1 00:00:00,005 --> 00:00:05,229 [MUSIC]. 2 00:00:05,229 --> 00:00:09,351 Now that we know what a process is, let's see how we create processes. 3 00:00:09,351 --> 00:00:11,540 We're going to use a model called the fork-exec model. 4 00:00:11,540 --> 00:00:17,370 So fork creates a copy of the current process. 5 00:00:17,370 --> 00:00:21,858 And execve replaces the current process code and address space with a code for a 6 00:00:21,858 --> 00:00:27,026 different program, okay? So, fork creates a copy an entire copy of 7 00:00:27,026 --> 00:00:32,354 the current process and execve replaces the codes and the data, and the address 8 00:00:32,354 --> 00:00:39,760 space of the process with the code for a different program, okay? 9 00:00:39,760 --> 00:00:43,669 And these two servers are, we call system calls, okay? 10 00:00:43,669 --> 00:00:47,689 System calls is part of the operating system programming interface, okay? 11 00:00:47,689 --> 00:00:51,166 So note that we going to be doing what Linux provides here, and Windows is 12 00:00:51,166 --> 00:00:56,010 slightly different, but the concepts are essentially the same, okay? 13 00:00:56,010 --> 00:00:58,520 So, and there are other system calls to do process management. 14 00:00:58,520 --> 00:01:02,488 For example, there's getpid, which returns an ID of the current process, the 15 00:01:02,488 --> 00:01:07,604 process that's calling the system call. This exit, which just kills and, and ends 16 00:01:07,604 --> 00:01:11,029 a process. And there's wait and waitpid which is 17 00:01:11,029 --> 00:01:15,460 used to synchronize processes running, okay? 18 00:01:15,460 --> 00:01:18,170 So, let's see what fork does in more detail. 19 00:01:18,170 --> 00:01:23,207 So, it creates new processes and when you call fork, fork takes no parameters and 20 00:01:23,207 --> 00:01:27,913 it returns this PID type. Essentially, it's just the idea that the 21 00:01:27,913 --> 00:01:30,992 process was newly create that was newly created, okay? 22 00:01:30,992 --> 00:01:34,712 So, when you call, when a person, say that we have this example here, you can 23 00:01:34,712 --> 00:01:38,100 call fork. And what fork does is it creates a new 24 00:01:38,100 --> 00:01:44,230 process that's identical to the calling process, called the parent process. 25 00:01:44,230 --> 00:01:48,850 And we turned 0 to the child process. And we turn, returns the child, the child 26 00:01:48,850 --> 00:01:54,114 process idea to the parent process, okay? I know that this can get a little 27 00:01:54,114 --> 00:02:00,129 confusing because it kind of is. and the reason is that fork is unique 28 00:02:00,129 --> 00:02:04,760 because he called once, but it returns twice. 29 00:02:04,760 --> 00:02:08,108 Because when you call, when you're calling, you have a processor in it, you 30 00:02:08,108 --> 00:02:11,660 called fork, okay? Now, we're going to replicate the process 31 00:02:11,660 --> 00:02:14,545 and going to have two different processes here. 32 00:02:14,545 --> 00:02:19,604 This is the parent and this is the child. It is going to return the process ID to 33 00:02:19,604 --> 00:02:24,804 the parent and it's going to return 0 to the child. 34 00:02:24,804 --> 00:02:29,885 So in this example here, we call fork here then, and then we check. 35 00:02:29,885 --> 00:02:34,547 If the pid return is 0, that means that this is going to be executing the child 36 00:02:34,547 --> 00:02:40,507 and if not 0, that's going to be executed in the parents, okay? 37 00:02:40,507 --> 00:02:42,788 So, let me go to just one more level of detail, okay? 38 00:02:42,788 --> 00:02:46,244 Suppose that we have a process n, and we're executing the piece of code we just 39 00:02:46,244 --> 00:02:51,611 showed you, okay, when we call fork, it's going to create a process called m, okay? 40 00:02:51,611 --> 00:02:55,853 And they run the same program, alright? Because it's an entire replica byte for 41 00:02:55,853 --> 00:02:59,593 byte, and, and the entire state[UNKNOWN] but now they are both running at the same 42 00:02:59,593 --> 00:03:05,350 time, okay? So now process n continues executing. 43 00:03:05,350 --> 00:03:09,781 So, it's going to relieve pid, right? The pid is going to recieve M which is 44 00:03:09,781 --> 00:03:13,190 the, the ID of, of the child process, okay? 45 00:03:13,190 --> 00:03:21,680 And now the child process is running and it's going to receive 0, okay? 46 00:03:21,680 --> 00:03:26,828 Great, so now since process n received m, it's going to execute this side 47 00:03:26,828 --> 00:03:31,483 of[UNKNOWN] okay? And this one's going to execute as well, 48 00:03:31,483 --> 00:03:34,942 so you're going to see hello from child, okay? 49 00:03:34,942 --> 00:03:40,300 Pretty cool, isn't it? So which one is first? 50 00:03:40,300 --> 00:03:45,392 Well, we definitely know that this fork here executed before anything else, but 51 00:03:45,392 --> 00:03:49,685 which one is first? Well, even, as soon as the child process 52 00:03:49,685 --> 00:03:54,125 is creating, it could be anything, it's non-deterministic really, okay,? 53 00:03:54,125 --> 00:03:57,942 So, one could be executing. But the only thing we know is that 54 00:03:57,942 --> 00:04:05,730 processing up to the point where the fork is executed process and executed first. 55 00:04:05,730 --> 00:04:11,060 So so, the example things to remember that the parent and child both run the 56 00:04:11,060 --> 00:04:15,781 same code, okay? So, when we distinguish the child by the 57 00:04:15,781 --> 00:04:20,411 value returned from fork, okay? So, which one runs after the fork, as I 58 00:04:20,411 --> 00:04:24,380 said before, is undefined, it's non-deterministic, okay? 59 00:04:24,380 --> 00:04:27,414 It has to be one of them. But you might run multiple times, you 60 00:04:27,414 --> 00:04:31,165 might see different things, okay? So, the one thing to note here is that 61 00:04:31,165 --> 00:04:34,545 they both start the same state but after the fork, they both have a private copy 62 00:04:34,545 --> 00:04:38,346 of the state, okay? Same variables, same call stack, same 63 00:04:38,346 --> 00:04:41,307 files descriptors and so on. But then, after the point, they can 64 00:04:41,307 --> 00:04:44,572 diverge the execution, right? Because that is what's going to make this 65 00:04:44,572 --> 00:04:46,670 interesting. And we are going to see how to use that 66 00:04:46,670 --> 00:04:47,770 in the next video.