IPC@CHIP Documentation - SC12 @CHIP-RTOS V1.10

Semaphore Services
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Semaphores are used to guarantee a task mutually exclusive access to a critical resource. Semaphores
synchronize asynchronous occurring activities. They are an essential part of a multitasking system. A
good description of multitasking systems and semaphores is available in the book "Operating systems"
from Andrew Tanenbaum.
The @CHIP-RTOS API provides two types of semaphores:
❍ A counting semaphore is a semaphore with an associated counter, which can be incremented
(signal) and decremented (wait). The resource controlled by the semaphore is free (available)
when the counter is greater than 0.
❍ A resource semaphore is a counting semaphore with a maximum of count of one. It can be
used to provide mutually exclusive access to a single resource. A resource semaphore is also
called a binary semaphore. It differs from a counting semaphore in one significant feature: The
resource ownership is tied to a specific task. No other task except the task owning the resource
is allowed to signal the associated semaphore to release the resource.
The counting and resource semaphores provide automatic timeout. Tasks can specify the maximum time
for waiting on a semaphore. The tasks wait in FIFO order for a resource. A semaphore is created with
RTX_CREATE_SEMservice call. The @CHIP-RTOS needs a unique four byte semaphore name and on
success returns a new semaphore ID (or handle) to the caller. This handle is needed for the other
semaphore services.
Using a counting semaphore:
A counting semaphore is created by specifying an initial count greater or equal to zero in the call to
RTX_CREATE_SEM. If a semaphore is initialized with a value n, it can be used to control access to n
resources, e.g. a counting semaphore with the initial value three assures that no more than three tasks
can own a resource at any one time. Access to a resource controlled by a counting semaphore is
acquired with a call to RTX_WAIT_SEMor RTX_GET_SEM. If the resource is available the @CHIP-
RTOS gives it to the task immediately. When the task is finished using the resource, it signals its release
by calling RTX_SIGNAL_SEM.
Using a resource semaphore:
A resource semaphore is created by specifying an initial count of -1 in the call of
RTX_CREATE_SEM. The @CHIP-RTOS creates a resource semaphore and automatically gives it an
initial value of one indicating that the resource is free. A resource is reserved by calling
RTX_RESERVE_RESwith the semaphore ID returned by RTX_CREATE_SEM. The resource is released
with a call of RTX_RELEASE_SEM.
Semaphore Services:
RTX_CREATE_SEM Create a semaphore
RTX_DELETE_SEM Delete a semaphore
RTX_FREE_RES Free a resource semaphore
RTX_GET_SEM Get use of a counting semaphore(no wait)
RTX_RELEASE_SEM Release a resource semaphore
RTX_RESERVE_RES Reserve a resource semaphore
RTX_SIGNAL_SEM Signal a counting semaphore
RTX_WAIT_SEM Wait on a counting semaphore (optional timeout)
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