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From: stevesu@copper.UUCP (Steve Summit)
Newsgroups: net.unix-wizards
Subject: Re: for adb hackers only
Message-ID: <334@copper.UUCP>
Date: Tue, 6-May-86 23:57:56 EDT
Article-I.D.: copper.334
Posted: Tue May  6 23:57:56 1986
Date-Received: Sat, 10-May-86 19:48:52 EDT
References: <4060004@csd2.UUCP>
Organization: Tektronix, Inc., Beaverton, OR
Lines: 67
Summary: What about conditional breakpoints?

I don't like to post without having a definitive answer, but
Wayne Berke posed an interesting question about adb about which
there has been no discussion, so I'd like to toss some
information in.

Wayne's article pointed out that adb won't automatically continue
after a breakpoint unless the internal variable adrflg is set.
As I understand the code, adrflg is set if an address expression
has been entered in the adb command string ("address" as in
"address,count command modifier", the "general form of a request"
mentioned on the second page of the adb tutorial).

I got bit by this bug when trying to do things like

	printf+2:b ,3$c;:c

I worked around it by doing something like

	printf+2,-1:b ,3$c;:c;environ/X

I am still not sure whether it was setting count to -1, examining
a nonzero address to set dot and adrflg, the :c itself, or some
combination, that was really getting me the free-running behavior
that I wanted.

It occurred to me a few days later that adb's testing things
before continuing after a breakpoint could be extremely useful if
you wanted to arrange that a breakpoint only caused a stop after
some complicated condition had been met.  For instance, if you
wanted to break only when some global variable had reached 0, you
could include an expression in your breakpoint action which
arranged to set dot to the value of your global variable.
If the breakpoint action otherwise tried to continue, it would
stop when your variable reached 0.

My reason for pointing this out is to make sure, if this
feature works and is useful, that nobody inadvertently breaks it
(no pun intended) when trying to fix breakpoints.

Another piece of information which I don't fully understand is
this footnote, from the bottom of page 6 or 7 of the adb
tutorial:

	* Owing to a bug in early versions of ADB (including the
	version distributed in Generic 3 UNIX) these statements
	must be written as:

		settab+4:b	settab,5?ia;0
		getc+4,3:b	main.c?C;0
		settab+4:b	settab,5?ia; ptab/o;0

	Note that ;0 will set dot to zero and stop at the breakpoint.

Anyhoo, although I certainly agree that the code looks fishy,
let's be careful before changing

	return(adrflg ANDF dot != 0);
to
	return(dot != 0)

One of the problems with these old programs that have twisted
control flow and lots of global variables is that it's very hard
to be sure that an apparently isolated change will not have
far-reaching and unintended effects.

                                         Steve Summit
                                         tektronix!copper!stevesu