Semaphores

Semaphores

As defined in Chapter 13, “Resource Management” on page 211, a semaphore is a protocol mechanism offered by most multitasking kernels. Semaphores were originally used to control access to shared resources. However, better mechanisms exist to protect access to shared resources, as described in Chapter 12. Semaphores are best used to synchronize an ISR to a task, or synchronize a task with another task as shown in Figure 14-1.

Note that the semaphore is drawn as a ?ag to indicate that it is used to signal the occurrence of an event. The initial value for the semaphore is typically zero (0), indicating the event has not yet occurred.

The value “N” next to the flag indicates that the semaphore can accumulate events or credits. An ISR (or a task) can post (or signal) multiple times to a semaphore and the semaphore will remember how many times it was posted. It is possible to initialize the semaphore with a value other than zero, indicating that the semaphore initially contains that number of events.

Also, the small hourglass close to the receiving task indicates that the task has an option to specify a timeout. This timeout indicates that the task is willing to wait for the semaphore to be signaled (or posted to) within a certain amount of time. If the semaphore is not signaled within that time, µC/OS-III resumes the task and returns an error code indicating that the task was made ready-to-run because of a timeout and not the semaphore was signaled.

1. 1. µC/OS-III Semaphore Services

There are a number of operations to perform on semaphores as summarized in Table 14-1 and Figure 14-1. However, in this chapter, we will only discuss the three functions used most often: OSSemCreate(), OSSemPend(), and OSSemPost(). The other functions are described in Appendix A, “µC/OS-III API Reference” on page 431. Also note that every semaphore function is callable from a task, but only OSSemPost() can be called by an ISR.

1. 1. Semaphore API summary

When used for synchronization, a semaphore keeps track of how many times it was signaled using a counter. The counter can take values between 0 and 255, 65,535, or 4,294,967,295, depending on whether the semaphore mechanism is implemented using 8, 16, or 32 bits, respectively. For µC/OS-III, the maximum value of a semaphore is determined by the data type OS_SEM_CTR (see os_type.h), which is changeable, as needed (assuming access to µC/OS-III’s source code). Along with the semaphore’s value, µC/OS-III keeps track of tasks waiting for the semaphore to be signaled.