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William Cheng: Good morning this is LIVE lecture number five on June 4

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William Cheng: If you have questions, feel free to ask

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Lalit Gupta: So I have a question. Sure.

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Lalit Gupta: So in another slide you are talking about like you sleep.

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Lalit Gupta: If you sleep at night, not the entrance, or in a lake or what

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William Cheng: I think you sleep is pretty safe to use under all kinds of conditions.

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Lalit Gupta: So you

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William Cheng: Save it all kinds of stuff.

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William Cheng: Okay, so

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William Cheng: As far as Ubuntu 16.4 is concerned, we don't know if it's true in, you know, in all the other operating system.

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William Cheng: But for one to 16.4 which is the only platform that your code should work on, you know, for that it's it's it's retention is safe. It's everything

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Lalit Gupta: So you're saying there's some there can be some problem with usually like ours aren't able to understand what what can be the problem that usually

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William Cheng: What it just that on some operating system. You know, it's ambiguous. Right. Will you say I want to say for, you know, three seconds. What do you feel running multithreading you know maybe you are evenly divided three seconds. Mom, I mean it sounds weird, but there are some system like that.

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William Cheng: Okay, so, so it's just not universal. But, you know, for for our case, since we only have to run all been to 16.4 then you know

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William Cheng: I think they they they think either the man page or somewhere says that

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William Cheng: You know, if you no matter how many thirds, you're running. If you say I want to wake up three seconds later. Well, the system will wake you up, you know, the current time plus three seconds later, but again, no guaranteed exactly what you're going to wake up.

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William Cheng: But it doesn't really matter how many so I do run in parallel.

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Lalit Gupta: And also, like is it does this when we do man pages or something.

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Lalit Gupta: Does the mandate also specify whether the code is the entrance or not. Do we have to like look up somewhere else.

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William Cheng: Well, usually you assume that the code is retention.

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William Cheng: You know all the UNIX libraries, they make a thread safe, long time ago. Right.

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William Cheng: So again in warm up to you if you're asking about warm up to warm up to, it's a multi threading assignment. So under multithreading re entering a thread safe. It's the same thing.

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William Cheng: Okay, so the UNIX Library and the library, you know, they made it safe.

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William Cheng: You know, long time ago. But of course, you're not there are certain things. You're not allowed to do, right, for example, if your thread.

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William Cheng: You know,

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William Cheng: Like the example that I mentioned before, if you have to threats are joined with the same thread, well then you, you have a bug in your code.

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William Cheng: Right. Some people say, Oh, no, the code is not retention. The code is not thread safe but but it's maybe a simple as, you know, you have a bug in your code.

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William Cheng: For example, if you free, the same data structure toil you so if if you free, the same point or twice. Well, you know, again, it's a bug in your code. It's not that because free is not safe.

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William Cheng: Did you know what do you know what I mean.

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Lalit Gupta: You know, can you read one more time.

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William Cheng: Well, so for example if you call Mela you get a pointer. Right. And now you share this pointer with two threads.

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William Cheng: Oh yeah, both let's call free on it.

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William Cheng: And you'll program crash. Some people say, oh, they are. That's because three three is not safe. No, it's because there's a buggy.

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Lalit Gupta: Yeah, once you once one, one of the tech calls

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Lalit Gupta: Freedom other college should not call free

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William Cheng: Right. And also, you know, for the pizza library. Right. So. So for example, if you call piece of meat has unlocked when you don't have to meet x lock, then it's are safe, right, because you have a bug in your code.

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William Cheng: Yes. Okay, so. So again, just because things are thread safe doesn't mean that you can use it anywhere you want. You still have to use it correctly.

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Lalit Gupta: Okay, and

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Lalit Gupta: During the main page we can figure out what is how, what is the function actually do. And suppose I want to figure out like how what

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Lalit Gupta: For example, you're talking about like struck time. Well, is it possible to do man of that also

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William Cheng: On. No, no. So data structure don't have their man pages, the function that uses them has a man page. Okay, so what you will do is that you can you can do a man get time of day.

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William Cheng: Okay, so good time of day is going to sort of to tell you what the structure look like

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William Cheng: They're also function like you know time or something like that. See time, right, so remember see time also work with the same or no. Yes.

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William Cheng: No, no, sorry. See time. Start with the integer and you convert that into a time, nevermind.

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William Cheng: So I think if you look at the time of day and then it will talk about struck time Val. And then what you do is that you look at the main page and you scroll all the way to the bottom at the bottom, there's something called. See also

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William Cheng: Okay, so in the. See also function, they will mention other men pages that my talk about something similar.

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William Cheng: Okay, and they can do men on the other end on, on the other function. So maybe some men pages. That's more documentation on the data structure and some some doesn't

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William Cheng: Okay, you can you can also just ask me, right, because you know struck time val is a very simple data structure. He has two integers in them. One is a number of seconds. And the other was number of microseconds that that's all it is.

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William Cheng: So it just, it's you know, it's not even an abstract data type, he just, he just really pretty.

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Lalit Gupta: Simple. Yes.

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William Cheng: Yes. So you so you need to look at the main page of the function are going to use that because what they will do that they will manipulate the data inside the you know the data structure. Okay.

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William Cheng: Thank you. Okay.

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Wookyum Kim: Purpose arrival constraints.

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Wookyum Kim: In Slide

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William Cheng: 2727 okay

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Wookyum Kim: It's about the conditioner variables. So,

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Wookyum Kim: Yeah, I was.

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Wookyum Kim: kind of confused that condition variable is a queue. Right.

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Wookyum Kim: But

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Wookyum Kim: The queue have to be I think you have to be place the outside of the serial Asian box right

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William Cheng: So I, I joined. I joined like this intentionally right so this way when you're asleep outside asleep inside the queue, then it's clear that it's not inside the serialization box.

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But

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Wookyum Kim: Variable shouldn't be in their cereal box. I mean,

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William Cheng: Right. So this one to say that you can only touch the condition variable, just like you know when you when you try to touch a, b, and c, you have to have the new tax law.

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William Cheng: Okay, so, so if you're outside or you try to touch the CV. Well then, then, then you're going to cause trouble.

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Wookyum Kim: Okay.

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Wookyum Kim: So,

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Wookyum Kim: Okay, because we are using the the piece rat library. So we don't really care about the conditioner variable should be inside the text or not. Right.

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Wookyum Kim: We don't, we don't really need to care care about because that it's built in the library.

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William Cheng: Yes, but, but, you know, on the right hand side, this picture. I mean, you don't know, there's no serialization box civilization bug is just conceptual

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Wookyum Kim: Yeah yeah yeah yeah

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William Cheng: So you don't you don't really put anything inside of a that's just the way you should think about it is that if I if I, you know, if I have this tool.

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William Cheng: That show you this picture. It's a reminder that you shouldn't touch the CV, unless you are, you know, unless you are the owner of the new tax.

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Wookyum Kim: Oh, okay. So serialization boxes, not on mute tax.

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William Cheng: The civilization.

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William Cheng: The serialization, but he's just a notation.

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Wookyum Kim: Yeah.

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William Cheng: This is the new tax, right. So, so every new tax right every new tax has a new text queue and conceptually, you know, there's a civilization box.

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Wookyum Kim: Okay, okay.

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William Cheng: So you need to decide what variable to put inside the civilization box.

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William Cheng: And they follow the rules.

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Wookyum Kim: Yeah yeah

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William Cheng: So I guess that that's why I put civilization box in double quotes, so it's

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William Cheng: Just like quoting

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William Cheng: I see.

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Wookyum Kim: And then slide 31 okay yeah oh

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Wookyum Kim: Yeah so piece thread conditioner condition. Can't wait functions. The last time you text and wait for events should be must be at to me in here is, but I don't really understand that. Why have to be

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Wookyum Kim: At Tommy in here.

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William Cheng: Yeah, so that's a few slides down over here. Right. Right here. Right. It says that if piece that condition where it is not atomic

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William Cheng: Then it is possible. Somebody will signal Brock has the condition in between lock unlock in a new tab and wait for The Event.

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Wookyum Kim: I'm okay. But maybe later, there will be another Seguin there. But that's still a race conditions right what

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William Cheng: If this

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William Cheng: If this signal is a once in a lifetime signal. Well, they mess it

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Okay.

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William Cheng: So in order for a man, you know, in order for your code to be perfectly safe, then you know working every time you have to do it. Right, right. So in this case, the Peter library also have to implemented correctly.

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Wookyum Kim: Is that the only reason for it should be that to me.

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William Cheng: What this is just an example where things won't work. Right. So, so if there's one case that things won't work. Then, according to Murphy's Law, it only happens when something is very important.

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Wookyum Kim: Okay. Okay. Thank you.

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Wookyum Kim: Yeah, slight

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William Cheng: 5757 okay

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William Cheng: Okay.

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Wookyum Kim: Yeah, in here. Um, yeah, so

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Wookyum Kim: So it's wrong code right it it doesn't

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William Cheng: It doesn't work. Yeah.

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Wookyum Kim: Yeah, but I don't understand why it doesn't work.

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Wookyum Kim: Oh,

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William Cheng: Yeah so. So let's say that you have only one CPU. So I mean, when the code works, it should work with one CPU two CPUs. Many CPUs. Right.

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William Cheng: Yeah, so if you only have one CPU. So let's say the n equals two, five guys have four threads right want 123443 sleeping in the barrier. Q When the fifth rest comes in. Over here, it will execute as Co Peter condition broadcasts.

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William Cheng: Okay, so they will reset the counter zero and now the fifth threat number number five is going to leave the barrier synchronization.

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William Cheng: And then all these four says when they wake up, they will see count equals zero. So they all go back to sleep.

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Wookyum Kim: What

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William Cheng: Right, so. So again, if you have, if you have only one CPU. You can only will execute one third of the time, right.

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Wookyum Kim: So when you

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William Cheng: Execute the thread. Number five. Number five is going to, you know, wake up all these four threads. But when you wake them up. They're still sleeping right because the CPU can only do one thing at a time.

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Wookyum Kim: If, then, what if, what happens if that there is a vertical CPU, then maybe words.

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William Cheng: So in this case, again, it works in multiple CPUs are also working on a single CPU. The reason that it works in multiple CPU is that thread number five over here has a metric locked

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William Cheng: So therefore, even if you if you work out the other four threads. The other four threads. They don't have the new tax.

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Wookyum Kim: Yeah.

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William Cheng: So therefore, they will not execute right okay when certain number five copies me test unlock. Then another thread will start executing critical section code.

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William Cheng: Okay.

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Wookyum Kim: So that's that is going to be either 3123 or four and that's that will return from piece of a condition. Wait, rather, this will you return from Peter of initial way, you have the new tax law, but it. I mean, but it or Luddy are but broadcast is that waking up that that

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Wookyum Kim: Cue up the new tax not cure the barrier here.

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William Cheng: Yeah, you move them up.

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William Cheng: And you want to. So there's still that all these four threads waiting for the new tax.

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William Cheng: The when one of the threads 1234 get the new tax it will return from Peter condition. Wait.

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Wookyum Kim: Oh no wait, um,

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Wookyum Kim: But in here barrier accuse the condition be reversed. Right.

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William Cheng: Yeah, the bear a few over here is a condition variable. Yeah.

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Wookyum Kim: Yeah, then why don't

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Wookyum Kim: Condition broadcast cannot wake up the error, who in here.

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William Cheng: Yeah, there are woken up but they're sitting in the new tech skew.

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Wookyum Kim: Okay, but then why you why. And then counter counter becomes zero and then

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Wookyum Kim: Oh,

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Wookyum Kim: Sorry, I will. I'm confused now.

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William Cheng: Right, so, so let's take a look at this picture, and here's a serialization box. Right. Okay.

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William Cheng: Right here is the condition variable. Q. We have 31234 sleeping over here. Let's cue over here is empty. Right.

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William Cheng: Yeah, so number five is executing inside of civilization box.

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William Cheng: Okay okay so so square number five says piece or a condition broadcast. So all these 1234 are gone. They all they all come here right 1234 and I'm still here. Okay. So yeah, that's right. Number five is going to execute piece for me is unlocked. So thread. Number five is gonna is gonna leave

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William Cheng: So in this example, it will bring in three number one. So step number one is going to be execute here. Okay.

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William Cheng: Okay, so what kosher to execute with their number one come into the sterilization box.

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Wookyum Kim: Um,

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Wookyum Kim: If count. I mean, our peace read new tax law, and then he counts.

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William Cheng: Was also no no it doesn't return on piece of meat as he returned from Peter condition way right

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William Cheng: Because last

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William Cheng: Three number one did was copy through a condition way they unlock the new tabs and go to sleep in a condition variable killing one atomic operation.

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William Cheng: Okay, though, so it's a function called right so they will return from the function call.

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William Cheng: So when that one.

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William Cheng: start executing again, you know, it will return from Peter condition. Wait.

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William Cheng: Oh, and then he will check counties. What a zero and is equal to five. So therefore, they will go back to sleep again. Right, so one over here is going to go back here and threat to is going to come in. So I to also go to sleep and thread three so they all do the same thing.

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Wookyum Kim: Oh,

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Wookyum Kim: Because it's while I see

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William Cheng: One of them can be executed in critical section co at a time.

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Oh,

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Wookyum Kim: I see. So in that previous one. That was our so that the wrong was but that can be like work because it's not a while states ones in here if that's if conditions. So,

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Wookyum Kim: Yeah, so, so, so this function can return for no reason. And now this thread, don't leave the barrier synchronization while it was never woken up

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William Cheng: Right. Nobody called Peter condition signal a piece of content broadcast. So in that case, the co executing correctly.

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Wookyum Kim: Okay. That's because

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Wookyum Kim: That's just like the piece right libraries, not like built in this way. Right.

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William Cheng: Well,

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William Cheng: So again, the piece of the library implemented to say that, well, you know, the correct way to use piece or a condition way is to do while Nagar piece recognition way. Okay, so this is a programming bug right

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Okay.

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William Cheng: Thank you.

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Wookyum Kim: Okay.

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William Cheng: You can also ask question about, you know, warm up to, you know, lecture slides. Anything you want to hit anything you want to ask

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William Cheng: No, nobody else has questions.

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William Cheng: Okay, if nobody else has questions is, okay, that we close the lecture or you're still scanning some slides or something like that, trying to find questions.

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William Cheng: Okay, so in that case, I'm going to close the lecture. So, I still have officer at one to 2PM so so feel free to come to office hour today. Okay. Oh.

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Professor

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Wookyum Kim: Because I'm still like some troubles with that finding a team members so

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Um,

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Wookyum Kim: Because we're not really like go into the classrooms to get along with the other students or so. So like, um, maybe can we like like talk to each other after the class like in the zoom meetings.

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William Cheng: Even in the you and I talked about it.

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Wookyum Kim: I mean i mean i mean i mean i mean we the other students like just have the students can you like talk about the team.

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William Cheng: Okay.

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William Cheng: All right, so, so you want me to watch this meeting is recorded. There's no you know so

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Wookyum Kim: Oh really,

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William Cheng: Okay. One thing that I can do that. I can turn off recording

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William Cheng: Yeah so i don't i think that that won't kill the meeting. So, so you can talk to other students. Okay.

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William Cheng: Okay. Alright, so I'm going to stop the recording and then I'm going to leave and then anybody want to visit me in the office are please go ahead

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Wookyum Kim: Yeah, thank you.

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William Cheng: Okay, bye.