WEBVTT 00:04.950 --> 00:09.450 So guys, next we will going to understand the Linux except system call. 00:09.720 --> 00:14.370 So except system call plays a very vital role in socket programming. 00:14.820 --> 00:21.870 It is the accept system call which establishes the duplex or bidirectional communication between the 00:21.870 --> 00:26.610 client and the server processes which wants to communicate. 00:27.960 --> 00:33.300 So you can see in this diagram we have a server and we have already discussed that when a server boots 00:33.300 --> 00:35.410 up, that is when the server starts. 00:35.430 --> 00:42.000 The first thing the server process needs to do is to create a master socket file descriptor because 00:42.000 --> 00:47.250 it is the master socket file descriptor using which the server will accept the connection. 00:47.870 --> 00:54.440 Because the purpose of master socket file descriptor is only to establish the connection with the new 00:54.440 --> 00:55.250 clients. 00:56.720 --> 00:57.440 Right. 00:57.440 --> 01:04.070 So we assume that the server has booted up and it has created the master socket file descriptor already. 01:04.640 --> 01:12.590 Now when the server receives a new connection initiation request message from the new client, this 01:12.590 --> 01:19.550 connection initiation request message goes on a master socket maintained by the server that is master 01:19.550 --> 01:20.810 socket file descriptor. 01:20.990 --> 01:27.440 And whenever any message arrives on any socket, in this case the master socket file descriptor, that 01:27.440 --> 01:30.680 particular master socket file descriptor is activated. 01:31.610 --> 01:38.180 So you can see that the client C one sends the connection initiation request to the server. 01:38.300 --> 01:44.060 This connection initiation request always arrives at the master socket file descriptor of the server. 01:44.360 --> 01:47.840 This is done by the operating system itself. 01:47.840 --> 01:54.860 The underlying operating system always redirects the connection initiation request messages to the master 01:54.860 --> 01:56.930 socket file descriptor of the server. 01:58.560 --> 01:59.220 Right. 01:59.220 --> 02:05.160 So in the next step, when the server receives connection, initiation requests from the new client 02:05.160 --> 02:10.800 server invokes the accept system call to establish the bi directional communication. 02:10.980 --> 02:15.750 So in the next step, the server has invoked the accept system call. 02:17.530 --> 02:22.570 So remember, whenever the server receives the connection initiation request on a master socket file 02:22.570 --> 02:28.060 descriptor, the next step is always to invoke the accept system call. 02:28.270 --> 02:36.880 And as a result of invoking the accept system call the server creates a client handle or communication 02:36.880 --> 02:37.880 file descriptor. 02:37.900 --> 02:44.200 So the return value of accept system call is a client handle which we also call as communication file 02:44.200 --> 02:45.040 descriptor. 02:45.520 --> 02:46.160 Right. 02:46.180 --> 02:49.060 So remember, it is always an integer value. 02:49.060 --> 02:52.750 It could be six, it could be seven, eight or anything else. 02:52.780 --> 02:59.230 So when we will be discussing the code, we will see that file descriptors, whether it is communication 02:59.230 --> 03:04.060 file descriptor or master socket file descriptors are always integer values. 03:06.350 --> 03:12.650 Now, once the communication file descriptor for client C1 is created, the server will use this handle 03:12.680 --> 03:15.740 to carry out actual data exchange with the client. 03:15.770 --> 03:16.400 C1. 03:18.270 --> 03:18.800 Right. 03:18.810 --> 03:25.800 The server will use this handle to send the data as well as to receive the data from the client. 03:25.840 --> 03:26.520 C1. 03:27.720 --> 03:34.470 So you can see here that the client handles are used for actual communication with the client. 03:36.460 --> 03:36.990 Right. 03:37.570 --> 03:42.880 Also accept system call is used only for connection oriented communication. 03:42.880 --> 03:46.330 It is not used for connectionless communication. 03:46.330 --> 03:50.170 So there are two types of communication between the client and the server. 03:50.560 --> 03:56.560 The first type of communication is called connection oriented communication, and the second type of 03:56.560 --> 04:00.040 communication is called Connectionless communication. 04:01.550 --> 04:08.090 We shall discuss in detail that what are the properties of connection oriented communication and connectionless 04:08.090 --> 04:10.820 communication in the next slide. 04:11.600 --> 04:16.970 But remember, except system call is used only when the type of communication between the client and 04:16.970 --> 04:20.540 the server is connection oriented communication. 04:22.170 --> 04:22.800 Right. 04:22.800 --> 04:29.160 So likewise, going forward, suppose a client C two, which is now a new client, also sends the connection 04:29.160 --> 04:31.350 initiation request to the server. 04:31.380 --> 04:35.700 The master socket will be activated and the accept system call will be invoked. 04:35.700 --> 04:41.790 And as a result, a new client handle will be created which will be used by the server for carrying 04:41.790 --> 04:44.520 out data exchange with the client. 04:44.550 --> 04:45.210 C two. 04:47.620 --> 04:53.020 So you can see here that except system call is used to create client handles.