class Semaphore { }

Protect your shared code, data or device access using semaphores. An example is a printer manager managing a pool of printers without the need of storing print jobs when all printers are occupied. The next job is just blocked until a printer becomes available.

class print-manager {
  has Array $!printers;
  has Semaphore $!print-control;
 
  method BUILD( Int:D :$nbr-printers ) {
    for ^$nbr-printers -> $pc {
      $!printers[$pc] = { :name{"printer-$pc"} };
    }
 
    $!print-control .= new($nbr-printers);
  }
 
  method find-available-printer-and-print-it($job) { say "Is printed!"; }
 
  method print( $print-job ) {
    $!print-control.acquire;
 
    self.find-available-printer-and-print-it($print-job);
 
    $!print-control.release;
  }
}

Another example is a protection around code updating sensitive data. In such a case the semaphore is typically initialized to 1.

It is important to have a release on every exit of your program! While this is obvious, it is easy to fall in traps such as throwing an exception caused by some event. When the program dies there is no problem. When the exception is caught your program might eventually come back to the acquire method and will hang indefinitely.

Methods

method new

method new( int $permits )

Initialize the semaphore with the number of permitted accesses. E.g. when set to 2, program threads can pass the acquire method twice until it blocks on the third time acquire is called.

method acquire

method acquire()

Acquire access. When other threads have called the method before and the number of permits are used up, the process blocks until threads passed before releases the semaphore.

method try_acquire

method try_acquire(--> Bool)

Same as acquire but will not block. Instead it returns True if access is permitted or False otherwise.

method release

method release()

Release the semaphore raising the number of permissions. Any blocked thread will get access after that.