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From: brianr@tekig4.TEK.COM (Brian Rhodefer)
Newsgroups: comp.sys.amiga
Subject: Task Spawning Difficulties
Message-ID: <1592@tekig4.TEK.COM>
Date: Thu, 18-Jun-87 21:29:56 EDT
Article-I.D.: tekig4.1592
Posted: Thu Jun 18 21:29:56 1987
Date-Received: Mon, 22-Jun-87 01:10:41 EDT
Organization: Tektronix Inc., Beaverton, Or.
Lines: 69
Keywords: task spawn crash guru debug manx


A few days ago, I posted a plea for help with the problem I had
trying to spawn a new task from a Manx Aztec C program (compiled
with ver 3.40a) on the Amiga: the Amiga would crash during the
AddTask library function call.

Although Ali Ozer (sp?) very kindly sent me a working example
of task spawning,  no one suggested what my difficulty was.
Is it possible then, that the solution merits posting?

With apologies to millions of detective story fans: "DeBUGGER DID IT!"

Since I had never created a new task before, I ran my program
under the Manx debugger, "db", so I could watch what went on
very closely.  I followed my program's execution to a point just
prior to its call on "CreateTask" (A Manx c.lib routine).  I then
set a breakpoint on the instruction following the call, and another
breakpoint on the first instruction of the task I was adding.
(It was merely a subroutine in the code already loaded, so it was
already resident).  The new task was being "Create"d at priority
22, higher than anything else in the system, because I wanted to
ensure that it started immediately.  I expected, when I told the
debugger to "go", that the breakpoint at the beginning of the
new task would be hit; instead, the Amiga crashed (Gurued).

Repeating the proceedure, but single-stepping through "CreateTask"
disclosed that the crash came somewhere in the middle of "CreateTask"s
call on "AddTask".

Was something wrong in the Task structure that "CreateTask" sets up?
I re-ran the program, stopping short of "AddTask", and dumped out the
contents of the structure to a log file.  Since Ali's program ran OK,
I ran the debugger on IT, so I could dump ITS Task structure for
comparison.  It was virtually identical to mine.

Puzzled, I hit "go", AND THE AMIGA CRASHED AGAIN!

I ran my test program by itself, (without the debugger)
and it worked fine.

Further experimentation suggested that the crash occurred BECAUSE
I SET A BREAKPOINT IN A DIFFERENT TASK THAN THE ONE THE DEBUGGER
HAD STARTED UP ON.

Right about then, I figured that a thorough reading of the debugger
manual was in order.  While I could find no explicit caution against
the perils of setting breakpoints in other tasks, I DID find commands
described for opening new debugger windows,

  "..to allow debugging of multiple tasks..".

Using these hitherto ignored commands ("an" and "as"), I was able
to debug my newly created task without crashing. 

I don't know how Manx' debugger works, but It BEHAVES like the
in-circuit emulators I'm familiar with.  To an in-circuit emulator,
a breakpoint is a breakpoint, in fine egalitarian fashion.
In "db", though, breakpoints and tasks are interrelated somehow.
I don't mind having to use multiple windows, even if I don't understand
why it's necessary, except that it seems like the code between the
start of a new task and the first time it does a Wait is
irrevocably inaccessible to the debugger.

Summarizing (with an apology for longwindedness):

  *** ONLY  DEBUG  ONE  TASK  PER  DEBUG  WINDOW! *** 


Brian Rhodefer    ....tektronix!tekig4!brianr