Debugging with GDB (September 2007)

8 Debugging with GDB

1.9 Listing Source Code

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.

(gdb) 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.

(gdb) n

539 len_rquote = strlen(lquote);

(gdb) n

540 }

(gdb) p len lquote

$3 = 9

(gdb) p len rquote

$4 = 7

1.10 Setting Variable Values During a Session

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.

(gdb) p len lquote=strlen(lquote)

$5 = 7

(gdb) 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:

(gdb) 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 to exit by giving it

an EOF as input: