Are lock, mutex, and semaphore for between threads or between processes

lockprocessthread

I think lock, mutex, semaphore are used for synchronized multiple (threads or processes?) to access something simultaneously.

Must this "something" be some shared memory between multiple (threads or processes)?

If yes, does that mean lock, mutex, semaphore are only used for multiple threads of a process, not for multiple processes, because multiple processes don't share memory, while multiple threads of the same process do?

Thanks.

Best Answer

Are lock, mutex, and semaphore for between threads or between processes?

You'll find examples of locking primitives for both situations. For example, pthread mutexes are used for mutual exclusion between threads of the same process. On the other hand, System V IPC (man svipc) semaphores can be used across processes. Filesystem-level locks (on files or parts of files) can also used to coordinate between multiple processes.

Must this "something" be some shared memory between multiple (threads or processes)?

Not necessarily shared memory. Any sort of resource can be protected with synchronization primitives. A piece of shared memory (shared between processes and/or threads) is perhaps the most common example, but other things like file-handles (locking a whole file, or parts of a file, or other sorts of "handles" to unique resources) and access to hardware devices are also possible.

If yes, does that mean lock, mutex, semaphore are only used for multiple threads of a process, not for multiple processes, because multiple processes don't share memory, while multiple threads of the same process do?

Well no. Processes can share memory. (And in fact most of them do, if only in a read-only fashion - pages containing shared library code are shared between processes. That doesn't need protection though. Copy-on-write shared mappings done when forking do need protection, but the kernel handles that transparently - this involves locks on shared memory mappings, which is conceptually a bit different from ordinary locks protecting a memory region/datastructure.)

Anything else that processes or threads can share (when at least one of them can write) needs protection/coordination, files and devices being the two most common I think.

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