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c-user: Clarify simple binary semaphore release

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Close #2725.
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Sebastian Huber 2017-01-23 13:44:43 +01:00
parent 7b52e8f0f6
commit 31157bcd47
1 changed files with 11 additions and 9 deletions
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c-user/semaphore_manager.rst
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@ -43,15 +43,17 @@ only with the ``rtems_semaphore_create``, ``rtems_semaphore_obtain``, and
counting semaphores. A binary semaphore is restricted to values of zero or one, counting semaphores. A binary semaphore is restricted to values of zero or one,
while a counting semaphore can assume any non-negative integer value. while a counting semaphore can assume any non-negative integer value.
A binary semaphore can be used to control access to a single resource. In A binary semaphore (not a simple binary semaphore) can be used to control
particular, it can be used to enforce mutual exclusion for a critical section access to a single resource. In particular, it can be used to enforce mutual
in user code. In this instance, the semaphore would be created with an initial exclusion for a critical section in user code (mutex). In this instance, the
count of one to indicate that no task is executing the critical section of semaphore would be created with an initial count of one to indicate that no
code. Upon entry to the critical section, a task must issue the task is executing the critical section of code. Upon entry to the critical
``rtems_semaphore_obtain`` directive to prevent other tasks from entering the section, a task must issue the ``rtems_semaphore_obtain`` directive to prevent
critical section. Upon exit from the critical section, the task must issue the other tasks from entering the critical section. Upon exit from the critical
section, the task that obtained the binary semaphore must issue the
``rtems_semaphore_release`` directive to allow another task to execute the ``rtems_semaphore_release`` directive to allow another task to execute the
critical section. critical section. A binary semaphore must be released by the task that
obtained it.
A counting semaphore can be used to control access to a pool of two or more A counting semaphore can be used to control access to a pool of two or more
resources. For example, access to three printers could be administered by a resources. For example, access to three printers could be administered by a
@ -367,7 +369,7 @@ The ``rtems_semaphore_release`` directive is used to release the specified
semaphore. A simplified version of the ``rtems_semaphore_release`` directive semaphore. A simplified version of the ``rtems_semaphore_release`` directive
can be described as follows: can be described as follows:
If there sre no tasks are waiting on this semaphore then increment the If there are no tasks are waiting on this semaphore then increment the
semaphore's count else assign semaphore to a waiting task and return semaphore's count else assign semaphore to a waiting task and return
SUCCESSFUL. SUCCESSFUL.