Macro for Singleton with GCD

gistfile1.c

#define SINGLETON(classname)                \
+(classname *)instance {                    \
    static dispatch_once_t onetime;         \
    static classname *shared = nil;         \
    dispatch_once(&onetime, ^{              \
        shared = [[classname alloc] init];  \
    });                                     \
    return shared;                          \
}