What does a semaphore count?¶
Level: 201 · anyone who needs at most N of something at once
Stub — a question and its expected answers, not yet a lesson. No program behind this page has been written or run, so nothing on it has been through the check that backs every finished page. The table is what the finished lesson is expected to find, and every cell is unverified until a program prints it. (Not machine-checked here.)
One line: A semaphore is a counter that blocks when it reaches zero — N permits for N concurrent users of a pool, a connection limit, a disk — and unlike a mutex it has no owner, so any thread may release what another acquired, which is both its use and its hazard.
The question¶
Ten workers, three database connections. A mutex allows one; a semaphore initialized to three allows three. The page runs ten workers through a semaphore of three, records that no more than three were ever inside at once, and then does what a mutex would forbid: releases from a thread that did not acquire, which is how a semaphore signals an event happened rather than I am done with the resource.
What each language is expected to say¶
| Expected answer, to be verified by the program | |
|---|---|
| Rust | no semaphore in std; a Mutex<usize> plus Condvar, or tokio::sync::Semaphore (link, not run) |
| Go | a buffered channel of capacity N: send to acquire, receive to release; golang.org/x/sync/semaphore is weighted |
| C | sem_init/sem_wait/sem_post from POSIX; unnamed on macOS is deprecated, so sem_open |
| C++ | std::counting_semaphore<N> (C++20) and binary_semaphore |
| Java | java.util.concurrent.Semaphore(n) with an optional fairness flag |
| Python | threading.Semaphore(n) and BoundedSemaphore, which refuses to be released past its initial count |
What the programs have to show¶
- ten workers, three permits: the peak concurrency observed, three, and every worker finished
- release from a different thread than acquired: allowed, and what it is for
BoundedSemaphoreraising on the extra release, the one language that checks
See also¶
- Before this: What happens when the signal comes before the wait?
- After this: How do N threads wait for each other?
- How do N threads wait for each other?
- What stops a fast producer from filling memory?
- The Go library's A buffered channel as a semaphore ↗
- Concepts: Semaphore · Synchronization · Buffered and bounded channels · Mutex