WEBVTT 00:04.900 --> 00:10.000 So guys, now let us make ourselves familiar with the very fundamental problem to multithreading and 00:10.000 --> 00:13.970 thread synchronization, and that is the spurious wakeups. 00:13.990 --> 00:19.600 So when you design multithreading solutions or when you write Multithreading code, you need to take 00:19.600 --> 00:22.590 care that you do not cause spurious wakeup problem. 00:22.630 --> 00:29.080 So we will first discuss what spurious wakeup are and how it can be handled or it can be resolved. 00:29.410 --> 00:33.510 It is the programmers responsibility to avoid spurious wakeup. 00:33.520 --> 00:38.880 As such, there is no API or no system call which can avoid spurious wakeup. 00:38.890 --> 00:44.780 It's a logical problem which a developer of a multithreading applications must handle. 00:44.800 --> 00:47.590 So a spurious wakeup is all about that. 00:47.590 --> 00:53.920 You wake up only to find whatever you were promised or told has been broken or is a lie. 00:54.220 --> 00:55.000 Right? 00:55.700 --> 00:59.120 So probably this statement make less sense to you as of now. 00:59.120 --> 01:01.790 So let us discuss with the help of our example. 01:02.120 --> 01:08.480 So let us suppose that you receive a phone call from your friend and he invites you for a booze party 01:08.480 --> 01:09.980 at a common friend's house. 01:10.010 --> 01:10.730 Right? 01:10.730 --> 01:17.420 So common friend is a third friend where you guys are planning to get together while you are on the 01:17.420 --> 01:18.080 way. 01:18.080 --> 01:24.130 The common friend leaves the town out of some emergency and he did not even inform you. 01:24.140 --> 01:31.090 So what will happen that you will eventually reach his home only to find that the common friend is nowhere? 01:31.100 --> 01:31.790 Right. 01:31.790 --> 01:35.600 So this is a situation of spurious wakeup to you. 01:38.340 --> 01:40.290 You eventually find the truth. 01:40.290 --> 01:43.590 And the truth is no less surprise to you. 01:43.950 --> 01:49.800 It takes finite amount of time for you to reach your friend's house, and within that time interval, 01:49.800 --> 01:55.860 the situation changed such that it was futile for you to reach your friend's house eventually. 01:55.860 --> 01:56.430 Right? 01:57.400 --> 02:01.390 Let us take one more example which demonstrates the spurious wakeup. 02:01.510 --> 02:02.650 Your father. 02:02.680 --> 02:09.730 Let us suppose bought sweets, which is exclusive for you, and he advised you to consume those sweets 02:09.730 --> 02:11.080 after taking a bath. 02:11.110 --> 02:16.090 So you go for a bath and your sibling is somewhat notorious guy. 02:16.090 --> 02:22.690 He overheard this conversation between you and your father, and he eat all the sweets in the box, 02:22.690 --> 02:24.130 leaving the box empty. 02:24.160 --> 02:24.940 Right? 02:25.030 --> 02:32.260 Now you come out of the washroom after taking a bath and you open the sweet box only to find that the 02:32.260 --> 02:33.750 sweet box is empty. 02:33.760 --> 02:34.450 Right. 02:34.450 --> 02:35.500 So note the word. 02:35.500 --> 02:39.070 It is something against your expectation in the previous example. 02:39.070 --> 02:45.790 Also you reach your friend's house only to find that the friend was not at his house. 02:45.790 --> 02:49.590 So it was also something that was against your expectation. 02:49.600 --> 02:56.230 So in the second example also when you find the sweet box empty, then it was the spurious wakeup for 02:56.230 --> 02:56.930 you. 02:57.410 --> 03:00.410 So as a result, you are sad and angry. 03:00.420 --> 03:05.930 So when you wake up spuriously you find that the situation is against your expectations. 03:05.930 --> 03:12.620 In both of these examples, you ended up with a situation which was against your expectations, right? 03:12.920 --> 03:20.000 You think you have been given wrong information by your friend in the first example or by your father 03:20.000 --> 03:24.230 in the second example, but actually they did not give you wrong information. 03:24.230 --> 03:28.550 It just happened that somebody else cheated you. 03:28.550 --> 03:29.180 Right. 03:29.180 --> 03:35.870 In both of these examples, in the first example, the common friend cheated you knowingly or unknowingly, 03:35.870 --> 03:38.960 but your friend gave you the right information. 03:38.960 --> 03:39.620 Right? 03:39.650 --> 03:42.950 Similarly, in the second example, your father never cheated you. 03:42.980 --> 03:47.390 He gave you the right information to have a sweets in the sweet box. 03:47.390 --> 03:52.340 But it so happened that you eventually find the sweet box empty. 03:52.340 --> 03:55.640 And you think that your father lied to you, right? 03:55.640 --> 03:57.050 So now let us see that. 03:57.050 --> 04:01.580 How this spurious wakeup is relevant in the world of Multithreading. 04:02.190 --> 04:09.420 So when it comes to multithreading, when a thread gets unblocked, that is the thread which was blocked 04:09.450 --> 04:11.110 on the wait system call. 04:11.130 --> 04:18.240 So when the thread gets unblocked because it has received a signal, it can resume its execution due 04:18.240 --> 04:21.140 to a reason which is no more valid. 04:21.150 --> 04:21.810 Right. 04:21.810 --> 04:27.090 So in the next lecture video we will going to understand this point with the help of an example, the 04:27.090 --> 04:31.100 thread blocks because some condition was not fulfilled. 04:31.110 --> 04:38.670 But when a thread receives a signal and starts its execution, that condition, because of which it 04:38.670 --> 04:41.740 was blocked, is still not fulfilled. 04:41.760 --> 04:44.970 That is called spurious wakeup for a thread. 04:46.270 --> 04:53.110 So now let us see in the next lecture video how threads can be subjected to spurious wakeups and what 04:53.110 --> 04:58.960 shall be the problem that would happen if the threads are subjected to spurious wakeups? 04:58.960 --> 05:05.170 And as a developer or programmer, what you should do in order to prevent spurious wakeups of blocked 05:05.200 --> 05:06.040 threads.