The difference between GCD multithreading-dispatch_async and Dispatch_sync

From the literal distinction between async and sync, async is the asynchronous abbreviation, and sync is the abbreviation for synchronization.

1, dispatch_async means asynchronous processing, the main program does not wait for block blocks to execute the results.

int Main (intconstChar * argv[]) {    = dispatch_get_main_queue ();    ^{        NSLog (@ "+++++++");});    NSLog (@ "hahahaha");     return 0 ;}

Output Result: hahahaha

No output +++++++ in results

Result Analysis : Asynchronous execution does not affect the execution of the main thread, and the block is added to the main queue at the end. But because it runs directly to the return line, the main thread is killed and the block blocks added to the main thread are not executed. If you are in an app project, you will first output hahahaha and then output +++++++.

2, dispatch_sync means that synchronous processing, the main program must wait until block blocks are executed, can continue to execute down.

    dispatch_queue_t Queuemain = dispatch_get_main_queue ();     ^{        NSLog (@ "+++++++");});    NSLog (@ "hahahaha");

The program stops at the Dispatch_sync () line.

result Analysis : Because the main thread executes to Dispatch_sync (), it is in a wait state. Add the block block to the main queue and wait for the current Mian thread to execute the block block after the task has finished executing. Cause both sides are waiting, so there is a deadlock, the program stuck.

GCD multithreading-dispatch_async differs from Dispatch_sync