SemaphoreHandle_t i2c_lock = NULL;

// A task that creates a semaphore.
void vATask( void * pvParameters )
{
   // Create the semaphore to guard a shared resource.
   vSemaphoreCreateBinary( i2c_lock );
}

// A task that uses the semaphore.
void vAnotherTask( void * pvParameters )
{
   // ... Do other things.

   if( i2c_lock != 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( i2c_lock, ( 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( i2c_lock );
       }
       else
       {
           // We could not obtain the semaphore and can therefore not access
           // the shared resource safely.
       }
   }
}