android分析之Condition

Condition的含義是條件變量，其實現依賴於系統，一般都要配合Mutex使用，使用步驟為：給mutex上鎖（Lock），調用wait等待“條件”發生，如果沒有發生則re-wait（），最后釋放mutex（unlock），並繼續執行。所有等待（wait)同一個“條件變量(condition)”的線程都要使用相同的一把鎖——這樣相當於互斥操作該Condition。

// ---------------------------------------------------------------------------
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,//指定釋放跨進程共享
        SHARED = 1
    };

    Condition();
    Condition(int type);
    ~Condition();
    // Wait on the condition variable.  Lock the mutex before calling.
    status_t wait(Mutex& mutex);//通過Mutex來對“條件變量”實現互斥訪問——多線程、多進程場景
    // 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類型
#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) {//系統調用wait包含：釋放鎖、進入休眠等待、喚醒后重新獲取鎖
    return -pthread_cond_wait(&mCond, &mutex.mMutex);
}
inline status_t Condition::waitRelative(Mutex& mutex, nsecs_t reltime) {//具有時間控制
#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

　　Barrier是利用上面Condition的一個例子：

class Barrier
{
public:
    inline Barrier() : state(CLOSED) { }//state就是所謂的“條件”
    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);//臨時對象_l，用lock來構造，在AutoLock的構造函數里已給lock加鎖（調用lock（）函數）——該wait()函數執行完畢，會自動釋放lock(這個場景會使得其他線程再次修改state，產生不安全因素。不過由於Barrier的使用場景的特殊性，其用在線程初始化時，故OK。)
        while (state == CLOSED) {//while語句：不斷輪詢，直到state==OPENED
            cv.wait(lock);
        }
    }
private:
    enum { OPENED, CLOSED };
    mutable     Mutex       lock;//持有一個互斥鎖
    mutable     Condition   cv;//持有一個條件變量
    volatile    int         state;//每次都從內存更新的“條件”
};