Question

I am using a pretty decent tutotial I have found for GCD, in it it shows you how to declare a new dispatch queue.

Grand Central Dispatch operates using queues. Queues are a C typedef:
dispatch_queue_t. To get a new global queue, we call
dispatch_get_global_queue(), which takes two arguments: a long for
priority and an unsigned long for options, which is unused, so we’ll
pass 0ul. Here’s how we get a high-priority queue:

In it it says to pass 0ul in as a parameter like so

dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0ul);

Im hoping someone can explain to me what 0ul is? and why its included?

Answer 1

It’s just 0 and the ul is telling the compiler that you want it to be an unsigned long to match the function signature.

dispatch_queue_t dispatch_get_global_queue(long priority, unsigned long flags);

If you don’t add the ul the 0 will get treated as in integer which may cause a compiler warning.

Please look at the documentation here.

https://developer.apple.com/library/mac/#documentation/Performance/Reference/GCD_libdispatch_Ref/Reference/reference.html