WEBVTT 00:06.990 --> 00:12.690 So guys, now continuing from the previous lecture video, let us take one more round of discussion 00:12.690 --> 00:16.050 discussing the difference between condition variables and mutex. 00:16.470 --> 00:21.540 So Mutex is used to grant an access to the resource if it is not locked already. 00:21.540 --> 00:22.260 Right. 00:22.620 --> 00:29.520 So now this statement is an obvious to us now condition Variables, on the other hand, allows thread 00:29.550 --> 00:31.920 to inspect the resource state. 00:31.950 --> 00:38.040 Right and decide if the thread wants to wait for favorable resource state. 00:38.070 --> 00:43.710 That is, if the resource is not in a position to be accessed by the thread, then in that situation 00:43.740 --> 00:51.300 a thread may choose to block itself and it will continue to be blocked until until the state of the 00:51.300 --> 00:56.130 resource is changed such that the thread can now access the resource. 00:56.310 --> 01:01.140 So let us try to discuss this point with the help of an example you can see on the screen. 01:01.140 --> 01:03.000 The screen is divided into two parts. 01:03.030 --> 01:04.140 The left hand side. 01:04.140 --> 01:07.330 I will try to explain Mutexes and the right hand side. 01:07.330 --> 01:12.790 I will try to explain condition variables and try to contrast the difference between mere access and 01:12.790 --> 01:14.050 condition variables. 01:14.050 --> 01:20.110 So going with the example mutex is all about granting an access to the laptop. 01:20.110 --> 01:24.370 For example, if the laptop is not being used by somebody else. 01:24.370 --> 01:25.090 Right. 01:25.090 --> 01:27.070 So this is as simple as that. 01:27.100 --> 01:32.650 Here the resource is the laptop and if the laptop is not being used by somebody else, then you can 01:32.650 --> 01:34.210 very well access the laptop. 01:34.540 --> 01:35.230 Condition. 01:35.230 --> 01:42.040 Variables, on the other hand, grants an access to the laptop only if it is not being used by somebody 01:42.040 --> 01:46.690 else and if it has an internet connection. 01:46.810 --> 01:52.690 So you can see in the case of condition variables, we have added an additional condition. 01:52.690 --> 01:53.440 Right? 01:55.420 --> 02:02.020 That is, you are allowed to access the laptop only and only if the laptop is not being used by somebody 02:02.020 --> 02:02.680 else. 02:02.710 --> 02:05.650 As well as the laptop has an internet connection. 02:05.890 --> 02:11.830 So you can see that the mutex only provides mutual exclusion, whereas condition variables provides 02:11.830 --> 02:17.190 mutual exclusion plus the custom condition based access to the user. 02:17.200 --> 02:17.950 Right. 02:18.310 --> 02:24.700 When you translate the mutexes and the condition variables usage into the code, you will see that the 02:24.700 --> 02:28.060 code on the left hand side demonstrates the use of mutexes. 02:28.060 --> 02:32.980 And now, while you have progressed this much into this course, you must be pretty much familiar with 02:32.980 --> 02:34.060 this piece of code. 02:34.210 --> 02:41.230 Once the thread t is able to grant a lock on the laptop murex the thread can perform any operation on 02:41.260 --> 02:42.100 that laptop. 02:42.100 --> 02:42.730 Right? 02:42.730 --> 02:44.770 So this is the critical section. 02:44.770 --> 02:52.780 Whereas on the other hand, in case of condition variables, a thread not only get an exclusive access 02:52.780 --> 02:59.270 on the laptop that is entered into the critical section, but also the thread checks whether the laptop 02:59.270 --> 03:01.370 has an internet connection or not. 03:01.490 --> 03:07.430 Now, in this case, if the laptop do not have an internet connection, the thread t decides to block 03:07.430 --> 03:08.180 itself. 03:08.180 --> 03:08.960 Right. 03:09.110 --> 03:15.020 So in the case of condition variable, you can see that we are using mutex as well as as well as the 03:15.020 --> 03:16.220 condition variable. 03:16.310 --> 03:19.880 The mutex helps in providing the mutual exclusion. 03:20.000 --> 03:23.840 That is, while the thread is inspecting the resource state. 03:23.870 --> 03:27.920 No other thread of the process should change the resource state. 03:27.920 --> 03:28.700 Right. 03:28.820 --> 03:36.650 So once the thread t gets a lock on the mutex, no other thread can change the state of internet connectivity 03:36.650 --> 03:37.730 of this laptop. 03:37.730 --> 03:38.480 Right. 03:38.480 --> 03:41.780 So it is for this reason that this mutex is required. 03:42.110 --> 03:47.840 Mutex helps the thread to exclusively check the resource state. 03:49.430 --> 03:52.520 Then the second part is the usage of this condition variable. 03:52.550 --> 03:57.840 This condition variables allows the calling thread to get blocked. 03:57.860 --> 04:04.160 If the thread do not find the resource in its favorable condition here, the favorable condition is 04:04.160 --> 04:07.130 that that the laptop should have an internet connection. 04:07.310 --> 04:14.030 But here if the thread t do not find the laptop having an internet connection, the thread can choose 04:14.030 --> 04:16.450 to block itself using condition variable. 04:16.460 --> 04:17.240 Right? 04:18.110 --> 04:24.170 So the whole essence here is that that the mutex only provides mutual exclusion, whereas condition 04:24.170 --> 04:31.430 variable is all about accessing the resource only when all the conditions which are in favor of providing 04:31.430 --> 04:34.340 the access of the resource to the thread are met. 04:35.150 --> 04:39.970 So note here is that that condition variable cannot be used without Mirax. 04:39.980 --> 04:43.310 There should be a paired mutex with the condition variable. 04:43.310 --> 04:44.090 Right. 04:44.820 --> 04:51.150 In other words, condition variable and mutex is a combo to be used together always. 04:51.450 --> 04:57.750 You need a mutual exclusion using mutex so that while at the time of testing the state of the resource, 04:57.780 --> 05:02.610 no other thread of the process changes the state of the resource. 05:02.640 --> 05:08.400 So summarizing the points in the condition variables condition variables are not used for mutual exclusion, 05:08.400 --> 05:10.320 but they are used for coordination. 05:10.320 --> 05:13.050 That is the coordination between the threads. 05:13.410 --> 05:16.170 So this is what we were going to discuss next. 05:16.410 --> 05:20.940 First, let us understand the basics of condition variables and how they are used. 05:20.970 --> 05:25.140 Then we will deep dive into the advanced applications of condition variables. 05:25.140 --> 05:31.800 So to put it simple, using condition variables, a thread can block itself if certain condition is 05:31.800 --> 05:32.610 not met. 05:32.610 --> 05:33.210 Right? 05:33.240 --> 05:36.690 For example, laptop not having an internet connectivity. 05:36.690 --> 05:42.810 So the API used by a thread to block itself using condition variable is p thread condition. 05:42.810 --> 05:43.500 Wait. 05:43.500 --> 05:44.250 Right. 05:44.940 --> 05:46.560 And using condition variables. 05:46.590 --> 05:52.250 A thread can signal another already blocked thread on a condition variable to resume. 05:52.260 --> 05:56.110 For that, the thread has to use an API thread condition signal. 05:56.130 --> 06:02.820 So if you have a thread t1 and in the execution flow of the thread t1 the thread t1 choose to block 06:02.820 --> 06:05.850 at the point p and to block at the point p. 06:05.880 --> 06:08.370 The thread uses an api p thread condition. 06:08.370 --> 06:09.070 Wait. 06:09.090 --> 06:11.640 So for short form I am just writing. 06:11.640 --> 06:12.240 Wait. 06:12.240 --> 06:12.960 Right. 06:13.560 --> 06:19.680 And while the threat even is blocked at the point P because the threat T one has invoked p thread condition 06:19.680 --> 06:27.210 wait call the other thread of the process during the course of its execution may choose at some random 06:27.210 --> 06:36.420 point Q to send a signal to the thread t one and when this signal is received by the thread t one the 06:36.420 --> 06:39.610 thread t one then resumes its execution. 06:39.630 --> 06:40.370 Right? 06:40.380 --> 06:44.970 So this signal is sent using this api p thread condition signal. 06:45.720 --> 06:50.760 So in the next lecture video we will going to discuss the concept of wait and signals. 06:50.760 --> 06:56.910 And these wait and signals are nothing, but these are the end result of invention of condition variables. 06:56.910 --> 06:57.660 Right? 06:57.930 --> 07:06.540 Using wait the thread block itself and using signals one thread unblocks the other already blocked thread.