How does synchronized lock unlock in Objective-C

Question

Does  synchronized not use  lock  and  unlock  to achieve mutual exclusion  How does it do lock unlock then   The output of the following program is only  Hello World     interface MyLock  NSLock lt NSLocking gt   end   implementation MyLock  -  id init       return  super init      -  void lock       NSLog   before lock         super lock       NSLog   after lock       -  void unlock       NSLog   before unlock         super unlock       NSLog   after unlock        end   int main  int argc  const char   argv          NSAutoreleasePool   pool     NSAutoreleasePool alloc  init        MyLock  lock     MyLock new  autorelease        synchronized lock            NSLog   Hello World                pool drain

User · Answer

It just associates a semaphore with every object  and uses that

User · Answer

Actually       synchronized self        return   myString retain  autorelease           transforms directly into      needs  import  lt objc objc-sync h gt      objc sync enter self      id retVal     myString retain  autorelease     objc sync exit self     return retVal      This API available since iOS 2 0 and imported using      import  lt objc objc-sync h gt

User · Answer

Apple s implementation of  synchronized is open source and it can be found here  Mike ash wrote two really interesting post about this subject    Locks  Thread Safety  and Swift Let s Build  synchronized   In a nutshell it has a table that maps object pointers  using their memory addresses as keys  to pthread mutex t locks  which are locked and unlocked as needed

User · Answer

The Objective-C language level synchronization uses the mutex  just like NSLock does  Semantically there are some small technical differences  but it is basically correct to think of them as two separate interfaces implemented on top of a common  more primitive  entity   In particular with a NSLock you have an explicit lock whereas with  synchronized you have an implicit lock associated with the object you are using to synchronize  The benefit of the language level locking is the compiler understands it so it can deal with scoping issues  but mechanically they behave basically the same   You can think of  synchronized as a compiler rewrite   -  NSString   myString      synchronized self        return   myString retain  autorelease           is transformed into   -  NSString   myString     NSString  retval   nil    pthread mutex t  self mutex   LOOK UP MUTEX self     pthread mutex lock self mutex     retval     myString retain  autorelease     pthread mutex unlock self mutex     return retval      That is not exactly correct because the actual transform is more complex and uses recursive locks  but it should get the point across

User · Answer

In Objective-C  a  synchronized block handles locking and unlocking  as well as possible exceptions  automatically for you  The runtime dynamically essentially generates an NSRecursiveLock that is associated with the object you re synchronizing on  This Apple documentation explains it in more detail  This is why you re not seeing the log messages from your NSLock subclass     the object you synchronize on can be anything  not just an NSLock   Basically   synchronized       is a convenience construct that streamlines your code  Like most simplifying abstractions  it has associated overhead  think of it as a hidden cost   and it s good to be aware of that  but raw performance is probably not the supreme goal when using such constructs anyway

[objective-c] How does @synchronized lock/unlock in Objective-C?

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