semaphore_acquire

Builtin index · Concurrency

semaphore_acquire(s)

Input: s · Output: no output

What it is

Blocks the calling thread until a permit is free on semaphore handle s, then takes it (decrementing the available count by one).

Overloads

One call shape — this function is not overloaded.

Examples

From the documentation, and run by the build -- the output and the figure below are what the engine actually produced. Simplest first.

s = semaphore(2)                       # 2 permits available

print(semaphore_available(s))          # 2

semaphore_acquire(s)                   # take one

print(semaphore_available(s))          # 1

semaphore_release(s)                   # give it back

print(semaphore_available(s))          # 2

2
1
2

from concurrency

How it works

semaphore_acquire(s) — blocks until a permit is free, then takes it. Unlike the mutex_* family, there's no self-contained atomic operation to offer here (the work done while holding the permit is arbitrary Qu code between the acquire and release calls), so — with no RAII/finally in Qu — releasing on an error path is the caller's job: semaphore_acquire(s) try ...critical work... catch err semaphore_release(s) error(err.message) end semaphore_release(s)

From the note above this builtin in the interpreter, where the reasoning was written down as it was built.