Re: Thread safety question
Re: Thread safety question
- Subject: Re: Thread safety question
- From: Ernesto Corvi <email@hidden>
- Date: Fri, 18 Feb 2005 11:33:32 -0500
Hi Steve.
The problem you might encounter here, if you have global register allocation turned on, is that the compiler might cache 'variable_initialized' in a register.
So let's contemplate this scenario:
Thread1 -> Caches variable_initialized into R2, gets swapped out between the 'if ( !variable_initialized )' and the '[myLock lock]'.
Thread2 -> Executes the entire function and initializes the variable.
Thread1 -> Resumes execution, but since it cached 'variable_initialized' into R2, it still thinks its unitialized, and initializes again.
A portable solution to this problem is to declare 'variable_initialized' as:
volatile BOOL variable_initialized=NO;
The 'volatile' keyword hints the compiler not to cache the variable and force direct access to the variable every time it's accessed.
Ernesto.
On Feb 18, 2005, at 5:47 AM, Steve Gehrman wrote:
I have an object that lazily initializes it's variables when accessed and I want it to be thread safe, but I would like to avoid locking and unlocking the lock everytime every method is called.
Below is one solution, but I don't think it is thread safe. I'm assuming that if variable_initialized is YES, then dataPtr must be valid, but I'm afraid that the OS might not guarantee when and how data is flushed, so could it be possible for variable_initialized and myVariable to be invalid?
BOOL variable_initialized=NO;
void* myVariable;
NSLock* myLock; // assume lock has been initialized in main
- (void)isThisThreadSafe;
{
if ( !variable_initialized )
{
[myLock lock];
if ( !variable_initialized )
myVariable = allocateStuff();
variable_initialized = YES;
[myLock unlock];
}
return myVariable;
}
Can anyone confirm this, or offer advice on an alternate solution?
Thanks.
-steve
_______________________________________________
Do not post admin requests to the list. They will be ignored.
Darwin-dev mailing list (email@hidden)
Help/Unsubscribe/Update your Subscription:
This email sent to email@hidden