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.
| (semaphore? v) → boolean? |
| v : any/c |
| (make-semaphore [init]) → semaphore? |
| init : exact-nonnegative-integer? = 0 |
Creates and returns a new semaphore with the counter initially set to
init. If init is larger than a semaphore’s maximum
internal counter value, the exn:fail exception is raised.
| (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.
| ||||||||||||||||||||||||||||
| sema : semaphore? | ||||||||||||||||||||||||||||
| proc : procedure? | ||||||||||||||||||||||||||||
| try-fail-thunk : (or/c (-> any) #f) = #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.
| ||||||||||||||||||||||||||||
| sema : semaphore? | ||||||||||||||||||||||||||||
| proc : procedure? | ||||||||||||||||||||||||||||
| try-fail-thunk : (or/c (-> any) #f) = #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.