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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 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.
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 µC/OS-III API 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.
Function Name | Operation |
---|---|
OSSemCreate() | Create a semaphore. |
OSSemDel() | Delete a semaphore. |
OSSemPend() | Wait on a semaphore. |
OSSemPendAbort() | Abort the wait on a semaphore. |
OSSemPost() | Release or signal a semaphore. |
OSSemSet() | Force the semaphore count to a desired value. |