10.2.3 Semaphores

A semaphore has an internal counter; when this counter is zero, the semaphore can block a thread’s execution (through semaphore-wait) until another thread increments the counter (using semaphore-post). The maximum value for a semaphore’s internal counter is platform-specific, but always at least 10000.

A semaphore’s counter is updated in a single-threaded manner, so that semaphores can be used for reliable synchronization. Semaphore waiting is fair: if a thread is blocked on a semaphore and the semaphore’s internal value is non-zero infinitely often, then the thread is eventually unblocked.

In addition to its use with semaphore-specific procedures, semaphores can be used as events; see Events.

(make-semaphore [init]) → semaphore?

init : nonnegative-exact-integer? = 0

Creates and returns a new semaphore with the counter initially set to init. If init-k is larger than a semaphore’s maximum internal counter value, the exn:fail exception is raised.

(semaphore? v) → boolean?

v : any/c

Returns #t if v is a semaphore created by make-semaphore, #f otherwise.

(semaphore-post sema) → void?

sema : semaphore?

Increments the semaphore’s internal counter and returns #<void>. If the semaphore’s internal counter has already reached its maximum value, the exn:fail exception is raised.

(semaphore-wait sema) → void?

sema : semaphore?

Blocks until the internal counter for semaphore sema is non-zero. When the counter is non-zero, it is decremented and semaphore-wait returns #<void>.

(semaphore-try-wait? sema) → boolean?

sema : semaphore?

Like semaphore-wait, but semaphore-try-wait? never blocks execution. If sema’s internal counter is zero, semaphore-try-wait? returns #f immediately without decrementing the counter. If sema’s counter is positive, it is decremented and #t is returned.

(semaphore-wait/enable-break sema) → void?

sema : semaphore?

Like semaphore-wait, but breaking is enabled (see Breaks) while waiting on sema. If breaking is disabled when semaphore-wait/enable-break is called, then either the semaphore’s counter is decremented or the exn:break exception is raised, but not both.

(semaphore-peek-evt sema) → evt?

sema : semaphore?

Creates and returns a new synchronizable event (for use with sync, for example) that is ready when sema is ready, but synchronizing the event does not decrement sema’s internal count.

(call-with-semaphore   sema             proc            [ try-fail-thunk]             arg ...)   →   any

sema : semaphore?

proc : procedure?

try-fail-thunk : (or/c (-> any) false/c) = #f

arg : any/c

Waits on sema using semaphore-wait, calls proc with all args, and then posts to sema. A continuation barrier blocks full continuation jumps into or out of proc (see Prompts, Delimited Continuations, and Barriers), but escape jumps are allowed, and sema is posted on escape. If try-fail-thunk is provided and is not #f, then semaphore-try-wait? is called on sema instead of semaphore-wait, and try-fail-thunk is called if the wait fails.

(call-with-semaphore/enable-break   sema             proc            [ try-fail-thunk]             arg ...)   →   any

sema : semaphore?

proc : procedure?

try-fail-thunk : (or/c (-> any) false/c) = #f

arg : any/c

Like call-with-semaphore, except that semaphore-wait/enable-break is used with sema in non-try mode. When try-fail-thunk is provided and not #f, then breaks are enabled around the use of semaphore-try-wait? on sema.