Sempahores¶

xSemaphoreCreateBinary xQueueGenericCreate( ( UBaseType_t ) 1, semSEMAPHORE_QUEUE_ITEM_LENGTH, queueQUEUE_TYPE_BINARY_SEMAPHORE )¶

semphr.h

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

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 xSemaphoreCreateBinary() then the required memory is automatically dynamically allocated inside the xSemaphoreCreateBinary() 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.

The old vSemaphoreCreateBinary() macro is now deprecated in favour of this xSemaphoreCreateBinary() function. Note that binary semaphores created using the vSemaphoreCreateBinary() macro are created in a state such that the first call to ‘take’ the semaphore would pass, whereas binary semaphores created using xSemaphoreCreateBinary() are created in a state such that the the semaphore must first be ‘given’ before it can be ‘taken’.

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 xSemaphoreCreateMutex().

Example usage:

SemaphoreHandle_t xSemaphore = NULL;

void vATask( void * pvParameters )
{
   // Semaphore cannot be used before a call to xSemaphoreCreateBinary().
   // This is a macro so pass the variable in directly.
   xSemaphore = xSemaphoreCreateBinary();

   if( xSemaphore != NULL )
   {
       // The semaphore was created successfully.
       // The semaphore can now be used.
   }
}

Return: Handle to the created semaphore, or NULL if the memory required to hold the semaphore’s data structures could not be allocated.

xSemaphoreCreateBinaryStatic(pxStaticSemaphore) xQueueGenericCreateStatic( ( UBaseType_t ) 1, semSEMAPHORE_QUEUE_ITEM_LENGTH, NULL, pxStaticSemaphore, queueQUEUE_TYPE_BINARY_SEMAPHORE )¶

semphr.h

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 xSemaphoreCreateBinary() then the required memory is automatically dynamically allocated inside the xSemaphoreCreateBinary() 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 xSemaphoreCreateMutex().

Example usage:

SemaphoreHandle_t xSemaphore = NULL;
StaticSemaphore_t xSemaphoreBuffer;

void vATask( void * pvParameters )
{
   // Semaphore cannot be used before a call to xSemaphoreCreateBinary().
   // 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 = xSemaphoreCreateBinary( &xSemaphoreBuffer );

   // 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

pxSemaphoreBuffer: 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.

xSemaphoreCreateCounting(uxMaxCount, uxInitialCount) xQueueCreateCountingSemaphore( ( uxMaxCount ), ( uxInitialCount ) )¶

semphr.h

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

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

Internally, within the FreeRTOS implementation, counting semaphores use a block of memory, in which the counting semaphore structure is stored. If a counting semaphore is created using xSemaphoreCreateCounting() then the required memory is automatically dynamically allocated inside the xSemaphoreCreateCounting() function. (see http://www.freertos.org/a00111.html). If a counting semaphore is created using xSemaphoreCreateCountingStatic() then the application writer can instead optionally provide the memory that will get used by the counting semaphore. xSemaphoreCreateCountingStatic() therefore allows a counting semaphore to be created without using any dynamic memory allocation.

Counting semaphores are typically used for two things:

1) Counting events.

In this usage scenario an event handler will ‘give’ a semaphore each time an event occurs (incrementing the semaphore count value), and a handler task will ‘take’ a semaphore each time it processes an event (decrementing the semaphore count value). The count value is therefore the difference between the number of events that have occurred and the number that have been processed. In this case it is desirable for the initial count value to be zero.

2) Resource management.

In this usage scenario the count value indicates the number of resources available. To obtain control of a resource a task must first obtain a semaphore - decrementing the semaphore count value. When the count value reaches zero there are no free resources. When a task finishes with the resource it ‘gives’ the semaphore back - incrementing the semaphore count value. In this case it is desirable for the initial count value to be equal to the maximum count value, indicating that all resources are free.

Example usage:

SemaphoreHandle_t xSemaphore;

void vATask( void * pvParameters )
{
SemaphoreHandle_t xSemaphore = NULL;

   // Semaphore cannot be used before a call to xSemaphoreCreateCounting().
   // The max value to which the semaphore can count should be 10, and the
   // initial value assigned to the count should be 0.
   xSemaphore = xSemaphoreCreateCounting( 10, 0 );

   if( xSemaphore != NULL )
   {
       // The semaphore was created successfully.
       // The semaphore can now be used.
   }
}

Return

Handle to the created semaphore. Null if the semaphore could not be created.

Parameters

uxMaxCount: The maximum count value that can be reached. When the semaphore reaches this value it can no longer be ‘given’.
uxInitialCount: The count value assigned to the semaphore when it is created.

xSemaphoreCreateCountingStatic(uxMaxCount, uxInitialCount, pxSemaphoreBuffer) xQueueCreateCountingSemaphoreStatic( ( uxMaxCount ), ( uxInitialCount ), ( pxSemaphoreBuffer ) )¶

semphr.h

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

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

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

Counting semaphores are typically used for two things:

1) Counting events.

In this usage scenario an event handler will ‘give’ a semaphore each time an event occurs (incrementing the semaphore count value), and a handler task will ‘take’ a semaphore each time it processes an event (decrementing the semaphore count value). The count value is therefore the difference between the number of events that have occurred and the number that have been processed. In this case it is desirable for the initial count value to be zero.

2) Resource management.

In this usage scenario the count value indicates the number of resources available. To obtain control of a resource a task must first obtain a semaphore - decrementing the semaphore count value. When the count value reaches zero there are no free resources. When a task finishes with the resource it ‘gives’ the semaphore back - incrementing the semaphore count value. In this case it is desirable for the initial count value to be equal to the maximum count value, indicating that all resources are free.

Example usage:

SemaphoreHandle_t xSemaphore;
StaticSemaphore_t xSemaphoreBuffer;

void vATask( void * pvParameters )
{
SemaphoreHandle_t xSemaphore = NULL;

   // Counting semaphore cannot be used before they have been created.  Create
   // a counting semaphore using xSemaphoreCreateCountingStatic().  The max
   // value to which the semaphore can count is 10, and the initial value
   // assigned to the count will be 0.  The address of xSemaphoreBuffer is
   // passed in and will be used to hold the semaphore structure, so no dynamic
   // memory allocation will be used.
   xSemaphore = xSemaphoreCreateCounting( 10, 0, &xSemaphoreBuffer );

   // No memory allocation was attempted so xSemaphore cannot be NULL, so there
   // is no need to check its value.
}

Return

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

Parameters

uxMaxCount: The maximum count value that can be reached. When the semaphore reaches this value it can no longer be ‘given’.
uxInitialCount: The count value assigned to the semaphore when it is created.
pxSemaphoreBuffer: 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.

xSemaphoreCreateMutex xQueueCreateMutex( queueQUEUE_TYPE_MUTEX )¶

semphr.h

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

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

Mutexes created using this function can be accessed using the xSemaphoreTake() and xSemaphoreGive() macros. The xSemaphoreTakeRecursive() and xSemaphoreGiveRecursive() macros must not be used.

This type of semaphore uses a priority inheritance mechanism so a task ‘taking’ a semaphore MUST ALWAYS ‘give’ the semaphore back once the semaphore it is no longer required.

Mutex type semaphores cannot be used from within interrupt service routines.

See xSemaphoreCreateBinary() for an alternative implementation that can be used for pure synchronisation (where one task or interrupt always ‘gives’ the semaphore and another always ‘takes’ the semaphore) and from within interrupt service routines.

Example usage:

SemaphoreHandle_t xSemaphore;

void vATask( void * pvParameters )
{
   // Semaphore cannot be used before a call to xSemaphoreCreateMutex().
   // This is a macro so pass the variable in directly.
   xSemaphore = xSemaphoreCreateMutex();

   if( xSemaphore != NULL )
   {
       // The semaphore was created successfully.
       // The semaphore can now be used.
   }
}

Return: If the mutex was successfully created then a handle to the created semaphore is returned. If there was not enough heap to allocate the mutex data structures then NULL is returned.

xSemaphoreCreateMutexStatic(pxMutexBuffer) xQueueCreateMutexStatic( queueQUEUE_TYPE_MUTEX, ( pxMutexBuffer ) )¶

semphr.h

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

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

Mutexes created using this function can be accessed using the xSemaphoreTake() and xSemaphoreGive() macros. The xSemaphoreTakeRecursive() and xSemaphoreGiveRecursive() macros must not be used.

This type of semaphore uses a priority inheritance mechanism so a task ‘taking’ a semaphore MUST ALWAYS ‘give’ the semaphore back once the semaphore it is no longer required.

Mutex type semaphores cannot be used from within interrupt service routines.

See xSemaphoreCreateBinary() for an alternative implementation that can be used for pure synchronisation (where one task or interrupt always ‘gives’ the semaphore and another always ‘takes’ the semaphore) and from within interrupt service routines.

Example usage:

SemaphoreHandle_t xSemaphore;
StaticSemaphore_t xMutexBuffer;

void vATask( void * pvParameters )
{
   // A mutex cannot be used before it has been created.  xMutexBuffer is
   // into xSemaphoreCreateMutexStatic() so no dynamic memory allocation is
   // attempted.
   xSemaphore = xSemaphoreCreateMutexStatic( &xMutexBuffer );

   // As no dynamic memory allocation was performed, xSemaphore cannot be NULL,
   // so there is no need to check it.
}

Return

If the mutex was successfully created then a handle to the created mutex is returned. If pxMutexBuffer was NULL then NULL is returned.

Parameters

pxMutexBuffer: Must point to a variable of type StaticSemaphore_t, which will be used to hold the mutex’s data structure, removing the need for the memory to be allocated dynamically.

xSemaphoreCreateRecursiveMutex xQueueCreateMutex( queueQUEUE_TYPE_RECURSIVE_MUTEX )¶

semphr.h

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

Internally, within the FreeRTOS implementation, recursive mutexs use a block of memory, in which the mutex structure is stored. If a recursive mutex is created using xSemaphoreCreateRecursiveMutex() then the required memory is automatically dynamically allocated inside the xSemaphoreCreateRecursiveMutex() function. (see http://www.freertos.org/a00111.html). If a recursive mutex is created using xSemaphoreCreateRecursiveMutexStatic() then the application writer must provide the memory that will get used by the mutex. xSemaphoreCreateRecursiveMutexStatic() therefore allows a recursive mutex to be created without using any dynamic memory allocation.

Mutexes created using this macro can be accessed using the xSemaphoreTakeRecursive() and xSemaphoreGiveRecursive() macros. The xSemaphoreTake() and xSemaphoreGive() macros must not be used.

A mutex used recursively can be ‘taken’ repeatedly by the owner. The mutex doesn’t become available again until the owner has called xSemaphoreGiveRecursive() for each successful ‘take’ request. For example, if a task successfully ‘takes’ the same mutex 5 times then the mutex will not be available to any other task until it has also ‘given’ the mutex back exactly five times.

This type of semaphore uses a priority inheritance mechanism so a task ‘taking’ a semaphore MUST ALWAYS ‘give’ the semaphore back once the semaphore it is no longer required.

Mutex type semaphores cannot be used from within interrupt service routines.

See xSemaphoreCreateBinary() for an alternative implementation that can be used for pure synchronisation (where one task or interrupt always ‘gives’ the semaphore and another always ‘takes’ the semaphore) and from within interrupt service routines.

Example usage:

SemaphoreHandle_t xSemaphore;

void vATask( void * pvParameters )
{
   // Semaphore cannot be used before a call to xSemaphoreCreateMutex().
   // This is a macro so pass the variable in directly.
   xSemaphore = xSemaphoreCreateRecursiveMutex();

   if( xSemaphore != NULL )
   {
       // The semaphore was created successfully.
       // The semaphore can now be used.
   }
}

Return: xSemaphore Handle to the created mutex semaphore. Should be of type SemaphoreHandle_t.

xSemaphoreCreateRecursiveMutexStatic(pxStaticSemaphore) xQueueCreateMutexStatic( queueQUEUE_TYPE_RECURSIVE_MUTEX, pxStaticSemaphore )¶

semphr.h

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

Internally, within the FreeRTOS implementation, recursive mutexs use a block of memory, in which the mutex structure is stored. If a recursive mutex is created using xSemaphoreCreateRecursiveMutex() then the required memory is automatically dynamically allocated inside the xSemaphoreCreateRecursiveMutex() function. (see http://www.freertos.org/a00111.html). If a recursive mutex is created using xSemaphoreCreateRecursiveMutexStatic() then the application writer must provide the memory that will get used by the mutex. xSemaphoreCreateRecursiveMutexStatic() therefore allows a recursive mutex to be created without using any dynamic memory allocation.

Mutexes created using this macro can be accessed using the xSemaphoreTakeRecursive() and xSemaphoreGiveRecursive() macros. The xSemaphoreTake() and xSemaphoreGive() macros must not be used.

A mutex used recursively can be ‘taken’ repeatedly by the owner. The mutex doesn’t become available again until the owner has called xSemaphoreGiveRecursive() for each successful ‘take’ request. For example, if a task successfully ‘takes’ the same mutex 5 times then the mutex will not be available to any other task until it has also ‘given’ the mutex back exactly five times.

This type of semaphore uses a priority inheritance mechanism so a task ‘taking’ a semaphore MUST ALWAYS ‘give’ the semaphore back once the semaphore it is no longer required.

Mutex type semaphores cannot be used from within interrupt service routines.

See xSemaphoreCreateBinary() for an alternative implementation that can be used for pure synchronisation (where one task or interrupt always ‘gives’ the semaphore and another always ‘takes’ the semaphore) and from within interrupt service routines.

Example usage:

SemaphoreHandle_t xSemaphore;
StaticSemaphore_t xMutexBuffer;

void vATask( void * pvParameters )
{
   // A recursive semaphore cannot be used before it is created.  Here a
   // recursive mutex is created using xSemaphoreCreateRecursiveMutexStatic().
   // The address of xMutexBuffer is passed into the function, and will hold
   // the mutexes data structures - so no dynamic memory allocation will be
   // attempted.
   xSemaphore = xSemaphoreCreateRecursiveMutexStatic( &xMutexBuffer );

   // As no dynamic memory allocation was performed, xSemaphore cannot be NULL,
   // so there is no need to check it.
}

Return

If the recursive mutex was successfully created then a handle to the created recursive mutex is returned. If pxMutexBuffer was NULL then NULL is returned.

Parameters

pxMutexBuffer: Must point to a variable of type StaticSemaphore_t, which will then be used to hold the recursive mutex’s data structure, removing the need for the memory to be allocated dynamically.

vSemaphoreDelete(xSemaphore) vQueueDelete( ( QueueHandle_t ) ( xSemaphore ) )¶

semphr.h

Delete a semaphore. This function must be used with care. For example, do not delete a mutex type semaphore if the mutex is held by a task.

Parameters

xSemaphore: A handle to the semaphore to be deleted.

xSemaphoreGetMutexHolder(xSemaphore) xQueueGetMutexHolder( ( xSemaphore ) )¶

semphr.h

If xMutex is indeed a mutex type semaphore, return the current mutex holder. If xMutex is not a mutex type semaphore, or the mutex is available (not held by a task), return NULL.

Note: This is a good way of determining if the calling task is the mutex holder, but not a good way of determining the identity of the mutex holder as the holder may change between the function exiting and the returned value being tested.

xSemaphoreTake(xSemaphore, xBlockTime) xQueueGenericReceive( ( QueueHandle_t ) ( xSemaphore ), NULL, ( xBlockTime ), pdFALSE )¶

semphr.h

Macro to obtain a semaphore. The semaphore must have previously been created with a call to xSemaphoreCreateBinary(), xSemaphoreCreateMutex() or xSemaphoreCreateCounting().

Example usage:

SemaphoreHandle_t xSemaphore = NULL;

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

// 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 becoming available.

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).

xSemaphoreTakeFromISR(xSemaphore, pxHigherPriorityTaskWoken) xQueueReceiveFromISR( ( QueueHandle_t ) ( xSemaphore ), NULL, ( pxHigherPriorityTaskWoken ) )¶

semphr.h

Macro to take a semaphore from an ISR. The semaphore must have previously been created with a call to xSemaphoreCreateBinary() or xSemaphoreCreateCounting().

Mutex type semaphores (those created using a call to xSemaphoreCreateMutex()) must not be used with this macro.

This macro can be used from an ISR, however taking a semaphore from an ISR is not a common operation. It is likely to only be useful when taking a counting semaphore when an interrupt is obtaining an object from a resource pool (when the semaphore count indicates the number of resources available).

Return

pdTRUE if the semaphore was successfully taken, otherwise pdFALSE

Parameters

xSemaphore: A handle to the semaphore being taken. This is the handle returned when the semaphore was created.
pxHigherPriorityTaskWoken: xSemaphoreTakeFromISR() will set *pxHigherPriorityTaskWoken to pdTRUE if taking the semaphore caused a task to unblock, and the unblocked task has a priority higher than the currently running task. If xSemaphoreTakeFromISR() sets this value to pdTRUE then a context switch should be requested before the interrupt is exited.

xSemaphoreTakeRecursive(xMutex, xBlockTime) xQueueTakeMutexRecursive( ( xMutex ), ( xBlockTime ) )¶

semphr.h

Macro to recursively obtain, or ‘take’, a mutex type semaphore. The mutex must have previously been created using a call to xSemaphoreCreateRecursiveMutex();

configUSE_RECURSIVE_MUTEXES must be set to 1 in FreeRTOSConfig.h for this macro to be available.

This macro must not be used on mutexes created using xSemaphoreCreateMutex().

A mutex used recursively can be ‘taken’ repeatedly by the owner. The mutex doesn’t become available again until the owner has called xSemaphoreGiveRecursive() for each successful ‘take’ request. For example, if a task successfully ‘takes’ the same mutex 5 times then the mutex will not be available to any other task until it has also ‘given’ the mutex back exactly five times.

Example usage:

SemaphoreHandle_t xMutex = NULL;

// A task that creates a mutex.
void vATask( void * pvParameters )
{
   // Create the mutex to guard a shared resource.
   xMutex = xSemaphoreCreateRecursiveMutex();
}

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

   if( xMutex != NULL )
   {
       // See if we can obtain the mutex.  If the mutex is not available
       // wait 10 ticks to see if it becomes free.
       if( xSemaphoreTakeRecursive( xSemaphore, ( TickType_t ) 10 ) == pdTRUE )
       {
           // We were able to obtain the mutex and can now access the
           // shared resource.

           // ...
           // For some reason due to the nature of the code further calls to
        // xSemaphoreTakeRecursive() are made on the same mutex.  In real
        // code these would not be just sequential calls as this would make
        // no sense.  Instead the calls are likely to be buried inside
        // a more complex call structure.
           xSemaphoreTakeRecursive( xMutex, ( TickType_t ) 10 );
           xSemaphoreTakeRecursive( xMutex, ( TickType_t ) 10 );

           // The mutex has now been 'taken' three times, so will not be
        // available to another task until it has also been given back
        // three times.  Again it is unlikely that real code would have
        // these calls sequentially, but instead buried in a more complex
        // call structure.  This is just for illustrative purposes.
           xSemaphoreGiveRecursive( xMutex );
        xSemaphoreGiveRecursive( xMutex );
        xSemaphoreGiveRecursive( xMutex );

        // Now the mutex can be taken by other tasks.
       }
       else
       {
           // We could not obtain the mutex and can therefore not access
           // the shared resource safely.
       }
   }
}

Return

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

Parameters

xMutex: A handle to the mutex being obtained. This is the handle returned by xSemaphoreCreateRecursiveMutex();
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. If the task already owns the semaphore then xSemaphoreTakeRecursive() will return immediately no matter what the value of xBlockTime.

xSemaphoreGive(xSemaphore) xQueueGenericSend( ( QueueHandle_t ) ( xSemaphore ), NULL, semGIVE_BLOCK_TIME, queueSEND_TO_BACK )¶

semphr.h

Macro to release a semaphore. The semaphore must have previously been created with a call to xSemaphoreCreateBinary(), xSemaphoreCreateMutex() or xSemaphoreCreateCounting(). and obtained using sSemaphoreTake().

This macro must not be used from an ISR. See xSemaphoreGiveFromISR () for an alternative which can be used from an ISR.

This macro must also not be used on semaphores created using xSemaphoreCreateRecursiveMutex().

Example usage:

SemaphoreHandle_t xSemaphore = NULL;

void vATask( void * pvParameters )
{
   // Create the semaphore to guard a shared resource.
   xSemaphore = vSemaphoreCreateBinary();

   if( xSemaphore != NULL )
   {
       if( xSemaphoreGive( xSemaphore ) != pdTRUE )
       {
           // We would expect this call to fail because we cannot give
           // a semaphore without first "taking" it!
       }

       // Obtain the semaphore - don't block if the semaphore is not
       // immediately available.
       if( xSemaphoreTake( xSemaphore, ( TickType_t ) 0 ) )
       {
           // We now have the semaphore and can access the shared resource.

           // ...

           // We have finished accessing the shared resource so can free the
           // semaphore.
           if( xSemaphoreGive( xSemaphore ) != pdTRUE )
           {
               // We would not expect this call to fail because we must have
               // obtained the semaphore to get here.
           }
       }
   }
}

Return

pdTRUE if the semaphore was released. pdFALSE if an error occurred. Semaphores are implemented using queues. An error can occur if there is no space on the queue to post a message - indicating that the semaphore was not first obtained correctly.

Parameters

xSemaphore: A handle to the semaphore being released. This is the handle returned when the semaphore was created.

xSemaphoreGiveRecursive(xMutex) xQueueGiveMutexRecursive( ( xMutex ) )¶

semphr.h

Macro to recursively release, or ‘give’, a mutex type semaphore. The mutex must have previously been created using a call to xSemaphoreCreateRecursiveMutex();

configUSE_RECURSIVE_MUTEXES must be set to 1 in FreeRTOSConfig.h for this macro to be available.

This macro must not be used on mutexes created using xSemaphoreCreateMutex().

A mutex used recursively can be ‘taken’ repeatedly by the owner. The mutex doesn’t become available again until the owner has called xSemaphoreGiveRecursive() for each successful ‘take’ request. For example, if a task successfully ‘takes’ the same mutex 5 times then the mutex will not be available to any other task until it has also ‘given’ the mutex back exactly five times.

Example usage:

SemaphoreHandle_t xMutex = NULL;

// A task that creates a mutex.
void vATask( void * pvParameters )
{
   // Create the mutex to guard a shared resource.
   xMutex = xSemaphoreCreateRecursiveMutex();
}

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

   if( xMutex != NULL )
   {
       // See if we can obtain the mutex.  If the mutex is not available
       // wait 10 ticks to see if it becomes free.
       if( xSemaphoreTakeRecursive( xMutex, ( TickType_t ) 10 ) == pdTRUE )
       {
           // We were able to obtain the mutex and can now access the
           // shared resource.

           // ...
           // For some reason due to the nature of the code further calls to
        // xSemaphoreTakeRecursive() are made on the same mutex.  In real
        // code these would not be just sequential calls as this would make
        // no sense.  Instead the calls are likely to be buried inside
        // a more complex call structure.
           xSemaphoreTakeRecursive( xMutex, ( TickType_t ) 10 );
           xSemaphoreTakeRecursive( xMutex, ( TickType_t ) 10 );

           // The mutex has now been 'taken' three times, so will not be
        // available to another task until it has also been given back
        // three times.  Again it is unlikely that real code would have
        // these calls sequentially, it would be more likely that the calls
        // to xSemaphoreGiveRecursive() would be called as a call stack
        // unwound.  This is just for demonstrative purposes.
           xSemaphoreGiveRecursive( xMutex );
        xSemaphoreGiveRecursive( xMutex );
        xSemaphoreGiveRecursive( xMutex );

        // Now the mutex can be taken by other tasks.
       }
       else
       {
           // We could not obtain the mutex and can therefore not access
           // the shared resource safely.
       }
   }
}

Return

pdTRUE if the semaphore was given.

Parameters

xMutex: A handle to the mutex being released, or ‘given’. This is the handle returned by xSemaphoreCreateMutex();

xSemaphoreGiveFromISR(xSemaphore, pxHigherPriorityTaskWoken) xQueueGiveFromISR( ( QueueHandle_t ) ( xSemaphore ), ( pxHigherPriorityTaskWoken ) )¶

semphr.h

Macro to release a semaphore. The semaphore must have previously been created with a call to xSemaphoreCreateBinary() or xSemaphoreCreateCounting().

Mutex type semaphores (those created using a call to xSemaphoreCreateMutex()) must not be used with this macro.

This macro can be used from an ISR.

Example usage:

\#define LONG_TIME 0xffff
\#define TICKS_TO_WAIT  10
SemaphoreHandle_t xSemaphore = NULL;

// Repetitive task.
void vATask( void * pvParameters )
{
   for( ;; )
   {
       // We want this task to run every 10 ticks of a timer.  The semaphore
       // was created before this task was started.

       // Block waiting for the semaphore to become available.
       if( xSemaphoreTake( xSemaphore, LONG_TIME ) == pdTRUE )
       {
           // It is time to execute.

           // ...

           // We have finished our task.  Return to the top of the loop where
           // we will block on the semaphore until it is time to execute
           // again.  Note when using the semaphore for synchronisation with an
        // ISR in this manner there is no need to 'give' the semaphore back.
       }
   }
}

// Timer ISR
void vTimerISR( void * pvParameters )
{
static uint8_t ucLocalTickCount = 0;
static BaseType_t xHigherPriorityTaskWoken;

   // A timer tick has occurred.

   // ... Do other time functions.

   // Is it time for vATask () to run?
   xHigherPriorityTaskWoken = pdFALSE;
   ucLocalTickCount++;
   if( ucLocalTickCount >= TICKS_TO_WAIT )
   {
       // Unblock the task by releasing the semaphore.
       xSemaphoreGiveFromISR( xSemaphore, &xHigherPriorityTaskWoken );

       // Reset the count so we release the semaphore again in 10 ticks time.
       ucLocalTickCount = 0;
   }

   if( xHigherPriorityTaskWoken != pdFALSE )
   {
       // We can force a context switch here.  Context switching from an
       // ISR uses port specific syntax.  Check the demo task for your port
       // to find the syntax required.
   }
}

Return

pdTRUE if the semaphore was successfully given, otherwise errQUEUE_FULL.

Parameters

xSemaphore: A handle to the semaphore being released. This is the handle returned when the semaphore was created.
pxHigherPriorityTaskWoken: xSemaphoreGiveFromISR() will set *pxHigherPriorityTaskWoken to pdTRUE if giving the semaphore caused a task to unblock, and the unblocked task has a priority higher than the currently running task. If xSemaphoreGiveFromISR() sets this value to pdTRUE then a context switch should be requested before the interrupt is exited.

uxSemaphoreGetCount(xSemaphore) uxQueueMessagesWaiting( ( QueueHandle_t ) ( xSemaphore ) )¶

semphr.h

If the semaphore is a counting semaphore then uxSemaphoreGetCount() returns its current count value. If the semaphore is a binary semaphore then uxSemaphoreGetCount() returns 1 if the semaphore is available, and 0 if the semaphore is not available.