xpcom/base/StaticMutex.h

   1 #ifndef mozilla_StaticMutex_h
   2 #define mozilla_StaticMutex_h
   3
   4 #include "mozilla/Atomics.h"
   5 #include "mozilla/Mutex.h"
   6
   7 namespace mozilla {
   8
   9 /**
  10  * StaticMutex is a Mutex that can (and in fact, must) be used as a
  11  * global/static variable.
  12  *
  13  * The main reason to use StaticMutex as opposed to
  14  * StaticAutoPtr<OffTheBooksMutex> is that we instantiate the StaticMutex in a
  15  * thread-safe manner the first time it's used.
  16  *
  17  * The same caveats that apply to StaticAutoPtr apply to StaticMutex.  In
  18  * particular, do not use StaticMutex as a stack variable or a class instance
  19  * variable, because this class relies on the fact that global variablies are
  20  * initialized to 0 in order to initialize mMutex.  It is only safe to use
  21  * StaticMutex as a global or static variable.
  22  */
  23 class StaticMutex
  24 {
  25 public:
  26   // In debug builds, check that mMutex is initialized for us as we expect by
  27   // the compiler.  In non-debug builds, don't declare a constructor so that
  28   // the compiler can see that the constructor is trivial.
  29 #ifdef DEBUG
  30   StaticMutex()
  31   {
  32     MOZ_ASSERT(!mMutex);
  33   }
  34 #endif
  35
  36   void Lock()
  37   {
  38     Mutex()->Lock();
  39   }
  40
  41   void Unlock()
  42   {
  43     Mutex()->Unlock();
  44   }
  45
  46   void AssertCurrentThreadOwns()
  47   {
  48 #ifdef DEBUG
  49     Mutex()->AssertCurrentThreadOwns();
  50 #endif
  51   }
  52
  53 private:
  54   OffTheBooksMutex* Mutex()
  55   {
  56     if (mMutex) {
  57       return mMutex;
  58     }
  59
  60     OffTheBooksMutex* mutex = new OffTheBooksMutex("StaticMutex");
  61     if (!mMutex.compareExchange(nullptr, mutex)) {
  62       delete mutex;
  63     }
  64
  65     return mMutex;
  66   }
  67
  68   Atomic<OffTheBooksMutex*> mMutex;
  69
  70
  71   // Disallow copy constructor, but only in debug mode.  We only define
  72   // a default constructor in debug mode (see above); if we declared
  73   // this constructor always, the compiler wouldn't generate a trivial
  74   // default constructor for us in non-debug mode.
  75 #ifdef DEBUG
  76   StaticMutex(StaticMutex& aOther);
  77 #endif
  78
  79   // Disallow these operators.
  80   StaticMutex& operator=(StaticMutex* aRhs);
  81   static void* operator new(size_t) CPP_THROW_NEW;
  82   static void operator delete(void*);
  83 };
  84
  85 typedef BaseAutoLock<StaticMutex> StaticMutexAutoLock;
  86 typedef BaseAutoUnlock<StaticMutex> StaticMutexAutoUnlock;
  87
  88 } // namespace mozilla
  89
  90 #endif