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712 lines
28 KiB
C
712 lines
28 KiB
C
/*
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FreeRTOS V6.0.4 - Copyright (C) 2010 Real Time Engineers Ltd.
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***************************************************************************
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* *
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* If you are: *
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* *
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* + New to FreeRTOS, *
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* + Wanting to learn FreeRTOS or multitasking in general quickly *
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* + Looking for basic training, *
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* + Wanting to improve your FreeRTOS skills and productivity *
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* *
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* then take a look at the FreeRTOS eBook *
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* *
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* "Using the FreeRTOS Real Time Kernel - a Practical Guide" *
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* http://www.FreeRTOS.org/Documentation *
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* *
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* A pdf reference manual is also available. Both are usually delivered *
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* to your inbox within 20 minutes to two hours when purchased between 8am *
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* and 8pm GMT (although please allow up to 24 hours in case of *
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* exceptional circumstances). Thank you for your support! *
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* *
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***************************************************************************
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This file is part of the FreeRTOS distribution.
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FreeRTOS is free software; you can redistribute it and/or modify it under
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the terms of the GNU General Public License (version 2) as published by the
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Free Software Foundation AND MODIFIED BY the FreeRTOS exception.
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***NOTE*** The exception to the GPL is included to allow you to distribute
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a combined work that includes FreeRTOS without being obliged to provide the
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source code for proprietary components outside of the FreeRTOS kernel.
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FreeRTOS is distributed in the hope that it will be useful, but WITHOUT
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ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
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FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for
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more details. You should have received a copy of the GNU General Public
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License and the FreeRTOS license exception along with FreeRTOS; if not it
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can be viewed here: http://www.freertos.org/a00114.html and also obtained
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by writing to Richard Barry, contact details for whom are available on the
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FreeRTOS WEB site.
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1 tab == 4 spaces!
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http://www.FreeRTOS.org - Documentation, latest information, license and
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contact details.
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http://www.SafeRTOS.com - A version that is certified for use in safety
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critical systems.
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http://www.OpenRTOS.com - Commercial support, development, porting,
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licensing and training services.
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*/
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#ifndef INC_FREERTOS_H
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#error "#include FreeRTOS.h" must appear in source files before "#include semphr.h"
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#endif
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#ifndef SEMAPHORE_H
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#define SEMAPHORE_H
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#include "queue.h"
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typedef xQueueHandle xSemaphoreHandle;
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#define semBINARY_SEMAPHORE_QUEUE_LENGTH ( ( unsigned char ) 1 )
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#define semSEMAPHORE_QUEUE_ITEM_LENGTH ( ( unsigned char ) 0 )
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#define semGIVE_BLOCK_TIME ( ( portTickType ) 0 )
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/**
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* semphr. h
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* <pre>vSemaphoreCreateBinary( xSemaphoreHandle xSemaphore )</pre>
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*
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* <i>Macro</i> that implements a semaphore by using the existing queue mechanism.
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* The queue length is 1 as this is a binary semaphore. The data size is 0
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* as we don't want to actually store any data - we just want to know if the
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* queue is empty or full.
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*
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* This type of semaphore can be used for pure synchronisation between tasks or
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* between an interrupt and a task. The semaphore need not be given back once
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* obtained, so one task/interrupt can continuously 'give' the semaphore while
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* another continuously 'takes' the semaphore. For this reason this type of
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* semaphore does not use a priority inheritance mechanism. For an alternative
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* that does use priority inheritance see xSemaphoreCreateMutex().
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*
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* @param xSemaphore Handle to the created semaphore. Should be of type xSemaphoreHandle.
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*
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* Example usage:
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<pre>
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xSemaphoreHandle xSemaphore;
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void vATask( void * pvParameters )
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{
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// Semaphore cannot be used before a call to vSemaphoreCreateBinary ().
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// This is a macro so pass the variable in directly.
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vSemaphoreCreateBinary( xSemaphore );
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if( xSemaphore != NULL )
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{
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// The semaphore was created successfully.
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// The semaphore can now be used.
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}
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}
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</pre>
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* \defgroup vSemaphoreCreateBinary vSemaphoreCreateBinary
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* \ingroup Semaphores
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*/
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#define vSemaphoreCreateBinary( xSemaphore ) { \
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xSemaphore = xQueueCreate( ( unsigned portBASE_TYPE ) 1, semSEMAPHORE_QUEUE_ITEM_LENGTH ); \
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if( xSemaphore != NULL ) \
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{ \
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xSemaphoreGive( xSemaphore ); \
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} \
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}
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/**
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* semphr. h
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* <pre>xSemaphoreTake(
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* xSemaphoreHandle xSemaphore,
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* portTickType xBlockTime
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* )</pre>
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*
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* <i>Macro</i> to obtain a semaphore. The semaphore must have previously been
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* created with a call to vSemaphoreCreateBinary(), xSemaphoreCreateMutex() or
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* xSemaphoreCreateCounting().
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*
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* @param xSemaphore A handle to the semaphore being taken - obtained when
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* the semaphore was created.
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*
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* @param xBlockTime The time in ticks to wait for the semaphore to become
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* available. The macro portTICK_RATE_MS can be used to convert this to a
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* real time. A block time of zero can be used to poll the semaphore. A block
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* time of portMAX_DELAY can be used to block indefinitely (provided
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* INCLUDE_vTaskSuspend is set to 1 in FreeRTOSConfig.h).
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*
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* @return pdTRUE if the semaphore was obtained. pdFALSE
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* if xBlockTime expired without the semaphore becoming available.
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*
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* Example usage:
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<pre>
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xSemaphoreHandle xSemaphore = NULL;
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// A task that creates a semaphore.
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void vATask( void * pvParameters )
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{
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// Create the semaphore to guard a shared resource.
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vSemaphoreCreateBinary( xSemaphore );
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}
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// A task that uses the semaphore.
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void vAnotherTask( void * pvParameters )
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{
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// ... Do other things.
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if( xSemaphore != NULL )
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{
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// See if we can obtain the semaphore. If the semaphore is not available
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// wait 10 ticks to see if it becomes free.
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if( xSemaphoreTake( xSemaphore, ( portTickType ) 10 ) == pdTRUE )
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{
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// We were able to obtain the semaphore and can now access the
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// shared resource.
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// ...
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// We have finished accessing the shared resource. Release the
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// semaphore.
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xSemaphoreGive( xSemaphore );
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}
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else
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{
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// We could not obtain the semaphore and can therefore not access
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// the shared resource safely.
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}
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}
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}
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</pre>
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* \defgroup xSemaphoreTake xSemaphoreTake
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* \ingroup Semaphores
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*/
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#define xSemaphoreTake( xSemaphore, xBlockTime ) xQueueGenericReceive( ( xQueueHandle ) xSemaphore, NULL, xBlockTime, pdFALSE )
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/**
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* semphr. h
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* xSemaphoreTakeRecursive(
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* xSemaphoreHandle xMutex,
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* portTickType xBlockTime
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* )
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*
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* <i>Macro</i> to recursively obtain, or 'take', a mutex type semaphore.
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* The mutex must have previously been created using a call to
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* xSemaphoreCreateRecursiveMutex();
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*
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* configUSE_RECURSIVE_MUTEXES must be set to 1 in FreeRTOSConfig.h for this
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* macro to be available.
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*
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* This macro must not be used on mutexes created using xSemaphoreCreateMutex().
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*
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* A mutex used recursively can be 'taken' repeatedly by the owner. The mutex
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* doesn't become available again until the owner has called
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* xSemaphoreGiveRecursive() for each successful 'take' request. For example,
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* if a task successfully 'takes' the same mutex 5 times then the mutex will
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* not be available to any other task until it has also 'given' the mutex back
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* exactly five times.
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*
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* @param xMutex A handle to the mutex being obtained. This is the
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* handle returned by xSemaphoreCreateRecursiveMutex();
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*
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* @param xBlockTime The time in ticks to wait for the semaphore to become
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* available. The macro portTICK_RATE_MS can be used to convert this to a
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* real time. A block time of zero can be used to poll the semaphore. If
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* the task already owns the semaphore then xSemaphoreTakeRecursive() will
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* return immediately no matter what the value of xBlockTime.
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*
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* @return pdTRUE if the semaphore was obtained. pdFALSE if xBlockTime
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* expired without the semaphore becoming available.
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*
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* Example usage:
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<pre>
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xSemaphoreHandle xMutex = NULL;
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// A task that creates a mutex.
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void vATask( void * pvParameters )
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{
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// Create the mutex to guard a shared resource.
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xMutex = xSemaphoreCreateRecursiveMutex();
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}
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// A task that uses the mutex.
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void vAnotherTask( void * pvParameters )
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{
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// ... Do other things.
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if( xMutex != NULL )
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{
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// See if we can obtain the mutex. If the mutex is not available
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// wait 10 ticks to see if it becomes free.
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if( xSemaphoreTakeRecursive( xSemaphore, ( portTickType ) 10 ) == pdTRUE )
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{
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// We were able to obtain the mutex and can now access the
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// shared resource.
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// ...
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// For some reason due to the nature of the code further calls to
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// xSemaphoreTakeRecursive() are made on the same mutex. In real
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// code these would not be just sequential calls as this would make
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// no sense. Instead the calls are likely to be buried inside
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// a more complex call structure.
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xSemaphoreTakeRecursive( xMutex, ( portTickType ) 10 );
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xSemaphoreTakeRecursive( xMutex, ( portTickType ) 10 );
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// The mutex has now been 'taken' three times, so will not be
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// available to another task until it has also been given back
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// three times. Again it is unlikely that real code would have
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// these calls sequentially, but instead buried in a more complex
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// call structure. This is just for illustrative purposes.
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xSemaphoreGiveRecursive( xMutex );
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xSemaphoreGiveRecursive( xMutex );
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xSemaphoreGiveRecursive( xMutex );
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// Now the mutex can be taken by other tasks.
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}
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else
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{
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// We could not obtain the mutex and can therefore not access
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// the shared resource safely.
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}
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}
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}
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</pre>
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* \defgroup xSemaphoreTakeRecursive xSemaphoreTakeRecursive
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* \ingroup Semaphores
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*/
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#define xSemaphoreTakeRecursive( xMutex, xBlockTime ) xQueueTakeMutexRecursive( xMutex, xBlockTime )
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/*
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* xSemaphoreAltTake() is an alternative version of xSemaphoreTake().
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*
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* The source code that implements the alternative (Alt) API is much
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* simpler because it executes everything from within a critical section.
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* This is the approach taken by many other RTOSes, but FreeRTOS.org has the
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* preferred fully featured API too. The fully featured API has more
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* complex code that takes longer to execute, but makes much less use of
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* critical sections. Therefore the alternative API sacrifices interrupt
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* responsiveness to gain execution speed, whereas the fully featured API
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* sacrifices execution speed to ensure better interrupt responsiveness.
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*/
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#define xSemaphoreAltTake( xSemaphore, xBlockTime ) xQueueAltGenericReceive( ( xQueueHandle ) xSemaphore, NULL, xBlockTime, pdFALSE )
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/**
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* semphr. h
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* <pre>xSemaphoreGive( xSemaphoreHandle xSemaphore )</pre>
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*
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* <i>Macro</i> to release a semaphore. The semaphore must have previously been
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* created with a call to vSemaphoreCreateBinary(), xSemaphoreCreateMutex() or
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* xSemaphoreCreateCounting(). and obtained using sSemaphoreTake().
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*
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* This macro must not be used from an ISR. See xSemaphoreGiveFromISR () for
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* an alternative which can be used from an ISR.
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*
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* This macro must also not be used on semaphores created using
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* xSemaphoreCreateRecursiveMutex().
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*
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* @param xSemaphore A handle to the semaphore being released. This is the
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* handle returned when the semaphore was created.
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*
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* @return pdTRUE if the semaphore was released. pdFALSE if an error occurred.
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* Semaphores are implemented using queues. An error can occur if there is
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* no space on the queue to post a message - indicating that the
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* semaphore was not first obtained correctly.
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*
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* Example usage:
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<pre>
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xSemaphoreHandle xSemaphore = NULL;
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void vATask( void * pvParameters )
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{
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// Create the semaphore to guard a shared resource.
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vSemaphoreCreateBinary( xSemaphore );
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if( xSemaphore != NULL )
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{
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if( xSemaphoreGive( xSemaphore ) != pdTRUE )
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{
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// We would expect this call to fail because we cannot give
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// a semaphore without first "taking" it!
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}
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// Obtain the semaphore - don't block if the semaphore is not
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// immediately available.
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if( xSemaphoreTake( xSemaphore, ( portTickType ) 0 ) )
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{
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// We now have the semaphore and can access the shared resource.
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// ...
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// We have finished accessing the shared resource so can free the
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// semaphore.
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if( xSemaphoreGive( xSemaphore ) != pdTRUE )
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{
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// We would not expect this call to fail because we must have
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// obtained the semaphore to get here.
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}
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}
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}
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}
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</pre>
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* \defgroup xSemaphoreGive xSemaphoreGive
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* \ingroup Semaphores
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*/
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#define xSemaphoreGive( xSemaphore ) xQueueGenericSend( ( xQueueHandle ) xSemaphore, NULL, semGIVE_BLOCK_TIME, queueSEND_TO_BACK )
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/**
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* semphr. h
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* <pre>xSemaphoreGiveRecursive( xSemaphoreHandle xMutex )</pre>
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*
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* <i>Macro</i> to recursively release, or 'give', a mutex type semaphore.
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* The mutex must have previously been created using a call to
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* xSemaphoreCreateRecursiveMutex();
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*
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* configUSE_RECURSIVE_MUTEXES must be set to 1 in FreeRTOSConfig.h for this
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* macro to be available.
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*
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* This macro must not be used on mutexes created using xSemaphoreCreateMutex().
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*
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* A mutex used recursively can be 'taken' repeatedly by the owner. The mutex
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* doesn't become available again until the owner has called
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* xSemaphoreGiveRecursive() for each successful 'take' request. For example,
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* if a task successfully 'takes' the same mutex 5 times then the mutex will
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* not be available to any other task until it has also 'given' the mutex back
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* exactly five times.
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*
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* @param xMutex A handle to the mutex being released, or 'given'. This is the
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* handle returned by xSemaphoreCreateMutex();
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*
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* @return pdTRUE if the semaphore was given.
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*
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* Example usage:
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<pre>
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xSemaphoreHandle xMutex = NULL;
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// A task that creates a mutex.
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void vATask( void * pvParameters )
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{
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// Create the mutex to guard a shared resource.
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xMutex = xSemaphoreCreateRecursiveMutex();
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}
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// A task that uses the mutex.
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void vAnotherTask( void * pvParameters )
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{
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// ... Do other things.
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if( xMutex != NULL )
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{
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// See if we can obtain the mutex. If the mutex is not available
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// wait 10 ticks to see if it becomes free.
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if( xSemaphoreTakeRecursive( xMutex, ( portTickType ) 10 ) == pdTRUE )
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{
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// We were able to obtain the mutex and can now access the
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// shared resource.
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// ...
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// For some reason due to the nature of the code further calls to
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// xSemaphoreTakeRecursive() are made on the same mutex. In real
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// code these would not be just sequential calls as this would make
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// no sense. Instead the calls are likely to be buried inside
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// a more complex call structure.
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xSemaphoreTakeRecursive( xMutex, ( portTickType ) 10 );
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xSemaphoreTakeRecursive( xMutex, ( portTickType ) 10 );
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// The mutex has now been 'taken' three times, so will not be
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// available to another task until it has also been given back
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// three times. Again it is unlikely that real code would have
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// these calls sequentially, it would be more likely that the calls
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// to xSemaphoreGiveRecursive() would be called as a call stack
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// unwound. This is just for demonstrative purposes.
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xSemaphoreGiveRecursive( xMutex );
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xSemaphoreGiveRecursive( xMutex );
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xSemaphoreGiveRecursive( xMutex );
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// Now the mutex can be taken by other tasks.
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}
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else
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{
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// We could not obtain the mutex and can therefore not access
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// the shared resource safely.
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}
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}
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}
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</pre>
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* \defgroup xSemaphoreGiveRecursive xSemaphoreGiveRecursive
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* \ingroup Semaphores
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*/
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#define xSemaphoreGiveRecursive( xMutex ) xQueueGiveMutexRecursive( xMutex )
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/*
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|
* xSemaphoreAltGive() is an alternative version of xSemaphoreGive().
|
|
*
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|
* 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
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|
* sacrifices execution speed to ensure better interrupt responsiveness.
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*/
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#define xSemaphoreAltGive( xSemaphore ) xQueueAltGenericSend( ( xQueueHandle ) xSemaphore, NULL, semGIVE_BLOCK_TIME, queueSEND_TO_BACK )
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/**
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* semphr. h
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|
* <pre>
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xSemaphoreGiveFromISR(
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xSemaphoreHandle xSemaphore,
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|
signed portBASE_TYPE *pxHigherPriorityTaskWoken
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|
)</pre>
|
|
*
|
|
* <i>Macro</i> 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.
|
|
*
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|
* This macro can be used from an ISR.
|
|
*
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|
* @param xSemaphore A handle to the semaphore being released. This is the
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|
* handle returned when the semaphore was created.
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|
*
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* @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:
|
|
<pre>
|
|
\#define LONG_TIME 0xffff
|
|
\#define TICKS_TO_WAIT 10
|
|
xSemaphoreHandle 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 unsigned char ucLocalTickCount = 0;
|
|
static signed portBASE_TYPE 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.
|
|
}
|
|
}
|
|
</pre>
|
|
* \defgroup xSemaphoreGiveFromISR xSemaphoreGiveFromISR
|
|
* \ingroup Semaphores
|
|
*/
|
|
#define xSemaphoreGiveFromISR( xSemaphore, pxHigherPriorityTaskWoken ) xQueueGenericSendFromISR( ( xQueueHandle ) xSemaphore, NULL, pxHigherPriorityTaskWoken, queueSEND_TO_BACK )
|
|
|
|
/**
|
|
* semphr. h
|
|
* <pre>xSemaphoreHandle xSemaphoreCreateMutex( void )</pre>
|
|
*
|
|
* <i>Macro</i> 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
|
|
* xSemaphoreHandle.
|
|
*
|
|
* Example usage:
|
|
<pre>
|
|
xSemaphoreHandle 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.
|
|
}
|
|
}
|
|
</pre>
|
|
* \defgroup vSemaphoreCreateMutex vSemaphoreCreateMutex
|
|
* \ingroup Semaphores
|
|
*/
|
|
#define xSemaphoreCreateMutex() xQueueCreateMutex()
|
|
|
|
|
|
/**
|
|
* semphr. h
|
|
* <pre>xSemaphoreHandle xSemaphoreCreateRecursiveMutex( void )</pre>
|
|
*
|
|
* <i>Macro</i> 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
|
|
* xSemaphoreHandle.
|
|
*
|
|
* Example usage:
|
|
<pre>
|
|
xSemaphoreHandle 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.
|
|
}
|
|
}
|
|
</pre>
|
|
* \defgroup vSemaphoreCreateMutex vSemaphoreCreateMutex
|
|
* \ingroup Semaphores
|
|
*/
|
|
#define xSemaphoreCreateRecursiveMutex() xQueueCreateMutex()
|
|
|
|
/**
|
|
* semphr. h
|
|
* <pre>xSemaphoreHandle xSemaphoreCreateCounting( unsigned portBASE_TYPE uxMaxCount, unsigned portBASE_TYPE uxInitialCount )</pre>
|
|
*
|
|
* <i>Macro</i> 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:
|
|
<pre>
|
|
xSemaphoreHandle xSemaphore;
|
|
|
|
void vATask( void * pvParameters )
|
|
{
|
|
xSemaphoreHandle 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.
|
|
}
|
|
}
|
|
</pre>
|
|
* \defgroup xSemaphoreCreateCounting xSemaphoreCreateCounting
|
|
* \ingroup Semaphores
|
|
*/
|
|
#define xSemaphoreCreateCounting( uxMaxCount, uxInitialCount ) xQueueCreateCountingSemaphore( uxMaxCount, uxInitialCount )
|
|
|
|
|
|
#endif /* SEMAPHORE_H */
|
|
|
|
|