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Anchor10538431053843 Semaphores Anchor10538441053844 As defined in Chapter 13, “Resource Management” on page 211, A semaphore was originally a mechanical signaling mechanism. The railroad industry used the device to provide a form of mutual exclusion for railroads tracks shared by more than one train. In this form, the semaphore signaled trains by closing a set of mechanical arms to block a train from a section of track that was currently in use. When the track became available, the arm would swing up and the waiting train would then proceed.

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 were , originally used to control access to shared resources. However, better mechanisms exist to protect access to shared resources, , but now they are used for synchronization 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. Anchor10538451053845 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. Anchor10538461053846 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. Anchor10544311054431 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. Anchor10810121081012 Image Removed

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Anchor10538491053849 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 oftenSynchronization. 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.

A semaphore was originally a “lock mechanism” and code acquired the key to this lock to continue execution. Acquiring the key means that the executing task has permission to enter the section of otherwise locked code. Entering a section of locked code causes the task to wait until the key becomes available.

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 μC/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, μC/OS-III also keeps track of tasks waiting for the semaphore’s availability.

Only tasks are allowed to use semaphores when semaphores are used for sharing resources; ISRs are not allowed.

A semaphore is a kernel object defined by the OS_SEM data type, which is defined by the structure os_sem (see os.h). The application can have any number of semaphores (limited only by the amount of RAM available).

There are a number of operations the application is able to perform on semaphores, summarized in the table below. In this chapter, only the three most used functions are discussed: OSSemCreate(), OSSemPend(), and OSSemPost(). The other functions are described in Appendix A, “µCµC/OS-III API Reference” on page 431. Also note that Reference. When semaphores are used for sharing resources, every semaphore function is callable must be called from a task , but only OSSemPost() can be called by an ISR. Anchor10813161081316  

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and never from an ISR. The same limitation does not apply when using semaphores for signaling.

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