What is a mutex in C? (pthread_mutex)
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- Опубликовано: 11 фев 2025
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The operations performed by a thread after acquiring a lock are called "critical section". This helps in achieving thread synchronisation. Thanks for the video!
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The lock variable + if block example is the best example I have even seen for this Mutex topic
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Great video, explained how lock and unlock work really well. I was having trouble understanding what it was with my professors explanation lol. Thanks for explaining it, you definitely deserve more views and subs!
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Thank you! And yes, the last lessons in the course are not yet decided so will add most of the suggestion from the community there (like this one ;) ).
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Just a slight correction at the end: You can have race conditions on single core processors. We can fake "parallel execution" by time slicing. If the two threads are sharing memory resources, then we can absolutely have a race condition during a context switch.
That is correct.
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Race conditions can also occur on single core processors like if we consider concurrent CPU and the operations are read, increment and write. So lets say P1 starts first then P1 reads increments and then it is preempted by another process P2, then P2 does read, increment and then it is preempted by P1. Now P1 writes the value and exits after which P2 comes and writes its value thus there is a race condition
You're right, I didn't want to go into much detail regarding this and gave a simple (albeit wrong) explanation
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Mutual Exception.
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race cond can also happen in single core if interrupts are enabledand a variable is modified.
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just a note mutuale exclusion problem can heppen even for single core processors , because when the quantum ends of a processoes , another process can modifie the variable
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Could you make a video about Condition Variables with threads?
Will do
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I have one comment about the likelihood of race condition happening for single-core processors. Even with a single core and a single processor but multiple threads (as it is the case for the FreeRTOS OS in embedded systems), race conditions will happen more often than not without a mutex, atomic instructions, etc.
Thanks! Interesting. I wonder now about single-core single-thread environments. Technically there still should be some race conditions but they might be encountered less often. Would you like to test that?
@@CodeVault
I think the only possibility for race conditions to happen for single-core single-threaded app, is when a hardware interrupt occurs and ISR gets executed.
If hardware interrupts were not present, I'm not sure race conditions can ever occur for a single-core, single-threaded app
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Even if I comment mutex_init and mutex_destroy, i can able to see mutex lock/unlock is working. Then what is the use of init/destroy function???
Well, init is surely needed when using a mutex. Although, probably, since the mutex is global in this video it gets its members automatically initialized to 0 and I think that coincides with what pthread_mutex_init sets the values to. If you try to use the a local mutex I think you'd have issues without the init function call. The pthread_mutex_destroy is not 100% necessary on some architectures but it's good practice
@@CodeVault Thank you for your clarity.
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Just curious a little bit from the last video. Base on my understanding, Race condition happen when 2 thread read memory at the same time. Is that possible if mutex lock data in exactly same time and race condition happen. If not why?
First things first: a race condition happens only in two cases:
1) You have one thread writing a piece of data while other threads are reading it
2) You have multiple threads writing the same piece of data
Two threads reading the same piece of data won't cause a race condition.
Although with mutexes a read and write could happen at the same time, they are designed so that each read and write on them is atomic, meaning there's no way other threads could read the data while another is in the process of writing it (only before it wrote to it or after that)
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2:09 what if the threads met a race condition at reading lock?
locks are thread-safe at the OS level
Great work
Bro small problem with your code.I took this as a refresher to threads.Locking and unlocking the mutex in each iteration of the loop introduces unnecessary overhead and reduces the potential benefits of multithreading. A better way is to lock and unlock it outside the loop.Anyway thanks for the video bro
Of course. But this was a short explanation and I wanted to show that the mutexes work even if multiple loops are executing at the same time. In a production environment, this would be very inefficient indeed
say in the routine() function I had 3 different operations, op1, op2, op3. I place a mutex_lock before op1, mutex_unlock after op1. Assume 3 threads. Will the other threads execute op2, and op3 when op1 is under lock ?
No, they won't. All threads will wait at the mutex_lock instruction until the lock is unlocked. Though, this still allows for race conditions on op2 and op3
Could you make a video about semaphores and pointing the differences between it and mutexes' use cases?
Yep, semaphores and barriers are also planned for this course
man, i wanna be your thread. such a good explanation
Maybe a minor misstatement when you say race conditions only happen on multicore systems. I think race conditions can also happen on one core processors. You could save your mails in thread 1 context switch to thread 2 and finish loop then go back to 1 and finish, then your result would only be 1,000,000 on write back. Great content though. Love this playlist.
Yes, it's definitely wrong. Race conditions can happen on any system. It's just much much rare to happen on single-core processors as the context switch would have to happen right between an assignment (which, from what I recall, is a very low chance). Maybe I will make a video on this topic to investigate what is the chance of that happening and whatnot
@@CodeVault that would be awesome 👍🏻
Race codnitions can very well occur in single core processors too. Think of pre-emption.
Exactly.
What is use case of second parameter of NULL when I create a thread. What are the use case i can change from NULL to some other?
It's just some attributes that you can set for the thread to run in a specific way. You can read more on this here: linux.die.net/man/3/pthread_attr_init
Bravo ! thanks for explaining
my question is : when we may need to create 2 threads with same function?
Whenever you want to split CPU intensive work between the threads, that's when the workload is exactly the same except for some indices.
Apparently there's another way of initializing a pthread_mutex:
pthread_mutex_t lock = PTHREAD_MUTEX_INITIALIZER;
I wonder what the difference is
They are basically the same. The first one could technically be faster since it only assigns some values but can only be used. Also, it guarantees that the lock is always initialized
Thanks for the wonderful video. I have a question. What happens if thread 1 throws an error before the unlock? Will thread 2 wait forever?
The lock is released, so thread 2 won't wait forever
Understood
Great 💣💯
i have encounter an error regarding pthread_mutex_destroy(&mutex, NULL);
but they are not receiving the int type for this function it just takes the refernce that's it. Hope u understand what i am trying to convey.
The pthread_mutex_destroy only has one parameter, the mutex you are trying to destroy
Can we say that mutex locking forces the multithreading routine into serial?
Yes. Basically the critical section that is surrounded by a mutex lock/unlock will always be serially executed
very underrated