multithreading - In what situation do you use a semaphore over a mutex in C++?

Question

Throughout the resources I've read about multithreading, mutex is more often used and discussed compared to a semaphore. My question is when do you use a semaphore over a mutex? I don't see semaphores in Boost thread. Does that mean semaphores no longer used much these days?

As far as I've understand, semaphores allow a resource to be shared by several threads. This is only possible if those threads are only reading the resource but not writing. Is this correct?

Answer 1

The typical use case for a mutex (allowing only one thread access to a resource at any time) is far more common than the typical uses if a semaphore. But a semaphore is actually the more general concept: A mutex is (almost) a special case of a semaphore.

Typical applications would be: You don't want to create more than (e.g.) 5 database connections. No matter how many worker threads there are, they have to share these 5 connections. Or, if you run on a N-core machine, you might want to make sure that certain CPU/memory-intensive tasks don't run in more than N threads at the same time (because that would only reduce throughput due to context switches and cache thrashing effects). You might even want to limit the number of parallel CPU/memory intensive tasks to N-1, so the rest of the system doesn't starve. Or imagine a certain task needs a lot of memory, so running more than N instances of that task at the same time would lead to paging. You could use a semaphore here, to make sure that no more than N instances of this particular task run at the same time.

EDIT/PS: From your question "This is only possible if those threads are only reading the resource but not writing. Is this correct?" and your comment, it seems to me as if you're thinking of a resource as a variable or a stream, that can be read or written and that can only be written to by one thread at a time. Don't. This is misleading in this context.

Think of resources like "water". You can use water to wash your dishes. I can use water to wash my dishes at the same time. We don't need any kind of synchronization for that, because there is enough water for both of us. We don't necessarily use the same water. (And you can't "read" or "write" water.) But the total amount of water is finite. So it's not possible for any number of parties to wash their dishes at the same time. This kind of synchronization is done with a semaphore. Only usually not with water but with other finite resources like memory, disk space, IO throughput or CPU cores.