c++ - Exception thrown when locking boost::unique_lock which is already locked in other thread -

there global object boost::unique_lock. 1 thread locks it. when other thread tries lock exception "already owns mutex" thrown. why happening? expect thread blocked until other thread call unlock.

boost::mutex mutex; boost::unique_lock<boost::mutex> lock(mutex);  static void* scheduller(void* arg) {     boost::this_thread::sleep(boost::posix_time::seconds(5));     lock.lock();         return 0; }  static void* usb(void* arg) {     lock.lock();     boost::this_thread::sleep(boost::posix_time::seconds(20));     return 0; }  int bevent_test() {     lock.unlock();     int = 0;     boost::thread thread1(usb,&a);     boost::thread thread2(scheduller,&a);     boost::this_thread::sleep(boost::posix_time::seconds(100));     return 0;  } 

in bevent_test function i'm doing unlock because unique lock locked in constructor. delays make sure scheduller thread starts after usb thread. when tries lock() exception thrown. why?

your use of unique_lock wrong, in particular objects not supposed shared between different threads, because not thread-safe. yes, sounds weird @ first, make sense, because being thread-safe , providing thread-safety 2 distinct things.

the idea of unique_lock manage locks on mutexes. that, create objects locally only. that's reason locked, seem "work around" unlocking it. however, should rather try use scope/lifetime manage lock.

now, need unique_lock in order use event. yes , no. need one, not same 1 every thread using event. instead, can use different, local lock instances different threads. btw: reason event needs lock while waiting event, lock must released. after receiving event (or timeout), has reacquire lock. in other words, lock serves proxy mutex, offering indirection , added guarantees.

the reason receive exception try lock unique_lock twice. don't think matters same thread, wouldn't same thread. however, when lock mutex (via unique_lock), entering critical section. implies before, weren't in critical section. if unique_lock finds lock held, means in code went wrong, because code seems confused whether inside critical section or not. reason, exception motivation fix code.