Whether it is unlocked or signaled first, each has advantages and disadvantages, which will be analyzed below. Also note that whether the signal function is unlocked or locked is different from wait. Now, take a look at what happened in the first thread. Not like that! At that time, it can immediately check the list to see the changes it is interested in. In fact, it is generally unlocked first.
Let's look at an example:. If there are sodas available, the code will just keep running past the down statement without a problem. If no sodas are available, the thread will sleep here waiting to be notified of when soda is made available again when there are more sodas in the machine. The vendor producer thread would essentially be waiting for the soda machine to be empty.
The vendor gets notified when the last soda is taken from the machine and one or more consumers are potentially waiting to get sodas out. The vendor would restock the soda machine with the semaphore up method, the available number of sodas would be incremented each time and thereby the waiting consumer threads would get notified that more soda is available.
The wait and signal methods of a synchronization variable tend to be hidden within the down and up operations of the semaphore. Certainly there's overlap between the two choices. There are many scenarios where a semaphore or a condition variable or set of condition variables could both serve your purposes.
Both semaphores and condition variables are associated with a lock object that they use to maintain mutual exclusion, but then they provide extra functionality on top of the lock for synchronizing thread execution.
It's mostly up to you to figure out which one makes the most sense for your situation. Conditional variable is essentially a wait-queue , that supports blocking-wait and wakeup operations, i. And it can be used as it is without external dependencies.
That's not necessarily the most technical description, but that's how it makes sense in my head. Improve this answer. Great answer, I would like to add from other so answers:Semaphore is used to control the number of threads executing. There will be fixed set of resources. The resource count will gets decremented every time when a thread owns the same. When the semaphore count reaches 0 then no other threads are allowed to acquire the resource.
The threads get blocked till other threads owning resource releases. In short, the main difference is how many threads are allowed to acquire the resource at once? Mutex --its ONE. Just to elaborate on why there's this while loop instead of a simple if: something called spurios wakeup. Quoting this wikipedia article : "One of the reasons for this is a spurious wakeup; that is, a thread might be awoken from its waiting state even though no thread signaled the condition variable" — Vladislavs Burakovs.
VladislavsBurakovs Good point! I think it's also helpful for the case where a broadcast wakes up more threads than there are resources available e. I wish I would upvote you answer until the queue is full ; Perfect Answer. This code could help figuring out semaphores csc. VladislavsBurakovs To clarify a bit, the reason that the condition may still be false to a thread that has just woken up resulting in a spurious wakeup is that there might have been a context switch before the thread got a chance to check the condition again, where some other scheduled thread made that condition false.
This is one reason that I know for a spurious wakeup, don't know if there are more. Show 3 more comments. Let's reveal what's under the hood. The description about the protocol is wrong! Dacav Dacav 12k 9 9 gold badges 55 55 silver badges 82 82 bronze badges. Danielle Danielle 1 1 silver badge 2 2 bronze badges. Condition Variables aren't really good for implementing Barriers, either. In particular, there is a race condition between a thread decrementing said counter and putting itself to sleep on the Condition Variable.
So one also needs a Mutex. Each thread has to first acquire the Mutex, then decrement and check the counter, then put itself to sleep on the Condition Variable while atomically releasing the Mutex. When later all threads wake up, they all need to reacquire that Mutex, but can only do so one thread at a time. Therefore, if the OS libs provide a Barrier primitive, then use that! Justin R Justin R 2 2 silver badges 8 8 bronze badges.
It includes two operations i. It just allows thread to be signaled when something of interest to that threads occurs and is mostly used when one wants to know when something happens. Attention reader! Difference between Semaphore and Condition Variable :. Skip to content. Change Language.
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