semphr. h

sem_t xSemaphoreCreateBinaryStatic( static_sem_s_t *pxSemaphoreBuffer )

Creates a new binary semaphore instance, and returns a handle by which the new semaphore can be referenced.

NOTE: In many usage scenarios it is faster and more memory efficient to use a direct to task notification in place of a binary semaphore! http://www.freertos.org/RTOS-task-notifications.html

Internally, within the FreeRTOS implementation, binary semaphores use a block of memory, in which the semaphore structure is stored. If a binary semaphore is created using sem_binary_create() then the required memory is automatically dynamically allocated inside the sem_binary_create() function. (see http://www.freertos.org/a00111.html). If a binary semaphore is created using xSemaphoreCreateBinaryStatic() then the application writer must provide the memory. xSemaphoreCreateBinaryStatic() therefore allows a binary semaphore to be created without using any dynamic memory allocation.

This type of semaphore can be used for pure synchronisation between tasks or between an interrupt and a task. The semaphore need not be given back once obtained, so one task/interrupt can continuously 'give' the semaphore while another continuously 'takes' the semaphore. For this reason this type of semaphore does not use a priority inheritance mechanism. For an alternative that does use priority inheritance see mutex_create().

Parameters

pxSemaphoreBuffer

Must point to a variable of type static_sem_s_t, which will then be used to hold the semaphore's data structure, removing the need for the memory to be allocated dynamically.

Returns

If the semaphore is created then a handle to the created semaphore is returned. If pxSemaphoreBuffer is NULL then NULL is returned.

Example usage:

sem_t xSemaphore = NULL;
static_sem_s_t xSemaphoreBuffer;

void vATask( void * pvParameters )
{
   // Semaphore cannot be used before a call to sem_binary_create().
   // The semaphore's data structures will be placed in the xSemaphoreBuffer
   // variable, the address of which is passed into the function.  The
   // function's parameter is not NULL, so the function will not attempt any
   // dynamic memory allocation, and therefore the function will not return
   // return NULL.
   xSemaphore = sem_binary_create( &xSemaphoreBuffer );

   // Rest of task code goes here.
}