// ---------------------------------------------------------------------------namespace android {// ---------------------------------------------------------------------------/* * 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,//指定释放跨进程共享        SHARED = 1    };    Condition();    Condition(int type);    ~Condition();    // Wait on the condition variable.  Lock the mutex before calling.    status_t wait(Mutex& mutex);//通过Mutex来对“条件变量”实现互斥访问——多线程、多进程场景    // same with relative timeout    status_t waitRelative(Mutex& mutex, nsecs_t reltime);    // Signal the condition variable, allowing one thread to continue.    void signal();    // Signal the condition variable, allowing all threads to continue.    void broadcast();private:#if defined(HAVE_PTHREADS)    pthread_cond_t mCond;//Linux的pthread_cond_t类型#else    void*   mState;#endif};// ---------------------------------------------------------------------------#if defined(HAVE_PTHREADS)inline Condition::Condition() {    pthread_cond_init(&mCond, NULL);}inline Condition::Condition(int type) {    if (type == SHARED) {        pthread_condattr_t attr;        pthread_condattr_init(&attr);        pthread_condattr_setpshared(&attr, PTHREAD_PROCESS_SHARED);        pthread_cond_init(&mCond, &attr);        pthread_condattr_destroy(&attr);    } else {        pthread_cond_init(&mCond, NULL);    }}inline Condition::~Condition() {    pthread_cond_destroy(&mCond);}inline status_t Condition::wait(Mutex& mutex) {//系统调用wait包含：释放锁、进入休眠等待、唤醒后重新获取锁    return -pthread_cond_wait(&mCond, &mutex.mMutex);}inline status_t Condition::waitRelative(Mutex& mutex, nsecs_t reltime) {//具有时间控制#if defined(HAVE_PTHREAD_COND_TIMEDWAIT_RELATIVE)    struct timespec ts;    ts.tv_sec  = reltime/1000000000;    ts.tv_nsec = reltime%1000000000;    return -pthread_cond_timedwait_relative_np(&mCond, &mutex.mMutex, &ts);#else // HAVE_PTHREAD_COND_TIMEDWAIT_RELATIVE    struct timespec ts;#if defined(HAVE_POSIX_CLOCKS)    clock_gettime(CLOCK_REALTIME, &ts);#else // HAVE_POSIX_CLOCKS    // we don‘t support the clocks here.    struct timeval t;    gettimeofday(&t, NULL);    ts.tv_sec = t.tv_sec;    ts.tv_nsec= t.tv_usec*1000;#endif // HAVE_POSIX_CLOCKS    ts.tv_sec += reltime/1000000000;    ts.tv_nsec+= reltime%1000000000;    if (ts.tv_nsec >= 1000000000) {        ts.tv_nsec -= 1000000000;        ts.tv_sec  += 1;    }    return -pthread_cond_timedwait(&mCond, &mutex.mMutex, &ts);#endif // HAVE_PTHREAD_COND_TIMEDWAIT_RELATIVE}inline void Condition::signal() {    pthread_cond_signal(&mCond);}inline void Condition::broadcast() {    pthread_cond_broadcast(&mCond);}#endif // HAVE_PTHREADS// ---------------------------------------------------------------------------}; // namespace android

class Barrier{public:    inline Barrier() : state(CLOSED) { }//state就是所谓的“条件”    inline ~Barrier() { }    void open() {        Mutex::Autolock _l(lock);        state = OPENED;        cv.broadcast();    }    void close() {        Mutex::Autolock _l(lock);        state = CLOSED;    }    void wait() const {        Mutex::Autolock _l(lock);//临时对象_l，用lock来构造，在AutoLock的构造函数里已给lock加锁（调用lock（）函数）——该wait()函数执行完毕，会自动释放lock(这个场景会使得其他线程再次修改state，产生不安全因素。不过由于Barrier的使用场景的特殊性，其用在线程初始化时，故OK。)        while (state == CLOSED) {//while语句：不断轮询，直到state==OPENED            cv.wait(lock);        }    }private:    enum { OPENED, CLOSED };    mutable     Mutex       lock;//持有一个互斥锁    mutable     Condition   cv;//持有一个条件变量    volatile    int         state;//每次都从内存更新的“条件”};