4.4.1 Semaphore
A semaphore is an object indicating the availability of a resource and its quantity as a numerical value. A semaphore is used to realize mutual exclusion control and synchronization when using a resource. Functions are provided for creating and deleting a semaphore, acquiring and returning resources corresponding to semaphores, and referencing semaphore status. A semaphore is an object identi.ed by an ID number called a semaphore ID.
A semaphore contains a resource count indicating whether the corresponding resource exists and in what quantity, and a queue of tasks waiting to acquire the resource. When a task (the task making event noti.cation) returns m resources, it increments the semaphore resource count by m. When a task (the task waiting for an event) acquires n resources, it decreases the semaphore resource count by n. If the number of semaphore resources is insu.cient (i.e., further reducing the semaphore resource count would cause it to go into negative territory), a task attempting to acquire resources goes into WAIT state until the next time resources are returned. A task waiting for semaphore resources is put in the semaphore queue.
To prevent too many resources from being returned to a semaphore, a maximum resource count can be set for each semaphore. Error is reported if it is attempted to return resources to a semaphore that would cause this maximum count to be exceeded.
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