Using the GNU Compiler Collection (GCC): Synchronization

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### 8.8 Synchronization

GNU Objective-C provides support for synchronized blocks:

      @synchronized (ObjCClass *guard) {
        …
      }

Upon entering the `@synchronized` block, a thread of execution shall first check whether a lock has been placed on the corresponding `guard` object by another thread. If it has, the current thread shall wait until the other thread relinquishes its lock. Once `guard` becomes available, the current thread will place its own lock on it, execute the code contained in the `@synchronized` block, and finally relinquish the lock (thereby making `guard` available to other threads).

Unlike Java, Objective-C does not allow for entire methods to be marked `@synchronized`. Note that throwing exceptions out of `@synchronized` blocks is allowed, and will cause the guarding object to be unlocked properly.

Because of the interactions between synchronization and exception handling, you can only use `@synchronized` when compiling with exceptions enabled, that is with the command line option `-fobjc-exceptions`.