How does dispatch_once manages to run only once?

Question

I have been using dispatch_once for shared instances but what i don't understand is how dispatch_once works, why it runs only once.

score 1 · Answer 1 · answered Feb 20 '19 at 05:41

Low level computing answer.

Different threads that try to access the critical section — the code passed to dispatch_once — while a thread is already in this section are blocked until the critical section completes.

there-can-only-be-one

It should be noted that this just makes access to the shared instance thread safe. It does not make the class thread safe, necessarily.

score 0 · Answer 2 · edited May 23 '17 at 10:29

0

dispatch_once() is synchronous process and all GCD methods do things asynchronously (case in point, dispatch_sync() is synchronous)

The entire idea of dispatch_once() is "perform something once and only once", which is precisely what we're doing.

Source from Kevin Ballard

dispatch_once that’s used to guarantee that something happens exactly once, no matter how violent the program’s threading becomes

dispatch_once upon a time

edited May 23 '17 at 10:29

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answered Sep 09 '16 at 10:49

user3182143

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This is an answer to what is dispatch_once() and what it does. There is no information for, "How it manages to run only once?" – caffieneToCode Feb 20 '19 at 05:33

How does dispatch_once manages to run only once?

2 Answers2