Re: Thread crashing problem
Re: Thread crashing problem
- Subject: Re: Thread crashing problem
- From: "Michael Ash" <email@hidden>
- Date: Mon, 22 Dec 2008 17:33:11 -0500
On Mon, Dec 22, 2008 at 10:22 AM, Ken Tozier <email@hidden> wrote:
> How would one share a lock? Should I make it a property of the class? And
> then what? According to the NSLock documentation, multiple calls to tryLock
> are a no no, so how does one determine the current state of a lock? I didn't
> see any methods like "isLocked"
I hate to say this, but You Are Doing It Wrong.
Asking how to determine the current state of a lock is asking the
wrong question. Determining the current state of the lock is (almost)
never useful. It's certainly never useful here.
Determining the current state of the lock is like determining the
current state of a stoplight, only you're looking at the state of the
light for the intersecting road. If it's green, obviously stop. If
it's red... well, who knows?
Instead what you should do is simply stop your car if *your* light is
red, and continue if *your* light is green.
The way you do this is to lock and unlock. You simply do [lock lock],
then [lock unlock]. In the middle, you put the stuff that needs to be
protected. The system takes care of ensuring that only one person at a
time is allowed in the middle. You don't have to worry about how it
works or what state the lock is in or anything of the sort. Lock,
modify or read, unlock. That's it. And note that *every access to any
shared data* must be locked. You can't just lock the "threaded"
access. The moment any data is shared between threads, *all* accesses
*anywhere* must be locked.
My personal recommendation would be to just copy the array and give up
on locking. Pass the copy into your thread and then you have no shared
data. Shared data is the bane of threading. The moment two threads
share data, the potential for misfortune goes up by a factor of at
least a million. Avoid it whenever you can. The cost of copying an
array is irrelevant by comparison.
Mike
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