How does .NET implement a mutex?

Question

I understand the concept of a mutex. It was very well explained here.

But now I want to know what a mutex really is. My guess is that .NET is taking some primitive system resource (maybe even just a memory address?) and wrapping it in an object that it calls a mutex.

Anyone know exactly how a mutex is achieved in .NET?

May also be related: [How are Mutexes Implemented, Language Agnostic](http://stackoverflow.com/questions/1485924/how-are-mutexes-implemented) — Brad Christie, Apr 11 '11 at 22:48
I'm pretty sure it just wraps a Win32 Mutex object. Are you looking for details on how the Win32 mutex is implemented? — Michael Burr, Apr 11 '11 at 22:49
@Michael: If that's true, can we drill that down then. How is a Win32 mutex implemented? — richard, Apr 11 '11 at 22:53
For native mutex implementations, folks have had a go answering over here: http://stackoverflow.com/questions/4414106/native-mutex-implementation — otherchirps, Apr 12 '11 at 00:15

score 5 · Accepted Answer · answered Apr 12 '11 at 00:09

How a mutex gets implemented is quite likely hardware-dependent. Most CPUs have some sort of atomic compare-and-swap instruction that provides the guts of the thing.

But yes, under the hood, it's just a semaphore — a thing (word, probably) whose value indicates whether its signaled or not. The OS provides a means for a thread or process to do an idle wait, waiting for the semaphore to enter the desired state. Most implementation, I believe don't guarantee the order in which a thread might gain ownership of a mutex — just because you were first in line, doesn't mean that you'll be the first to get it.

"it's just a semaphore" - That's what I thought! It seems like it would just be a simple semaphore that's just ratcheted down to one thread getting access to the resource. Is that right? — richard, Apr 12 '11 at 06:43

score 0 · Answer 2 · answered Jun 08 '11 at 01:44

http://msdn.microsoft.com/en-us/magazine/cc164040.aspx will give you the details. .NET Mutexes are based at the end of the day, on Win32 thread primitives, though some things are a bit different (remember that the 'lock' statement works on any object, so there are some things in the CLR called SyncBlocks that deal with that).

How does .NET implement a mutex?

2 Answers2