Difference between dispatch_async and dispatch_sync on serial queue?

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Yes. Using serial queue ensure the serial execution of tasks. The only difference is that dispatch_sync only return after the block is finished whereas dispatch_async return after it is added to the queue and may not finished.

for this code

dispatch_async(_serialQueue, ^{ printf("1"); });
printf("2");
dispatch_async(_serialQueue, ^{ printf("3"); });
printf("4");

It may print 2413 or 2143 or 1234 but 1 always before 3

for this code

dispatch_sync(_serialQueue, ^{ printf("1"); });
printf("2");
dispatch_sync(_serialQueue, ^{ printf("3"); });
printf("4");

it always print 1234

Note: For first code, it won’t print 1324. Because printf("3") is dispatched after printf("2") is executed. And a task can only be executed after it is dispatched.

The execution time of the tasks doesn’t change anything. This code always print 12

dispatch_async(_serialQueue, ^{ sleep(1000);printf("1"); });
dispatch_async(_serialQueue, ^{ printf("2"); });

What may happened is

  • Thread 1: dispatch_async a time consuming task (task 1) to serial queue
  • Thread 2: start executing task 1
  • Thread 1: dispatch_async another task (task 2) to serial queue
  • Thread 2: task 1 finished. start executing task 2
  • Thread 2: task 2 finished.

and you always see 12

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