Re: problems while debugger is attached

I've had this many times when coding in a multi-threaded environment.
What will be happening is the attachment of the debugger is enough to
skew the timings of the function calls so that a race condition is
exposed.

When you say 'blocks on a function call and never returns', what
function call are you talking about? A system call or one of your own
functions? If its one of your own functions thats blocking, its a bit
easier to debug, because you can introduce logging around it. A system
call is a harder to diagnose - usually the sub-system the system call
is dealing with has some internal blocking, but its harder to figure
out where the race condition is coming from.

Before I started using test driven development for these things, I
could spend ages beating my head against weird behaviour like that.
The most effective solution I found for debugging strange race
conditions was to introduce sleeps of random and significant duration
around the system, to exacerbate any problems which don't show up
unless locks are held for a long while. Combine that with logging
around lock acquisition/releases so you can see what was going on
immediately prior to the deadlock, without having to use breakpoints.
Breakpoints don't help at all in these cases, because they screw up
the timing, and the nature of debuggers is often to suspend other
threads entirely when single-stepping through a thread - often making
the problem disappear entirely.

ChrisC

On 23/07/06, Andras Balogh <[email protected]> wrote:
> I have a strange bug, where if I'm running the program without the
> debugger attached (CTRL-F5 in VS), the program would run correctly both in
> debug and release builds. But when I run the program with F5, so the
> debugger is attached, then it blocks on a function call and never returns.
> If I put a breakpoint before this call, and step through it, then
> everything seems to work fine, and the function returns as expected..
>
> Could this be a deadlocking problem? Any ideas why attaching a debugger
> makes any difference? And hints on how to debug this?
>
> Thanks,
>
>
>
> Andras
>
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