From the documentation: The SemaphoreSlim class doesn’t enforce thread or task identity on calls to the Wait, WaitAsync, and Release methods In other words, the class doesn’t look to see which thread is calling it. It’s just a simple counter. The same thread can acquire the semaphore multiple times, and that will be the same … Read more
malloc and printf usually use global structures, and employ lock-based synchronization internally. That’s why they’re not reentrant. The malloc function could either be thread-safe or thread-unsafe. Both are not reentrant: Malloc operates on a global heap, and it’s possible that two different invocations of malloc that happen at the same time, return the same memory … Read more
Reentrancy means that locks are acquired on a per-thread rather than per-invocation basis. That is a misleading definition. It is true (sort of), but it misses the real point. Reentrancy means (in general CS / IT terminology) that you do something, and while you are still doing it, you do it again. In the case … Read more
TL;DR: A function can be reentrant, thread-safe, both or neither. The Wikipedia articles for thread-safety and reentrancy are well worth reading. Here are a few citations: A function is thread-safe if: it only manipulates shared data structures in a manner that guarantees safe execution by multiple threads at the same time. A function is reentrant … Read more
The main difference is that a Lock can only be acquired once. It cannot be acquired again, until it is released. (After it’s been released, it can be re-acaquired by any thread). An RLock on the other hand, can be acquired multiple times, by the same thread. It needs to be released the same number … Read more
1. How is safely defined? Semantically. In this case, this is not a hard-defined term. It just mean “You can do that, without risk”. 2. If a program can be safely executed concurrently, does it always mean that it is reentrant? No. For example, let’s have a C++ function that takes both a lock, and … Read more