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The notion of using a semaphore in software as a means of mutual exclusion was invented by the Dutch computer scientist Edgser Dijkstra in 1959. In computer software, a semaphore is a protocol mechanism offered by most multitasking kernels. Semaphores, originally used to control access to shared resources, but now they are used for synchronization as described in Chapter 14, “Synchronization” Synchronization. However, it is useful to describe how semaphores can be used to share resources. The pitfalls of semaphores will be discussed in a later section.

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Typically, two types of semaphores exist: binary semaphores and counting semaphores. As its name implies, a binary semaphore can only take two values: 0 or 1. A counting semaphore allows for 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 µCμC/OS-III, the maximum value of a semaphore is determined by the data type OS_SEM_CTR (see os_type.h), which can be changed as needed. Along with the semaphore’s value, µCμC/OS-III also keeps track of tasks waiting for the semaphore’s availability.

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There are a number of operations the application is able to perform on semaphores, summarized in Table 13-2the table below. In this chapter, only the three most used functions used most often are discussed: OSSemCreate(), OSSemPend(), and OSSemPost(). Other The other functions are described in Appendix A, “µCµC/OS-III API Reference”Reference. When semaphores are used for sharing resources, every semaphore function must be called from a task and never from an ISR. The same limitation does not apply when using semaphores for signaling, as described later in Chapter 13.

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