Questions tagged [semaphore]
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17 questions
0 votes
1 answer
116 views
How to design a fifo coupon distribution system
I have an app written in java and using mysql as DB. I need to create a coupon distributed system where I have 100 coupons stored in DB and the first 100 customers who login should get those coupons. ...
- 109
0 votes
1 answer
159 views
Difference between outcome of using mutex/spinlock/semaphore in this deadlock situation?
I'm not sure if my answers to the following exercise is correct: Suppose the program is run with two threads. Further suppose that the following sequence of events occurs: Time Thread 0 ...
- 11
3 votes
3 answers
5k views
When should one utilize Atomic Operations?
I have never used atomic operations, and remain largely ignorant of how to utilize them. And yet, I often come across objects like this when peering into Qt's backend: https://doc.qt.io/qt-6/...
- 3,649
-4 votes
3 answers
789 views
What is "it is possible for one process to lock a binary semaphore and for another to unlock it" useful for?
Here are some discussions about mutex (lock) and binary semaphore from two OS books. Stalling's Operating Systems book says A concept related to the binary semaphore is the mutex . A key ...
- 5,555
1 vote
2 answers
2k views
Why is it that "the process that locks the mutex must be the one to unlock it "?
Here are some discussions about mutex (lock) and binary semaphore from two OS books. Stalling's Operating Systems book says A concept related to the binary semaphore is the mutex . A key ...
- 5,555
1 vote
1 answer
1k views
Is there still busy waiting in the process-blocking implementation of a semaphore?
Operating System Concepts discusses two implementations of a semaphore, by busy waiting in Section 5.5 and by blocking the current process in Section 5.6: Section 5.5 A semaphore S is an integer ...
- 5,555
-1 votes
1 answer
253 views
Why does "producers and consumers check availability before getting the mutex" avoid deadlock?
In Little Book of Semaphores: 4.1.6 Finite buffer producer-consumer solution Finite buffer consumer solution 1 items.wait () 2 mutex.wait () 3 event = buffer .get () 4 mutex. signal () 5 spaces . ...
- 5,555
0 votes
1 answer
2k views
Is a semaphore always initialized to the maximum value it can take?
According to Wikipedia: Semaphores are a useful tool in the prevention of race conditions; however, their use is by no means a guarantee that a program is free from these problems. Semaphores which ...
- 5,555
-1 votes
1 answer
205 views
is there difference between releasing two semaphores in the same or reverse order as they are acquired?
Generally, if two semaphores are acquired one after the other, is there difference between releasing them in the same order or the reverse order as they are acquired? In the following solution to the ...
- 5,555
28 votes
5 answers
7k views
Is "releases mutexes in reverse order" required to make this deadlock-prevention method work?
Operating System Concepts says 7.4.4 Circular Wait The fourth and final condition for deadlocks is the circular-wait condition. One way to ensure that this condition never holds is to impose a total ...
- 5,555
-2 votes
2 answers
1k views
what would happen if you signal() before wait()?
A bakery shop has to provide a stream of muffins for customers. The muffins are made by a baker in the kitchen and placed on a conveyor belt. The conveyor belt carries the muffins to where the ...
-1 votes
1 answer
542 views
Is it better to initialize semaphores in shared memory or give them as command line arguments? (C++)
I am on a project for which I have to spawn many different processes, all in different executables, and there will be some shared memory segments and some semaphores to synchronize them. I use an "...
- 119
-1 votes
1 answer
2k views
Is there a race condition for multiple threads on multiple cores trying to lock a global variable?
I understand in a single core system, multiple threads operate in sequence, scheduled by the OS. Hence it can't be possible to have a race condition since by definition they are taking turns locking a ...
- 101
4 votes
1 answer
1k views
Simple solution for calling a function only on one thread, queuing waiting calls?
Let’s assume there is a function EventHandler that is called on multiple threads at different points in time. EventHandler has to call SomeOtherFunction, but these calls shall only happen on one ...
- 476
16 votes
1 answer
2k views
Why is a semaphore called a semaphore?
It doesn't seem to me that a railway semaphore describes a semaphore better than any other generic signalling object, like a flag or a stoplight. Even something like a keyring (people take keys off ...
- 271