WEBVTT 00:06.550 --> 00:14.230 So guys, now let us write a one unified single API which will take the responsibility to send any type 00:14.230 --> 00:19.360 of message from our userspace program to kernel space via netlink socket. 00:19.360 --> 00:19.950 Right. 00:19.960 --> 00:23.860 So the name of the API is send netlink message to kernel. 00:24.100 --> 00:28.930 So as you can see that this API accepts five argument. 00:28.960 --> 00:35.800 The first argument is the file descriptor of the netlink socket which the user space program will create 00:35.800 --> 00:37.990 when the user space program boots up. 00:38.440 --> 00:39.190 Right. 00:39.220 --> 00:45.730 The second argument is the payload which the user space program will send to the kernel space. 00:45.850 --> 00:48.640 The third argument is the size of the payload. 00:48.640 --> 00:55.690 The fourth argument is the type of the netlink message which this user space program is sending to the 00:55.690 --> 00:56.710 kernel space. 00:56.710 --> 01:01.120 So the fourth field is the netlink message type field of the Netlink header. 01:01.120 --> 01:06.340 In our case, this field will be equal to the enumeration, which is great. 01:06.370 --> 01:07.180 Right? 01:07.180 --> 01:14.230 And finally, the fifth field is the flags, which user space program must specify in order to give 01:14.260 --> 01:19.120 directions to the kernel space regarding what to do with the message. 01:19.570 --> 01:24.670 So now let us discuss the implementation of this API in four simple steps. 01:26.050 --> 01:29.470 First, we will take an overview of these four simple steps. 01:29.470 --> 01:34.810 Then in the next lecture video, we will go through the actual implementation of this API. 01:35.450 --> 01:42.410 So as a part of writing this API, the very first step that we need to do is to prepare a message of 01:42.410 --> 01:45.980 type Netlink message header followed by the payload. 01:45.980 --> 01:46.730 Right. 01:47.120 --> 01:53.570 Remember any message that is exchanged between user space and kernel space should be exchanged by using 01:53.570 --> 01:56.210 netlink message header followed by payload. 01:56.390 --> 01:59.330 So now this diagram is not new to you. 01:59.480 --> 02:02.750 We will reformat the message in the following format. 02:02.750 --> 02:05.840 That is netlink message header followed by payload. 02:06.950 --> 02:12.710 Then in the next step we have to use the intermediate data structure called Iok. 02:12.920 --> 02:15.920 Iok stands for input output vector. 02:15.920 --> 02:21.770 So this data structure, as you can see, is a very simple data structure comprises of only two fields. 02:21.800 --> 02:29.270 The first field is a EOF underscore base and it is of data type void star and the second field is of 02:29.270 --> 02:31.000 data type sys underscore tty. 02:31.220 --> 02:31.940 Right. 02:32.030 --> 02:38.730 So what we will going to do is that we will store the address of the netlink message header in this 02:38.760 --> 02:45.180 eof underscore base field and the total length of this message that is starting from the Netlink message 02:45.180 --> 02:50.180 header to the end of the payload, we will going to specify in Ireland. 02:50.430 --> 02:57.720 So basically we have encapsulated the netlink message which we will going to send to the kernel space 02:57.720 --> 03:01.470 indirectly inside the vector data structure. 03:01.920 --> 03:04.170 But the story doesn't ends here. 03:04.170 --> 03:06.350 We will do one more level of wrapping. 03:06.360 --> 03:13.560 Now in the step three, we will going to wrap this vector data structure inside inside another structure 03:13.560 --> 03:15.390 called struct message header. 03:15.390 --> 03:15.960 Right? 03:15.960 --> 03:19.470 So let us call this structure as outer message header. 03:19.710 --> 03:24.290 So in this type of messages, we have to fill these four fields. 03:24.300 --> 03:30.780 So the first field is nothing, but it is the destination address which we will going to specify now 03:30.780 --> 03:36.300 how to specify the destination address that we will going to discuss when we will walk through the code. 03:36.390 --> 03:39.690 The second field contains the size of the destination address. 03:39.690 --> 03:44.640 Again, this We will discuss when we will be discussing the code in the third field. 03:44.640 --> 03:50.490 We have to supply the address of the vector data structure which we have defined in step two. 03:50.490 --> 03:51.270 Right. 03:51.270 --> 03:58.650 And in the fourth field we have to specify how many units of vector data structure we are sending to 03:58.650 --> 03:59.790 the kernel space. 03:59.790 --> 04:07.650 So in this example, we have taken only one vector data structure and therefore you can specify one 04:07.650 --> 04:08.730 in this field. 04:09.030 --> 04:16.080 Now to discuss about this destination address, this destination address is of data type struct soc 04:16.140 --> 04:18.200 address underscore channel. 04:18.210 --> 04:24.540 So this is a socket address data structure which is used for netlink sockets. 04:25.560 --> 04:29.940 And inside this data structure we have to specify only two fields. 04:29.940 --> 04:34.380 The Netlink family, which is the address family for the netlink socket. 04:34.380 --> 04:41.820 So always we have to specify a F underscore netlink and we have to specify the port ID of the destination 04:41.820 --> 04:42.600 process. 04:42.630 --> 04:49.800 Now in this example, because we are communicating from userspace to kernel space for kernel space, 04:49.800 --> 04:53.070 the port ID is always zero, right. 04:53.760 --> 05:00.300 So we have to specify only these two fields in the destination address and watch the data structure 05:00.300 --> 05:03.000 that is used as a destination address. 05:03.570 --> 05:08.070 This data structure is provided by the netlink socket header files. 05:09.600 --> 05:16.440 Now as a final step in step number four, we are good to go to send our user space message to our kernel 05:16.440 --> 05:18.540 space via netlink socket. 05:18.570 --> 05:24.450 The first argument we have to specify the file descriptor of the socket which our user space program 05:24.450 --> 05:26.020 has already created. 05:26.040 --> 05:30.540 The second argument is the address of this outer message header. 05:30.810 --> 05:34.740 And the third argument you can always specify as zero. 05:35.420 --> 05:42.350 So now that we have discussed the high level overview regarding the steps for sending the message from 05:42.350 --> 05:44.340 the user space to kernel space. 05:44.360 --> 05:50.990 Now let us discuss the step by step implementation and discuss the implementation of this API. 05:51.020 --> 05:53.690 We will have to use this API every time. 05:53.690 --> 05:58.640 We will going to send message from our user space program to kernel space. 05:58.640 --> 06:01.980 And you can see that this API is very generic. 06:02.000 --> 06:09.110 We can send any type of message or payload from user space to kernel space using this function. 06:09.110 --> 06:09.800 Right.