Chapter 2. API Reference
(continued from previous page)
// Rest of task code goes here.
}
Return If the semaphore is created then a handle to the created semaphore is returned. If pxSemaphoreBuffer
is NULL then NULL is returned.
Parameters
• pxStaticSemaphore: Must point to a variable of type StaticSemaphore_t, which will then be
used to hold the semaphore’s data structure, removing the need for the memory to be allocated
dynamically.
xSemaphoreTake(xSemaphore, xBlockTime)
Macro to obtain a semaphore. The semaphore must have previously been created with a call to xSemaphoreCre-
ateBinary(), xSemaphoreCreateMutex() or xSemaphoreCreateCounting().
param xSemaphore A handle to the semaphore being taken - obtained when the semaphore was created.
param xBlockTime The time in ticks to wait for the semaphore to become available. The macro
portTICK_PERIOD_MS can be used to convert this to a real time. A block time of zero can be used to
poll the semaphore. A block time of portMAX_DELAY can be used to block indefinitely (provided IN-
CLUDE_vTaskSuspend is set to 1 in FreeRTOSConfig.h).
Example usage:
SemaphoreHandle_t xSemaphore = NULL;
// A task that creates a semaphore.
void vATask( void * pvParameters )
{
// Create the semaphore to guard a shared resource.
vSemaphoreCreateBinary( xSemaphore );
}
// A task that uses the semaphore.
void vAnotherTask( void * pvParameters )
{
// ... Do other things.
if( xSemaphore != NULL )
{
// See if we can obtain the semaphore. If the semaphore is not available
// wait 10 ticks to see if it becomes free.
if( xSemaphoreTake( xSemaphore, ( TickType_t ) 10 ) == pdTRUE )
{
// We were able to obtain the semaphore and can now access the
// shared resource.
// ...
// We have finished accessing the shared resource. Release the
// semaphore.
xSemaphoreGive( xSemaphore );
}
else
{
// We could not obtain the semaphore and can therefore not access
// the shared resource safely.
}
}
}
Return pdTRUE if the semaphore was obtained. pdFALSE if xBlockTime expired without the semaphore
Espressif Systems 903
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