Signaling for condition variables (pthread_cond_signal vs pthread_cond_broadcast)

[@narasarajv5278]

Your videos are improving my thinking ability.Thank you ...

[@giorgospappas9101]

Thank you, I have found this, and the previous video on conditional variables extremely helpful. My lecture slides are too fast forward but you explain the concepts in a decent depth and speed. P.s. your speech is very clear which is also nice :)

[@user-yz7ts2fq9m]

This is incredible what you are doing now!

[@iulianbiro7974]

misto explicat, exact asta fac la facultate. esti un ajutor de nadejde in sesiune.

[@salmanforester]

Fantastic video to learn such a complex concept.

[@stellachen1741]

your video is a life saver!! i couldn't understand what my professor lectures in class. But i'm totally with your logic. thank you so much. now i can start working on my homework now

[@TheEzinal]

Thank you very much for these excellent quality videos, I have learned a great deal on conditinal variables to start my hw :)

[@TaimoorKhan-h6l]

Thank you very much for this helpful tutorial

[@Toccobass13]

Thank you endlessly.

[@norantarek7097]

Thank you so much sir, your videos are very helpful

[@wouthoupeline1149]

Great video!! Really helped me to understand the subject better.

[@ArkFreestyle]

Brilliant explanation.

[@danghuynhhai6589]

it's very useful for me, thank you so much.

[@fabiozucconi7068]

just a simple checking correction: running the code through valgrind --tools=helgrind i found a possible problem in running the broadcast outside of the lock, simply solvable unlocking the mutex after running pthread_cond_broadcast. Hope this is useful =)

[@Dave-cq6pu]

Very helpful and quality videos.

[@afsanehakhbari8880]

thank you for your amazing and helpful videos. I am wondering how in last example 2 cars are in lock? isn't the concept of mutex that only one thread in lock situation and others are waiting?

[@CodeVault]

Yep, you're right. The trick is with the pthread_cond_wait call. Behind the scenes it actually does the following: pthread_mutex_unlock wait_for_signal pthread_mutex_lock And because of that pthread_mutex_unlock that pthread_cond_wait executes other threads are allowed to lock that mutex. This is why you actually have to pass the mutex to pthread_cond_wait

[@kaal_bhairav_24]

since feuler has broadcasted, all the four car threads are signalled to check the while loop but one car at a time since the car locks the mutex and then the second car comes and so on correct me if I'm wrong

[@rajshah9129]

@@CodeVault if on waiting for signal it unlock then 2 case other car can take lock or fuel_filling if other car take lock then program will hault and if fuel_filling will take lock then it will emit signal cond_wait will receive and it will start execution or on recieving signal before taking lock other car can take lock but in any case only one car or fuel_filling can only take lock ??? can u explain this properly

[@saularaya2066]

Piola👍

[@yahmk3978]

Thank you!

[@martinmickels1478]

So the Filled fuel ... X is the amount of fuel in the car that gets filled from the fuel tank and the Now left: X is the amount of fuel in the fuel tank that gets filled from somewhere

[@coin5207]

Very helpful thanks 👍

[@animeshkumarsinha5954]

Could you also make a video on pthread_cond_timedwait(). Also could you help to cover scenario like: Thread2 goes in condition timed wait. Thread 1 takes mutex lock Thread2 pthread_cond_timedout is signalled due to timeout. What would happen in this scenario? As thread1 was still under mutex lock and now thread 2 due to timeout also is now under mutex lock. Is it a deadlock scenario?

[@3042640426]

thank you

[@compscienthusiast]

pthread_cond_signal vs pthread_cond_broadcast seem like great tools depending on your applications. I'm assuming broadcast is mainly used for times when multiple threads are accessing one shared pool/resource and for a threaded pipeline you'd stick with signal? I'm wondering due to a coding assignment for a pipeline that consisted of 4 threads and I assumed 3 mutexes. (read from file > parse characters and process one pattern > parse characters and parse another pattern > output exactly 80 chars from buffer. With a mutex between each part to keep them from speeding ahead) I had assumed only 3 mutexes each between the arrows, but I'm thinking I would need another mutex at the beginning to keep the reading thread from emptying the entire file and then closing the program early.

[@CodeVault]

Usually you use signal when you expect just one thread to start working (in a thread pool for example, when adding a new job to the list). You use broadcast when there's a possibility that the work requires more than 1 thread

[@kashishbhatia9468]

If one of the car thread does not get enough fuel and all the fuel-filling iterations are over, how can the waiting thread on the condition be terminated. Is there any giveup/retry logic that can be added for such waiting threads.

[@CodeVault]

You could add a separate thread that simply checks for activity and have the cars always write to some variable the times it started trying to get fuel and got fuel

[@spokiechris7685]

01:41 Not fully sure, but it seems like it only awakens 1 car. Note how you initialized 4 cars, and there's only three initializing car prints above the first fueling. I'm guessing that the fueling function printed faster than the slowest car function instead of the conditional signal awakening 2 cars.

[@CodeVault]

Note that the car function only executes once. After the car has been fueled the thread finishes its execution (thus, for the next iteration, we only get 3 messages)

[@rajshah9129]

i have one doubt from start out of 4 car only 1 car can have lock so if 1st car take lock and waiting fro cond_wait and its unlock before lock taken by fuel_filling if another car take lock then fuel_filling will also wait for lock then what will happen??

[@CodeVault]

When calling cond_wait the mutex actually gets unlocked so others can access it. The thread relocks the mutex once signal or broadcast is called

[@rajshah9129]

@@CodeVault i was saying like out of 4 car only 1 will be able to reach cond_wait condition right ? bcoz of lock n if it unlock before lock taken by fuel_filling does another thread can also take know is this possible?

[@CodeVault]

No. All of them will, simply because that, when waiting at pthread_cond_wait, the lock gets dropped and other threads can also lock and call pthread_cond_wait

[@antonfernando8409]

awesome

[@mingyudai7566]

Just wondering why in the last example, after filled 30, not 4 thread goes in (2 first time and 2 signaled)

[@CodeVault]

That is because the car threads were able to lock the mutex first due to the broadcast (to check once) before letting the other fuel thread execute

[@Starckoo]

Small question, in the filling_fuel function, should the broadcast/signal be done inside the mutex lock? I think i remember reading somewhere that it was best practice, or does it not matter?

[@CodeVault]

In this specific program it doesn't matter. If you use the mutex standalone (i.e. in a context that doesn't depend on this condition variable)... it will matter when it comes to priority. If the order is: Signal -> Unlock Then both the standalone and the pthread_cond_wait threads have basically the same time to lock the mutex. If the order is: Unlock -> Signal Then that standalone mutex has a biiit more time to lock the mutex (right before that pthread_cond_signal is executed) to lock the mutex. All in all, this priority shouldn't affect your program's correctness, but it's something to keep in mind

[@Starckoo]

@@CodeVault Makes sense, thank you!

[@marcw6875]

Would there be any way to use signals to get the result you get with broadcast? I ask because I'm working on a programming assignment with threads and the professor has told us that we can only use wait and signal. We cannot use broadcast.

[@CodeVault]

I don't think there is an easy way without having a counter of how many threads are waiting and simply call pthread_cond_signal in a for loop. Although... usually, you only need to use signal, not broadcast, what is the reason you absolutely need broadcast?

[@HarshVerma-oz5uo]

How to use conditional variable in detachable threads??

[@CodeVault]

Just like in any other threads. Detachable threads are just non-joinable, but they still share the memory so they still can make use of the tools you use in non-detachable threads

[@HarshVerma-oz5uo]

@@CodeVault I have run the code successfully now. Thank you for your support

[@ivankontra3446]

what happens if signal is executed before another thread reaches condition wait? is the condition wait executed or does it wait until another signal is executed while it's waiting?

[@CodeVault]

It should wait until another signal gets executed. Here are the docs: linux.die.net/man/3/pthread_cond_signal

[@ivankontra3446]

@@CodeVault thank you, uni slides were iffy about it, btw I think these tutorials are top 1%