Re: NSUserDefaults & thread safety
Re: NSUserDefaults & thread safety
- Subject: Re: NSUserDefaults & thread safety
- From: Derrick Bass <email@hidden>
- Date: Wed, 25 Jan 2006 06:22:05 -0600
On Jan 25, 2006, at 5:47 AM, Jonathan del Strother wrote:
According to the documentation, NSUserDefaults isn't thread safe.
Does this mean that any access to NSUserDefaults outside of the
main thread is a really bad idea, or am I safe to restrict write-
access to the main thread and let any thread have read-access?
In general "not thread safe" means that it is okay to use from
different threads, as long as you make to sure to use some sort of
synchronization so only one thread at a time is accessing the
resource. For most purposes, all threads are equal; mutithreading
issues come not from doing things off the main thread, but from the
possibility that multiple threads might try to access the same
resource at the same time and mess one another up. That said, there
are a few things that are only safe from the main thread, but that
should be documented (NSAppleScript and some parts of QuickTime are
the two examples that come to mind).
You can often get away with reading from multiple threads without
synchronization. But you can never guarantee it will work (because
the reading routines might write some data to a cache, for example)
unless the documentation explicitly says so. So it always best to use
synchronization even if you will only ever be writing. And it is
never okay to write from a thread if there could be other threads
that are reading (what if they choose to read while something is half-
written?) without using synchronization.
I think that enclosing user defaults code in @synchronized
([NSUserDefaults standardUserDefaults]) should work.
Derrick
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