sem_wait

semphr. h

sem_wait(
                    sem_t xSemaphore,
                    uint32_t xBlockTime
                )

Macro to obtain a semaphore. The semaphore must have previously been created with a call to sem_binary_create(), mutex_create() or sem_create().

Parameters

xSemaphore	A handle to the semaphore being taken - obtained when the semaphore was created.
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 INCLUDE_vTaskSuspend is set to 1 in FreeRTOSConfig.h).

Returns

pdTRUE if the semaphore was obtained. pdFALSE if xBlockTime expired without the semaphore becoming available.

Example usage:

sem_t xSemaphore = NULL;

// A task that creates a semaphore.
void vATask( void * pvParameters )
{
   // Create the semaphore to guard a shared resource.
   xSemaphore = sem_binary_create();
}

// 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( sem_wait( xSemaphore, ( uint32_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.
           sem_post( xSemaphore );
       }
       else
       {
           // We could not obtain the semaphore and can therefore not access
           // the shared resource safely.
       }
   }
}