Take a look at Multithreading Tutorial by John Kopplin. In the section Synchronization Between Threads , he explain the differences among event, lock, mutex, semaphore, waitable timer.
A mutex can be owned by only one thread at a time, enabling threads to coordinate mutually exclusive access to a shared resource. Critical section objects provide synchronization similar to that provided by mutex objects, except that critical section objects can be used only by the threads of a single process. Another difference between a mutex and a critical section is that if the critical section object is currently owned by another thread, EnterCriticalSection waits indefinitely for ownership whereas WaitForSingleObject , which is used with a mutex, allows you to specify a timeout.
A semaphore maintains a count between zero and some maximum value, limiting the number of threads that are simultaneously accessing a shared resource. Lock: One example where you would use lock would be a shared dictionary into which items that must have unique keys are added. The lock would ensure that one thread does not enter the mechanism of code that is checking for item being in dictionary while another thread that is in the critical section already has passed this check and is adding the item.
If another thread tries to enter a locked code, it will wait be blocked until the object is released. Semaphore: Let's say you have a pool of connections, then an single thread might reserve one element in the pool by waiting for the semaphore to get a connection. It then uses the connection and when work is done releases the connection by releasing the semaphore.
Code example that I love is one of bouncer given by Patric - here it goes:. Mutex It is pretty much Semaphore 1,1 and often used globally application wide otherwise arguably lock is more appropriate. One would use global Mutex when deleting node from a globally accessible list last thing you want another thread to do something while you are deleting the node. When you acquire Mutex if different thread tries to acquire the same Mutex it will be put to sleep till SAME thread that acquired the Mutex releases it.
Good example on creating global mutex is by deepee. Wikipedia has a great section on the differences between Semaphores and Mutexes :. A mutex is essentially the same thing as a binary semaphore and sometimes uses the same basic implementation. The differences between them are:. Mutexes have a concept of an owner, which is the process that locked the mutex. Only the process that locked the mutex can unlock it. In contrast, a semaphore has no concept of an owner. Any process can unlock a semaphore.
Unlike semaphores, mutexes provide priority inversion safety. Since the mutex knows its current owner, it is possible to promote the priority of the owner whenever a higher-priority task starts waiting on the mutex. Mutexes also provide deletion safety, where the process holding the mutex cannot be accidentally deleted.
Semaphores do not provide this. My understanding is that a mutex is only for use within a single process, but across its many threads, whereas a semaphore may be used across multiple processes, and across their corresponding sets of threads. Also, a mutex is binary it's either locked or unlocked , whereas a semaphore has a notion of counting, or a queue of more than one lock and unlock requests. Could someone verify my explanation? Only the thread that currently holds the mutex can unlock it.
When thread get a lock it becomes a single thread which is able to execute a block of code. All others thread are blocked. Only thread which owns the lock can unlock it. It is a kind of lock. On some languages it is inter-process mechanism, on some languages it is a synonym of lock. For example Java uses lock in synchronised and java. You can find that mutex also can be implemented by semaphore.
It is a standalone object which manage an access to shared resource. You can find that any thread can signal and unblock.
Also it is used for signalling. How are we doing? Please help us improve Stack Overflow. Take our short survey. Stack Overflow for Teams — Collaborate and share knowledge with a private group.
Create a free Team What is Teams? Collectives on Stack Overflow. Learn more. What is the difference between lock, mutex and semaphore?
Ask Question. Asked 11 years, 10 months ago. Active 1 month ago. Viewed k times. Improve this question. The best explanation I have ever seen: crystal. Possible duplicate of Difference between binary semaphore and mutex — user Add a comment. Active Oldest Votes.
A mutex is the same as a lock but it can be system wide shared by multiple processes. Improve this answer. Hence, it will then be allowed for other threads to take ownership of the lock.
The lock can only have two values at one time which are either 0 or 1. When the critical section is empty, the value of it will be 0. However, when there the critical section is occupied, the value of the lock is 1. Hence, whenever a process wants to enter the critical section, he checks this value and then makes the decision accordingly.
Below is a pseudo code to understand the use of values in the lock:. On the other hand, semaphores are much similar to locks but they tend to solve the critical section problem. There are two basic operations in the process of synchronization which are wait and signal. Wait operation is the sleep operation and the signal is the wake-up operation. Semaphores just like locks allow only one process at a time. There is a process called busy waiting in this as the process tends to wait until its turn.
Therefore, there is no resource wastage. Semaphores are of two types: binary semaphores and counting semaphores. Let us try to understand the concept with a problem in OS called producer-consumer problem. Let us assume that we have a buffer of byte length. If multiple processes are writing to a file at the same time, the result is non deterministic.
Within the UNIX kernel, only one write to the same file may proceed at any given time. Sections of a file are also called records, hence file and record locking.
You lock files, or portions of files, to prevent the errors that can occur when two or more users of a file try to update information at the same time. How to lock your screen. The purpose of a lock file is to lock down the versions of the dependencies specified in a package. This means that in a yarn.
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