gcc/doc/gcc/gnu-objective-c-features/synchronization.rst

   1 ..
   2   Copyright 1988-2022 Free Software Foundation, Inc.
   3   This is part of the GCC manual.
   4   For copying conditions, see the copyright.rst file.
   5
   6 .. _synchronization:
   7
   8 Synchronization
   9 ***************
  10
  11 GNU Objective-C provides support for synchronized blocks:
  12
  13 .. code-block:: objective-c
  14
  15     @synchronized (ObjCClass *guard) {
  16       ...
  17     }
  18
  19 Upon entering the ``@synchronized`` block, a thread of execution
  20 shall first check whether a lock has been placed on the corresponding
  21 ``guard`` object by another thread.  If it has, the current thread
  22 shall wait until the other thread relinquishes its lock.  Once
  23 ``guard`` becomes available, the current thread will place its own
  24 lock on it, execute the code contained in the ``@synchronized``
  25 block, and finally relinquish the lock (thereby making ``guard``
  26 available to other threads).
  27
  28 Unlike Java, Objective-C does not allow for entire methods to be
  29 marked ``@synchronized``.  Note that throwing exceptions out of
  30 ``@synchronized`` blocks is allowed, and will cause the guarding
  31 object to be unlocked properly.
  32
  33 Because of the interactions between synchronization and exception
  34 handling, you can only use ``@synchronized`` when compiling with
  35 exceptions enabled, that is with the command line option
  36 :option:`-fobjc-exceptions`.