A lockless way of setting a lazily-initialized static global value. It works by using a CompareAndSwap atomic operation with a memory barrier to sync threads' instruction & data caches. If another thread sets the value since we looked at __staticVar, Comp

gistfile1.m

#import <libkern/OSAtomic.h>

@implementation SomeClass

+ (id) someStaticValueComputedOnFirstAccess
{
    static volatile id __staticVar = nil;
    if ( __staticVar == nil )
    {
        id var = [[Something alloc] init];
        if ( OSAtomicCompareAndSwapPtrBarrier(nil, var, &__staticVar) == false )
            [var release];  // already set by another thread, so release this one
    }
    
    return ( __staticVar );
}