android分析之mutex

 

Android的锁是对Linux锁的一种包装：

// ---------------------------------------------------------------------------
namespace android {
// ---------------------------------------------------------------------------

class Condition;

/*
 * 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,//该锁为本进程内使用
        SHARED = 1//该锁跨进程使用
    };
    
                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 {//内部类，用来管理mutex的--相当于“智能指针”
    public:
        inline Autolock(Mutex& mutex) : mLock(mutex)  { mLock.lock(); }
        inline Autolock(Mutex* mutex) : mLock(*mutex) { mLock.lock(); }
        inline ~Autolock() { mLock.unlock(); }
    private:
        Mutex& mLock;//持有mutex的引用
    };

private:
    friend class Condition;//友元类，Condition是在mutex的基础上使用的
    
    // A mutex cannot be copied
                Mutex(const Mutex&);//设为私有，无法调用“拷贝构造函数”
    Mutex&      operator = (const Mutex&);
    
#if defined(HAVE_PTHREADS)
    pthread_mutex_t mMutex;//使用linux的pthread_mutex_t这个互斥锁
#else
    void    _init();
    void*   mState;
#endif
};

// ---------------------------------------------------------------------------

#if defined(HAVE_PTHREADS)

inline Mutex::Mutex() {
    pthread_mutex_init(&mMutex, NULL);
}
inline Mutex::Mutex(const char* name) {
    pthread_mutex_init(&mMutex, NULL);
}
inline Mutex::Mutex(int type, const char* name) {
    if (type == SHARED) {//if分支：根据传进来的值，决定是否构造“跨进程”的锁
        pthread_mutexattr_t attr;
        pthread_mutexattr_init(&attr);
        pthread_mutexattr_setpshared(&attr, PTHREAD_PROCESS_SHARED);
        pthread_mutex_init(&mMutex, &attr);
        pthread_mutexattr_destroy(&attr);
    } else {
        pthread_mutex_init(&mMutex, NULL);//if分支：本进程内的锁
    }
}
inline Mutex::~Mutex() {
    pthread_mutex_destroy(&mMutex);
}
inline status_t Mutex::lock() {
    return -pthread_mutex_lock(&mMutex);
}
inline void Mutex::unlock() {
    pthread_mutex_unlock(&mMutex);
}
inline status_t Mutex::tryLock() {
    return -pthread_mutex_trylock(&mMutex);
}

#endif // HAVE_PTHREADS

// ---------------------------------------------------------------------------

/*
 * 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;

// ---------------------------------------------------------------------------
}; // namespace android