[MUSIC]. Now that we know what a process is, let's see how we create processes. We're going to use a model called the fork-exec model. So fork creates a copy of the current process. And execve replaces the current process code and address space with a code for a different program, okay? So, fork creates a copy an entire copy of the current process and execve replaces the codes and the data, and the address space of the process with the code for a different program, okay? And these two servers are, we call system calls, okay? System calls is part of the operating system programming interface, okay? So note that we going to be doing what Linux provides here, and Windows is slightly different, but the concepts are essentially the same, okay? So, and there are other system calls to do process management. For example, there's getpid, which returns an ID of the current process, the process that's calling the system call. This exit, which just kills and, and ends a process. And there's wait and waitpid which is used to synchronize processes running, okay? So, let's see what fork does in more detail. So, it creates new processes and when you call fork, fork takes no parameters and it returns this PID type. Essentially, it's just the idea that the process was newly create that was newly created, okay? So, when you call, when a person, say that we have this example here, you can call fork. And what fork does is it creates a new process that's identical to the calling process, called the parent process. And we turned 0 to the child process. And we turn, returns the child, the child process idea to the parent process, okay? I know that this can get a little confusing because it kind of is. and the reason is that fork is unique because he called once, but it returns twice. Because when you call, when you're calling, you have a processor in it, you called fork, okay? Now, we're going to replicate the process and going to have two different processes here. This is the parent and this is the child. It is going to return the process ID to the parent and it's going to return 0 to the child. So in this example here, we call fork here then, and then we check. If the pid return is 0, that means that this is going to be executing the child and if not 0, that's going to be executed in the parents, okay? So, let me go to just one more level of detail, okay? Suppose that we have a process n, and we're executing the piece of code we just showed you, okay, when we call fork, it's going to create a process called m, okay? And they run the same program, alright? Because it's an entire replica byte for byte, and, and the entire state[UNKNOWN] but now they are both running at the same time, okay? So now process n continues executing. So, it's going to relieve pid, right? The pid is going to recieve M which is the, the ID of, of the child process, okay? And now the child process is running and it's going to receive 0, okay? Great, so now since process n received m, it's going to execute this side of[UNKNOWN] okay? And this one's going to execute as well, so you're going to see hello from child, okay? Pretty cool, isn't it? So which one is first? Well, we definitely know that this fork here executed before anything else, but which one is first? Well, even, as soon as the child process is creating, it could be anything, it's non-deterministic really, okay,? So, one could be executing. But the only thing we know is that processing up to the point where the fork is executed process and executed first. So so, the example things to remember that the parent and child both run the same code, okay? So, when we distinguish the child by the value returned from fork, okay? So, which one runs after the fork, as I said before, is undefined, it's non-deterministic, okay? It has to be one of them. But you might run multiple times, you might see different things, okay? So, the one thing to note here is that they both start the same state but after the fork, they both have a private copy of the state, okay? Same variables, same call stack, same files descriptors and so on. But then, after the point, they can diverge the execution, right? Because that is what's going to make this interesting. And we are going to see how to use that in the next video.