Android同步类：Mutex和Condition

 Mutex就是操作系统中的互斥

关于Mutex的使用，除了初始化外，最重要的是lock和unlock函数的使用，它们的用法如下：
 要想独占卫生间，必须先调用Mutex的lock函数。这样，这个区域就被锁住了。如果这块区域之前已被别人锁住，lock函数则会等待，直到可以进入这块区域为止。系统保证一次只有一个线程能lock成功。
 当你“方便”完毕，记得调用Mutex的unlock以释放互斥区域。这样，其他人的lock才可以成功返回。
 另外，Mutex还提供了一个trylock函数，该函数只是尝试去锁住该区域，使用者需要根据trylock的返回值来判断是否成功锁住了该区域。

system/core/include/utils/Mutex.h

/*
 * Simple mutex class.  The implementation is system-dependent.
 * The mutex must be unlocked by the thread that locked it.  They are not
 * recursive, i.e. the same thread can't lock it multiple times.
 */
class Mutex {
public:
    enum {
        PRIVATE = 0,
        SHARED = 1  //type如果是SHARED，则表明这个Mutex支持跨进程的线程同步。
    };

                Mutex();
                Mutex(const char* name);
                Mutex(int type, const char* name = NULL);
                ~Mutex();

    // lock or unlock the mutex
    status_t    lock();
    void        unlock();

    // lock if possible; returns 0 on success, error otherwise
    status_t    tryLock();

    // Manages the mutex automatically. It'll be locked when Autolock is
    // constructed and released when Autolock goes out of scope.
    class Autolock {
    public:
        inline Autolock(Mutex& mutex) : mLock(mutex)  { mLock.lock(); }
        inline Autolock(Mutex* mutex) : mLock(*mutex) { mLock.lock(); }
        inline ~Autolock() { mLock.unlock(); }//超过作用域自动解锁
    private:
        Mutex& mLock;
    };

private:
    friend class Condition;

    // A mutex cannot be copied
                Mutex(const Mutex&);
    Mutex&      operator = (const Mutex&);

#if defined(HAVE_PTHREADS)
    pthread_mutex_t mMutex;
#else
    void    _init();
    void*   mState;
#endif
};

/*
 * Automatic mutex.  Declare one of these at the top of a function.
 * When the function returns, it will go out of scope, and release the
 * mutex.
 */
typedef Mutex::Autolock AutoMutex;

 system/core/include/utils/Condition.h

/*
 * Condition variable class.  The implementation is system-dependent.
 *
 * Condition variables are paired up with mutexes.  Lock the mutex,
 * call wait(), then either re-wait() if things aren't quite what you want,
 * or unlock the mutex and continue.  All threads calling wait() must
 * use the same mutex for a given Condition.
 */
class Condition {
public:
    enum {
        PRIVATE = 0,
        SHARED = 1
    };

    enum WakeUpType {
        WAKE_UP_ONE = 0,
        WAKE_UP_ALL = 1
    };

    Condition();
    Condition(int type);
    ~Condition();
    // Wait on the condition variable.  Lock the mutex before calling.
    status_t wait(Mutex& mutex);
    // same with relative timeout
    status_t waitRelative(Mutex& mutex, nsecs_t reltime);
    // Signal the condition variable, allowing exactly one thread to continue.
    void signal();
    // Signal the condition variable, allowing one or all threads to continue.
    void signal(WakeUpType type) {
        if (type == WAKE_UP_ONE) {
            signal();
        } else {
            broadcast();
        }
    }
    // Signal the condition variable, allowing all threads to continue.
    void broadcast();

private:
#if defined(HAVE_PTHREADS)
    pthread_cond_t mCond;
#else
    void*   mState;
#endif
};