HP gnu source-level debugger 5992-4701 1.6Stepping into a subroutine, 1.7 Examining the Stack, 1.8Printing Variable Values

((gdb)) n

882 set_quotes((argc >= 2) ? TOKEN_DATA_TEXT(argv[1])\

:nil,

1.6Stepping 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 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.

((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);

1.8Printing 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.

1.6 Stepping into a subroutine 21