CODE>step</CODE>) instead of <CODE>next</CODE>.
<CODE>step</CODE> goes to the next line to be executed in <EM>any</EM>
subroutine, so it steps into <CODE>set_quotes</CODE>.
</P><P>
<TABLE><tr><td> </td><td class=smallexample><FONT SIZE=-1><pre>(gdb) <B>s</B>
set_quotes (lq=0x34c78 "<QUOTE>", rq=0x34c88 "<UNQUOTE>")
at input.c:530
530 if (lquote != def_lquote)
</FONT></pre></td></tr></table></P><P>
The display that shows the subroutine where <CODE>m4</CODE> is now
suspended (and its arguments) is called a stack frame display. It
shows a summary of the stack. We can use the <CODE>backtrace</CODE>
command (which can also be spelled <CODE>bt</CODE>), to see where we are
in the stack as a whole: the <CODE>backtrace</CODE> command displays a
stack frame for each active subroutine.
</P><P>
<TABLE><tr><td> </td><td class=smallexample><FONT SIZE=-1><pre>(gdb) <B>bt</B>
#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
</FONT></pre></td></tr></table></P><P>
We step through a few more lines to see what happens. The first two
times, we can use <SAMP>`s'</SAMP>; the next two times we use <CODE>n</CODE> to avoid
falling into the <CODE>xstrdup</CODE> subroutine.
</P><P>
<TABLE><tr><td> </td><td class=smallexample><FONT SIZE=-1><pre>(gdb) <B>s</B>
0x3b5c 532 if (rquote != def_rquote)
(gdb) <B>s</B>
0x3b80 535 lquote = (lq == nil || *lq == '\0') ? \
def_lquote : xstrdup(lq);
(gdb) <B>n</B>
536 rquote = (rq == nil || *rq == '\0') ? def_rquote\
: xstrdup(rq);
(gdb) <B>n</B>
538 len_lquote = strlen(rquote);
</FONT></pre></td></tr></table></P><P>
The last line displayed looks a little odd; we can examine the variables
<CODE>lquote</CODE> and <CODE>rquote</CODE> to see if they are in fact the new left
and right quotes we specified. We use the command <CODE>p</CODE>
(<CODE>print</CODE>) to see their values.
</P><P>
<TABLE><tr><td> </td><td class=smallexample><FONT SIZE=-1><pre>(gdb) <B>p lquote</B>
$1 = 0x35d40 "<QUOTE>"
(gdb) <B>p rquote</B>
$2 = 0x35d50 "<UNQUOTE>"
</FONT></pre></td></tr></table></P><P>
<CODE>lquote</CODE> and <CODE>rquote</CODE> 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 <CODE>l</CODE> (<CODE>list</CODE>) command.
</P><P>
<TABLE><tr><td> </td><td class=smallexample><FONT SIZE=-1><pre>(gdb) <B>l</B>
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
</FONT></pre></td></tr></table></P><P>
Let us step past the two lines that set <CODE>len_lquote</CODE> and
<CODE>len_rquote</CODE>, and then examine the values of those variables.
</P><P>
<TABLE><tr><td> </td><td class=smallexample><FONT SIZE=-1><pre>(gdb) <B>n</B>
539 len_rquote = strlen(lquote);
(gdb) <B>n</B>
540 }
(gdb) <B>p len_lquote</B>
$3 = 9
(gdb) <B>p len_rquote</B>
$4 = 7
</FONT></pre></td></tr></table></P><P>
That certainly looks wrong, assuming <CODE>len_lquote</CODE> and
<CODE>len_rquote</CODE> are meant to be the lengths of <CODE>lquote</CODE> and
<CODE>rquote</CODE> respectively. We can set them to better values using
the <CODE>p</CODE> command, since it can print the value of
any expression--and that expression can include subroutine calls and
assignments.
</P><P>
<TABLE><tr><td> </td><td class=smallexample><FONT SIZE=-1><pre>(gdb) <B>p len_lquote=strlen(lquote)</B>
$5 = 7
(gdb) <B>p len_rquote=strlen(rquote)</B>
$6 = 9
</FONT></pre></td></tr></table></P><P>
Is that enough to fix the problem of using the new quotes with the
<CODE>m4</CODE> built-in <CODE>defn</CODE>? We can allow <CODE>m4</CODE> to continue
executing with the <CODE>c</CODE> (<CODE>continue</CODE>) command, and then try the
example that caused trouble initially:
</P><P>
<TABLE><tr><td> </td><td class=smallexample><FONT SIZE=-1><pre>(gdb) <B>c</B>
Continuing.
<B>define(baz,defn(<QUOTE>foo<UNQUOTE>))</B>
baz
0000
</FONT></pre></td></tr></table></P><P>
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 <CODE>m4</CODE> exit by giving it an EOF as input:
</P><P>
<TABLE><tr><td> </td><td class=smallexample><FONT SIZE=-1><pre><B>Ctrl-d</B>
Program exited normally.
</FONT></pre></td></tr></table></P><P>
The message <SAMP>`Program exited normally.'</SAMP> is from GDB; it
indicates <CODE>m4</CODE> has finished executing. We can end our GDB
session with the GDB <CODE>quit</CODE> command.
</P><P>
<TABLE><tr><td> </td><td class=smallexample><FONT SIZE=-1><pre>(gdb) <B>quit</B>
</FONT></pre></td></tr></table></P><P>
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