/*
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***************************************************************************
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***************************************************************************
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*/
#ifndef SEMAPHORE_H
#define SEMAPHORE_H
#ifndef INC_FREERTOS_H
#error "include FreeRTOS.h" must appear in source files before "include semphr.h"
#endif
#include "queue.h"
typedef QueueHandle_t SemaphoreHandle_t;
#define semBINARY_SEMAPHORE_QUEUE_LENGTH ( ( uint8_t ) 1U )
#define semSEMAPHORE_QUEUE_ITEM_LENGTH ( ( uint8_t ) 0U )
#define semGIVE_BLOCK_TIME ( ( TickType_t ) 0U )
/**
* semphr. h
* vSemaphoreCreateBinary( SemaphoreHandle_t xSemaphore )
*
* This old vSemaphoreCreateBinary() macro is now deprecated in favour of the
* 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'.
*
* Macro that implements a semaphore by using the existing queue mechanism.
* The queue length is 1 as this is a binary semaphore. The data size is 0
* as we don't want to actually store any data - we just want to know if the
* queue is empty or full.
*
* 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().
*
* @param xSemaphore Handle to the created semaphore. Should be of type SemaphoreHandle_t.
*
* Example usage:
SemaphoreHandle_t xSemaphore = NULL;
void vATask( void * pvParameters )
{
// Semaphore cannot be used before a call to vSemaphoreCreateBinary ().
// This is a macro so pass the variable in directly.
vSemaphoreCreateBinary( xSemaphore );
if( xSemaphore != NULL )
{
// The semaphore was created successfully.
// The semaphore can now be used.
}
}
* \defgroup vSemaphoreCreateBinary vSemaphoreCreateBinary
* \ingroup Semaphores
*/
#define vSemaphoreCreateBinary( xSemaphore ) \
{ \
( xSemaphore ) = xQueueGenericCreate( ( UBaseType_t ) 1, semSEMAPHORE_QUEUE_ITEM_LENGTH, queueQUEUE_TYPE_BINARY_SEMAPHORE ); \
if( ( xSemaphore ) != NULL ) \
{ \
( void ) xSemaphoreGive( ( xSemaphore ) ); \
} \
}
/**
* semphr. h
* SemaphoreHandle_t xSemaphoreCreateBinary( void )
*
* 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'.
*
* Function that creates a semaphore by using the existing queue mechanism.
* The queue length is 1 as this is a binary semaphore. The data size is 0
* as nothing is actually stored - all that is important is whether the queue is
* empty or full (the binary semaphore is available or not).
*
* 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().
*
* @return Handle to the created semaphore.
*
* Example usage:
SemaphoreHandle_t xSemaphore = NULL;
void vATask( void * pvParameters )
{
// Semaphore cannot be used before a call to vSemaphoreCreateBinary ().
// 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.
}
}
* \defgroup vSemaphoreCreateBinary vSemaphoreCreateBinary
* \ingroup Semaphores
*/
#define xSemaphoreCreateBinary() xQueueGenericCreate( ( UBaseType_t ) 1, semSEMAPHORE_QUEUE_ITEM_LENGTH, queueQUEUE_TYPE_BINARY_SEMAPHORE )
/**
* semphr. h
* xSemaphoreTake(
* SemaphoreHandle_t xSemaphore,
* TickType_t xBlockTime
* )
*
* Macro to obtain a semaphore. The semaphore must have previously been
* created with a call to vSemaphoreCreateBinary(), 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
* INCLUDE_vTaskSuspend is set to 1 in FreeRTOSConfig.h).
*
* @return pdTRUE if the semaphore was obtained. pdFALSE
* if xBlockTime expired without the semaphore becoming available.
*
* 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.
}
}
}
* \defgroup xSemaphoreTake xSemaphoreTake
* \ingroup Semaphores
*/
#define xSemaphoreTake( xSemaphore, xBlockTime ) xQueueGenericReceive( ( QueueHandle_t ) ( xSemaphore ), NULL, ( xBlockTime ), pdFALSE )
/**
* semphr. h
* xSemaphoreTakeRecursive(
* SemaphoreHandle_t xMutex,
* TickType_t xBlockTime
* )
*
* 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.
*
* @param xMutex A handle to the mutex being obtained. This is the
* handle returned by xSemaphoreCreateRecursiveMutex();
*
* @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. If
* the task already owns the semaphore then xSemaphoreTakeRecursive() will
* return immediately no matter what the value of xBlockTime.
*
* @return pdTRUE if the semaphore was obtained. pdFALSE if xBlockTime
* expired without the semaphore becoming available.
*
* 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.
}
}
}
* \defgroup xSemaphoreTakeRecursive xSemaphoreTakeRecursive
* \ingroup Semaphores
*/
#define xSemaphoreTakeRecursive( xMutex, xBlockTime ) xQueueTakeMutexRecursive( ( xMutex ), ( xBlockTime ) )
/*
* xSemaphoreAltTake() is an alternative version of xSemaphoreTake().
*
* The source code that implements the alternative (Alt) API is much
* simpler because it executes everything from within a critical section.
* This is the approach taken by many other RTOSes, but FreeRTOS.org has the
* preferred fully featured API too. The fully featured API has more
* complex code that takes longer to execute, but makes much less use of
* critical sections. Therefore the alternative API sacrifices interrupt
* responsiveness to gain execution speed, whereas the fully featured API
* sacrifices execution speed to ensure better interrupt responsiveness.
*/
#define xSemaphoreAltTake( xSemaphore, xBlockTime ) xQueueAltGenericReceive( ( QueueHandle_t ) ( xSemaphore ), NULL, ( xBlockTime ), pdFALSE )
/**
* semphr. h
* xSemaphoreGive( SemaphoreHandle_t xSemaphore )
*
* Macro to release a semaphore. The semaphore must have previously been
* created with a call to vSemaphoreCreateBinary(), 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().
*
* @param xSemaphore A handle to the semaphore being released. This is the
* handle returned when the semaphore was created.
*
* @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.
*
* Example usage:
SemaphoreHandle_t xSemaphore = NULL;
void vATask( void * pvParameters )
{
// Create the semaphore to guard a shared resource.
vSemaphoreCreateBinary( xSemaphore );
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.
}
}
}
}
* \defgroup xSemaphoreGive xSemaphoreGive
* \ingroup Semaphores
*/
#define xSemaphoreGive( xSemaphore ) xQueueGenericSend( ( QueueHandle_t ) ( xSemaphore ), NULL, semGIVE_BLOCK_TIME, queueSEND_TO_BACK )
/**
* semphr. h
* xSemaphoreGiveRecursive( SemaphoreHandle_t xMutex )
*
* 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.
*
* @param xMutex A handle to the mutex being released, or 'given'. This is the
* handle returned by xSemaphoreCreateMutex();
*
* @return pdTRUE if the semaphore was given.
*
* 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.
}
}
}
* \defgroup xSemaphoreGiveRecursive xSemaphoreGiveRecursive
* \ingroup Semaphores
*/
#define xSemaphoreGiveRecursive( xMutex ) xQueueGiveMutexRecursive( ( xMutex ) )
/*
* xSemaphoreAltGive() is an alternative version of xSemaphoreGive().
*
* The source code that implements the alternative (Alt) API is much
* simpler because it executes everything from within a critical section.
* This is the approach taken by many other RTOSes, but FreeRTOS.org has the
* preferred fully featured API too. The fully featured API has more
* complex code that takes longer to execute, but makes much less use of
* critical sections. Therefore the alternative API sacrifices interrupt
* responsiveness to gain execution speed, whereas the fully featured API
* sacrifices execution speed to ensure better interrupt responsiveness.
*/
#define xSemaphoreAltGive( xSemaphore ) xQueueAltGenericSend( ( QueueHandle_t ) ( xSemaphore ), NULL, semGIVE_BLOCK_TIME, queueSEND_TO_BACK )
/**
* semphr. h
*
xSemaphoreGiveFromISR(
SemaphoreHandle_t xSemaphore,
BaseType_t *pxHigherPriorityTaskWoken
)
*
* Macro to release a semaphore. The semaphore must have previously been
* created with a call to vSemaphoreCreateBinary() 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.
*
* @param xSemaphore A handle to the semaphore being released. This is the
* handle returned when the semaphore was created.
*
* @param 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.
*
* @return pdTRUE if the semaphore was successfully given, otherwise errQUEUE_FULL.
*
* 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.
}
}
* \defgroup xSemaphoreGiveFromISR xSemaphoreGiveFromISR
* \ingroup Semaphores
*/
#define xSemaphoreGiveFromISR( xSemaphore, pxHigherPriorityTaskWoken ) xQueueGenericSendFromISR( ( QueueHandle_t ) ( xSemaphore ), NULL, ( pxHigherPriorityTaskWoken ), queueSEND_TO_BACK )
/**
* semphr. h
*
xSemaphoreTakeFromISR(
SemaphoreHandle_t xSemaphore,
BaseType_t *pxHigherPriorityTaskWoken
)
*
* Macro to take a semaphore from an ISR. The semaphore must have
* previously been created with a call to vSemaphoreCreateBinary() 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).
*
* @param xSemaphore A handle to the semaphore being taken. This is the
* handle returned when the semaphore was created.
*
* @param 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.
*
* @return pdTRUE if the semaphore was successfully taken, otherwise
* pdFALSE
*/
#define xSemaphoreTakeFromISR( xSemaphore, pxHigherPriorityTaskWoken ) xQueueReceiveFromISR( ( QueueHandle_t ) ( xSemaphore ), NULL, ( pxHigherPriorityTaskWoken ) )
/**
* semphr. h
* SemaphoreHandle_t xSemaphoreCreateMutex( void )
*
* Macro that implements a mutex semaphore by using the existing queue
* mechanism.
*
* Mutexes created using this macro can be accessed using the xSemaphoreTake()
* and xSemaphoreGive() macros. The xSemaphoreTakeRecursive() and
* xSemaphoreGiveRecursive() macros should 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 vSemaphoreCreateBinary() 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.
*
* @return xSemaphore Handle to the created mutex semaphore. Should be of type
* SemaphoreHandle_t.
*
* 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.
}
}
* \defgroup vSemaphoreCreateMutex vSemaphoreCreateMutex
* \ingroup Semaphores
*/
#define xSemaphoreCreateMutex() xQueueCreateMutex( queueQUEUE_TYPE_MUTEX )
/**
* semphr. h
* SemaphoreHandle_t xSemaphoreCreateRecursiveMutex( void )
*
* Macro that implements a recursive mutex by using the existing queue
* mechanism.
*
* Mutexes created using this macro can be accessed using the
* xSemaphoreTakeRecursive() and xSemaphoreGiveRecursive() macros. The
* xSemaphoreTake() and xSemaphoreGive() macros should 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 vSemaphoreCreateBinary() 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.
*
* @return xSemaphore Handle to the created mutex semaphore. Should be of type
* SemaphoreHandle_t.
*
* 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.
}
}
* \defgroup vSemaphoreCreateMutex vSemaphoreCreateMutex
* \ingroup Semaphores
*/
#define xSemaphoreCreateRecursiveMutex() xQueueCreateMutex( queueQUEUE_TYPE_RECURSIVE_MUTEX )
/**
* semphr. h
* SemaphoreHandle_t xSemaphoreCreateCounting( UBaseType_t uxMaxCount, UBaseType_t uxInitialCount )
*
* Macro that creates a counting semaphore by using the existing
* queue mechanism.
*
* 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.
*
* @param uxMaxCount The maximum count value that can be reached. When the
* semaphore reaches this value it can no longer be 'given'.
*
* @param uxInitialCount The count value assigned to the semaphore when it is
* created.
*
* @return Handle to the created semaphore. Null if the semaphore could not be
* created.
*
* 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.
}
}
* \defgroup xSemaphoreCreateCounting xSemaphoreCreateCounting
* \ingroup Semaphores
*/
#define xSemaphoreCreateCounting( uxMaxCount, uxInitialCount ) xQueueCreateCountingSemaphore( ( uxMaxCount ), ( uxInitialCount ) )
/**
* semphr. h
* void vSemaphoreDelete( SemaphoreHandle_t xSemaphore );
*
* 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.
*
* @param xSemaphore A handle to the semaphore to be deleted.
*
* \defgroup vSemaphoreDelete vSemaphoreDelete
* \ingroup Semaphores
*/
#define vSemaphoreDelete( xSemaphore ) vQueueDelete( ( QueueHandle_t ) ( xSemaphore ) )
/**
* semphr.h
* TaskHandle_t xSemaphoreGetMutexHolder( SemaphoreHandle_t xMutex );
*
* 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.
*/
#define xSemaphoreGetMutexHolder( xSemaphore ) xQueueGetMutexHolder( ( xSemaphore ) )
#endif /* SEMAPHORE_H */