Debugging with GDB (September 2007)

Chapter 1: A Sample GDB Session 7

1.6 Stepping into a subroutine

The 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.

(gdb) s

set_quotes (lq=0x34c78 "<QUOTE>", rq=0x34c88 "<UNQUOTE>")

at input.c:530

530 if (lquote != def_lquote)

1.7 Examining the Stack

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.

(gdb) 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 s ee what happens. The ﬁrst two times, we can use ‘s’;

the next two times we use n to avoid falling into the xstrdup subroutine.

(gdb) s

0x3b5c 532 if (rquote != def_rquote)

(gdb) s

0x3b80 535 lquote = (lq == nil || *lq == ’\0’) ? \

def_lquote : xstrdup(lq);

(gdb) n

536 rquote = (rq == nil || *rq == ’\0’) ? def_rquote\

: xstrdup(rq);

(gdb) n

538 len_lquote = strlen(rquote);

1.8 Printing Variable Values

The last line displayed looks a little odd in the listing above; 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 view their values.

(gdb) p lquote

$1 = 0x35d40 "<QUOTE>"

(gdb) p rquote

$2 = 0x35d50 "<UNQUOTE>"