xSemaphoreCreateBinary
[Semaphores]
semphr. h
SemaphoreHandle_t xSemaphoreCreateBinary( void );
Creates a binary semaphore, and returns a handle by which the semaphore can be
referenced.
configSUPPORT_DYNAMIC_ALLOCATION
must be set to 1 in FreeRTOSConfig.h, or left undefined (in which case it will
default to 1), for this RTOS API function to be available.
Each binary semaphore require a small amount of RAM that is used to hold the
semaphore's state. If a binary semaphore is created using xSemaphoreCreateBinary()
then the required RAM is automatically allocated from the FreeRTOS heap.
If a binary semaphore is created using xSemaphoreCreateBinaryStatic()
then the RAM is provided by the application writer, which requires an additional
parameter, but allows the RAM to be statically allocated at compile
time. See the Static Vs
Dynamic allocation page for more information.
The semaphore is created in the 'empty' state, meaning the semaphore must first
be given using the xSemaphoreGive() API function
before it can subsequently be taken (obtained) using the xSemaphoreTake() function.
Binary semaphores and mutexes are very similar but
have some subtle differences: Mutexes include a priority inheritance mechanism,
binary semaphores do not. This makes binary semaphores the better choice for
implementing synchronisation (between tasks or between tasks and an interrupt),
and mutexes the better choice for implementing simple mutual exclusion.
A binary semaphore need not be given back once obtained, so task synchronisation
can be implemented by one task/interrupt continuously 'giving' the semaphore
while another continuously 'takes' the semaphore. This is demonstrated by
the sample code on the xSemaphoreGiveFromISR() documentation page.
Note the same functionality can often be achieved in a more efficient way
using a direct to task notification.
The priority of a task that 'takes' a mutex can potentially be raised if another task of higher
priority attempts to obtain the same mutex. The task that owns the mutex 'inherits' the priority
of the task attempting to 'take' the same mutex. This means the mutex must always be 'given' back -
otherwise the higher priority task will never be able to obtain the mutex, and the lower priority
task will never 'disinherit' the priority. An example of a mutex being used to implement mutual
exclusion is provided on the xSemaphoreTake() documentation page.
Both mutex and binary semaphores are referenced by variables of type SemaphoreHandle_t and can be used
in any task level API function that takes a parameter of that type. Unlike mutexes,
binary semaphores can be used in interrupt service routines.
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Return values:
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NULL
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The semaphore could not be created because there was
insufficient FreeRTOS heap
available.
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Any other value
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The semaphore was created successfully. The returned
value is a handle by which the semaphore can be referenced.
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Example usage:
SemaphoreHandle_t xSemaphore;
void vATask( void * pvParameters )
{
/* Attempt to create a semaphore. */
xSemaphore = xSemaphoreCreateBinary();
if( xSemaphore == NULL )
{
/* There was insufficient FreeRTOS heap available for the semaphore to
be created successfully. */
}
else
{
/* The semaphore can now be used. Its handle is stored in the
xSemahore variable. Calling xSemaphoreTake() on the semaphore here
will fail until the semaphore has first been given. */
}
}
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