Difference between binary semaphore and mutex stack overflow

Mutex can be released only by thread that had acquired it, while you can signal semaphore from any other thread or processso semaphores are more suitable for some synchronization problems like producer-consumer.

The Toilet example is an enjoyable analogy:. Is a key to a toilet. One person can have the key - occupy the toilet - at the time. When finished, the person gives frees the key to the next person in the queue. A mutex object only allows one thread into a controlled section, forcing other threads which attempt to gain access to that section to wait until the first thread has exited from that section. Is the number of free identical toilet keys.

Example, say we have four toilets with identical locks and keys. The semaphore count - the count of keys - is set to 4 at beginning all four toilets are freethen the count value is decremented as people are coming in. If all toilets are full, ie. Threads can request access to the resource decrementing the semaphoreand can signal that they have finished using the resource incrementing the semaphore.

They are NOT the same thing. They difference between binary semaphore and mutex stack overflow used for different purposes! Mutual Exclusion Semaphores Mutual Exclusion semaphores are used to protect shared resources data structure, file, etc. A Mutex semaphore is "owned" by the task that takes it.

Binary Semaphore Binary Semaphore address a totally different question:. Note that with a binary semaphore, it is OK for B to take the semaphore and A to give it. Again, a binary semaphore is NOT protecting a resource from access. The act of Giving and Taking a semaphore are fundamentally decoupled. It typically makes little sense for the same task to do a give and a take on the same binary semaphore. The mutex is similar to the principles of the binary semaphore with one significant difference: Ownership is the simple concept that when a task locks acquires a mutex only it can unlock release it.

If the mutual exclusion object doesn't have ownership then, irrelevant of what it is called, it difference between binary semaphore and mutex stack overflow not a mutex. At a theoretical level, they are no different semantically. You can implement a mutex using semaphores or vice versa see here for an example. In practice, the implementation is different and they offer slightly different services. The practical difference in terms of the system services surrounding them is that the implementation of a mutex is aimed at being a more lightweight synchronisation mechanism.

In oracle-speak, mutexes are known as latches and semaphores are known as waits. At the lowest level, they use difference between binary semaphore and mutex stack overflow sort of atomic test and difference between binary semaphore and mutex stack overflow mechanism.

This reads the current value of a memory location, computes some sort of conditional and writes out a value at that location in a single instruction that cannot be interrupted. This means that you can acquire a mutex and test to see if anyone else had it before you. A typical mutex implementation has a process or thread executing the test-and-set instruction and evaluating whether anything else had set the mutex. A key point here is that there is no interaction with the schedulerso we have no idea and don't care who has set the lock.

Then we either give up our time slice and attempt it again when the task is re-scheduled or execute a spin-lock. A spin lock is an algorithm like:. When we have finished executing our protected code known as a critical difference between binary semaphore and mutex stack overflow we just set the mutex value to zero or whatever means 'clear.

Typically you would use mutexes to control a synchronised resource where exclusive access is only needed for very short periods of time, normally to make an update to a shared data structure. A semaphore is a synchronised data structure typically using a mutex that has a count and some system call wrappers that interact with the scheduler in a bit more depth than the mutex libraries would. Semaphores are incremented and decremented and used to block tasks until something else is ready.

Semaphores are initialised to some value - a binary semaphore difference between binary semaphore and mutex stack overflow just a special case where the semaphore is initialised to 1. Posting to a semaphore has the effect of waking up a waiting process. In the case of a binary semaphore the main practical difference between the two is the nature of the system services surrounding the actual data structure.

As evan has rightly pointed out, spinlocks will slow down a single processor machine. You would only use a spinlock on a multi-processor box because on a single processor the process holding the mutex will never reset it while another task is running. Spinlocks are only useful on multi-processor architectures. As such one can see a mutex as a token passed from task to tasks and a semaphore as traffic red-light it signals someone that it difference between binary semaphore and mutex stack overflow proceed.

You obviously use mutex to lock a data in one thread getting accessed by another thread at the same time. Assume that you have just called lock and in the process of accessing data. This means that you dont't expect any other thread or another instance of the same thread-code to access the same data locked by the same mutex.

That is, if it is the same thread-code getting executed on a different thread instance, hits the lock, then the lock should block the control flow there. This applies to a thread that uses a different thread-code, which is also accessing the same data and which is also locked by the same mutex.

In this case, you are still in the process of accessing the data and you may take, say, another 15 secs to reach the mutex unlock so that the other thread difference between binary semaphore and mutex stack overflow is getting blocked in mutex lock would unblock and would allow the control to access the data. Do you at any cost allow yet another thread to just unlock the same mutex, and in turn, allow the thread that is already waiting blocking in the mutex lock to unblock and access the data?

Hope you got what I am saying here? As per, agreed upon universal definition! So, if you are very particular about using binary-semaphore instead of mutex, then you should be very careful in 'scoping' the locks and unlocks.

I mean that every control-flow that hits every lock should hit an unlock call, also there shouldn't be any 'first unlock', rather it should be always 'first lock'. A mutex can only be released by the thread which has ownership, i. A semaphore can be released by any thread. A thread can call a wait function repeatedly on a mutex without blocking.

However, if you call a wait function twice on a binary semaphore without releasing the semaphore in between, the thread will block. A Mutex controls access to a single shared resource. It provides operations to acquire access to that resource and release it when done. A Semaphore controls access to a shared pool of resources. It provides operations to Difference between binary semaphore and mutex stack overflow until one of the resources in the pool becomes available, and Signal when it is given back to the pool.

When number of resources a Semaphore protects is greater than 1, it is called a Counting Semaphore. When it controls one resource, it is called a Boolean Semaphore. A boolean semaphore is equivalent to a mutex. Thus a Semaphore is a higher level abstraction than Mutex. A Mutex can be implemented using a Semaphore but not the other way around.

A semaphore can be a Mutex but a Mutex can never be semaphore. This simply means that a binary semaphore can be used as Mutex, but a Mutex can never exhibit the functionality of semaphore. No one owns semaphores, whereas Mutex are owned and the owner is held responsible for them. This is an important distinction from a debugging perspective. In case the of Mutex, the thread that owns the Mutex is responsible for freeing it.

However, in the case of semaphores, this condition is not required. Any other thread can signal to free the semaphore by using the s m p s function. A semaphore, by definition, restricts the number of simultaneous users of a shared resource up to a maximum number 6. The nature of semaphores makes it possible to use them in synchronizing related and unrelated process, as well as between threads.

Mutex can be used only in synchronizing between threads and at most between related processes the pthread implementation of the latest kernel comes with a feature that allows Mutex to be used between related process.

According to the kernel documentation, Mutex are lighter when compared to semaphores. What this means is that a program with semaphore usage has a higher memory footprint when compared to a program having Mutex.

From a usage perspective, Mutex has simpler semantics when compared to semaphores. Modified question is - What's the difference between A mutex and a "binary" semaphore in "Linux"? Following are the differences i 'Scope' The scope of mutex is within a process address space which has difference between binary semaphore and mutex stack overflow it and difference between binary semaphore and mutex stack overflow used for synchronization of threads.

Whereas semaphore can be used across process space and hence it can be used for interprocess synchronization. Other thread trying to acquire will block. Whereas in case of semaphore if same process tries to acquire it again it blocks as it can be acquired only once.

Mutex is locking mechanism used to synchronize access to a resource. Semaphore is signaling mechanism. Mutex are used for " Locking Mechanisms ". Semaphores are used for " Signaling Mechanisms " like "I am donenow can continue".

The answer may depend on the target OS. For example, at least one RTOS implementation I'm familiar with will allow multiple sequential "get" operations against a single OS mutex, so long as they're all from within the same thread context. The multiple gets must be replaced by an equal number of puts before another thread will be allowed to get the mutex. This differs from binary semaphores, for difference between binary semaphore and mutex stack overflow only a single get is allowed at a time, regardless of thread contexts.

The idea behind this type of mutex is that you protect an object by only allowing a single context to modify the data at a time. Of course, when using this feature, you must be certain that all accesses within a single thread really are safe! I'm not sure how common this approach is, or whether it applies outside of the systems with which I'm familiar.

We mainly Gustavo Pintowith a little help from myself and Weslley Torres have conducted a study on the most popular questions about concurrent programming on StackOverflow. Our goal with this study was to understand the practical problems faced by software developers when using concurrent programming abstractions. These are the 10 most popular questions:. What does it mean? There are also a couple of questions asking about general concepts 3,7 and one that may be thought of as language specific or not 8.

We used the same keywords as the well-known study by Shan Lu and colleagues to select the questions based on the tags associated to them: This produced a list with more than questions. We then ranked them by popularity, extracted only the most popular, and manually inspected those to ascertain that they were actually concurrency-related. We ended up with questions. To calculate the popularity of each question, we obtained, for each one: Each of these metrics was then normalized with respect to the average value considering all the questions in StackOverflow.

The P popularity measure is the result of calculating the geometric mean of these normalized metrics. Most of the questions pertained to either Java 77 or C Also, 28 pertained to mobile computing, focusing on one of the most popular platforms: Among all the questions, only one pertained to GPUs and not a single one asked specifically about improving performance, which is surprising. I could say that we learned three main things from this study:.

For more information, take a look at our paper. These are the 10 most popular questions: What are concurrency-related questions? How did we measure popularity? Summary Most of the questions pertained to either Java 77 or C We classified the questions in terms of 7 categories: I could say that we learned three main things from this study: Developers do not understand the problems that existing tools report about concurrency errors, because they often do not understand the errors themselves.

For example, a simple tool that indicates to developers which parts of the code execute atomically is straightforward to build and could be easily integrated into existing IDEs Developers want examples, both minimal examples of things that work correctly and minimal examples of code that help them understand concurrency problems.