Instances of class Semaphore are used for interprocess synchronization.
Semaphores can be thought of as containers for
signals. (Signals are not on/off switches; multiple signals can be queued up.) Sending the message
signal to a semaphore adds a signal, while sending the message
wait removes a signal. If a semaphore that has no signals is sent the
wait message, the process running the wait is suspended until the semaphore is sent a
signal message. If more than one process is waiting on a semaphore when a
signal is sent, the longest waiting process is resumed.
The following example illustrates the use of Semaphores. A process is forked at a high priority. It blocks, waiting on a
Semaphore. Each time the user interface signals the
Semaphore the forked process unblocks, increments a counter, and blocks on the
Semaphore again. The user interface process then prints the counter value. After the example is run, the
Semaphore becomes garbage, and the original process disappears with the
Semaphore.
Instances of class Delay are used to effect synchronization with the real-time clock, and to postpone process execution for a specified time period. Postponement can be specified in milliseconds, seconds, or until a specific time. Each
Delay has a
resumption time specified in milliseconds since midnight. When the
Delay is sent the
wait message, the process running the wait operation is suspended until the real-time clock advances to the resumption time specified by the
Delay.
