1 前言
任務(wù)之間的同步（同步就是任務(wù)之間做數(shù)據(jù)交互，或?yàn)閮蓚€(gè)任務(wù)之間的通訊），任務(wù)和中斷之間的同步都可以依靠消息隊(duì)列，從而實(shí)現(xiàn)異步處理，FreeRTOS的隊(duì)列采用FIFO（先進(jìn)先出）緩沖區(qū)，具體如下圖所示；
2 xQUEUE
FreeRTOS消息隊(duì)列的實(shí)現(xiàn)主要是queue.c，需要包含頭文件queue.h，下面先看一下queue.c中的數(shù)據(jù)類型xQUEUE，源碼如下所示；
typedef struct QueueDefinition
{
int8_t *pcHead; /*< Points to the beginning of the queue storage area. */
int8_t *pcTail; /*< Points to the byte at the end of the queue storage area. Once more byte is allocated than necessary to store the queue items, this is used as a marker. */
int8_t *pcWriteTo; /*< Points to the free next place in the storage area. */
union /* Use of a union is an exception to the coding standard to ensure two mutually exclusive structure members don't appear simultaneously (wasting RAM). */
{
int8_t *pcReadFrom; /*< Points to the last place that a queued item was read from when the structure is used as a queue. */
UBaseType_t uxRecursiveCallCount;/*< Maintains a count of the number of times a recursive mutex has been recursively 'taken' when the structure is used as a mutex. */
} u;
List_t xTasksWaitingToSend; /*< List of tasks that are blocked waiting to post onto this queue. Stored in priority order. */
List_t xTasksWaitingToReceive; /*< List of tasks that are blocked waiting to read from this queue. Stored in priority order. */
volatile UBaseType_t uxMessagesWaiting;/*< The number of items currently in the queue. */
UBaseType_t uxLength; /*< The length of the queue defined as the number of items it will hold, not the number of bytes. */
UBaseType_t uxItemSize; /*< The size of each items that the queue will hold. */
volatile int8_t cRxLock; /*< Stores the number of items received from the queue (removed from the queue) while the queue was locked. Set to queueUNLOCKED when the queue is not locked. */
volatile int8_t cTxLock; /*< Stores the number of items transmitted to the queue (added to the queue) while the queue was locked. Set to queueUNLOCKED when the queue is not locked. */
#if( ( configSUPPORT_STATIC_ALLOCATION == 1 ) && ( configSUPPORT_DYNAMIC_ALLOCATION == 1 ) )
uint8_t ucStaticallyAllocated; /*< Set to pdTRUE if the memory used by the queue was statically allocated to ensure no attempt is made to free the memory. */
#endif
#if ( configUSE_QUEUE_SETS == 1 )
struct QueueDefinition *pxQueueSetContainer;
#endif
#if ( configUSE_TRACE_FACILITY == 1 )
UBaseType_t uxQueueNumber;
uint8_t ucQueueType;
#endif
} xQUEUE;本文暫時(shí)不需要深入源碼，在queue.h的接口已經(jīng)封裝得相當(dāng)好，無需對(duì)細(xì)節(jié)太過于關(guān)注，下面會(huì)是對(duì)常用接口的使用的總結(jié)，如果英文好，直接看源碼中的函數(shù)注釋也是很好的選擇。
3 相關(guān)概念
3.1 數(shù)據(jù)結(jié)構(gòu)
隊(duì)列可以保存有限個(gè)具有確定長(zhǎng)度的數(shù)據(jù)單元。隊(duì)列可以保存的最大單元數(shù)目被稱為隊(duì)列的“深度”。在隊(duì)列創(chuàng)建時(shí)需要設(shè)定其深度和每個(gè)單元的大小。通常情況下，隊(duì)列被作為 FIFO(先進(jìn)先出)使用，即數(shù)據(jù)由隊(duì)列尾寫入，從隊(duì)列首讀出。當(dāng)然，由隊(duì)列首寫入也是可能的。往隊(duì)列寫入數(shù)據(jù)是通過字節(jié)拷貝把數(shù)據(jù)復(fù)制存儲(chǔ)到隊(duì)列中；從隊(duì)列讀出數(shù)據(jù)使得把隊(duì)列中的數(shù)據(jù)拷貝刪除。1 如下圖所示；
注意上面提到的數(shù)據(jù)單元可以是一個(gè)char或int類型的數(shù)，但是相對(duì)比較合理的設(shè)計(jì)是，封裝成一個(gè)合理的類，或者稱之為結(jié)構(gòu)體，可以明確當(dāng)前數(shù)據(jù)單元的數(shù)據(jù)類型，數(shù)據(jù)來源（來自哪個(gè)任務(wù)）等等，因?yàn)橐粋€(gè)隊(duì)列可以被多個(gè)任務(wù)進(jìn)行讀取和發(fā)送函數(shù)，這樣就避免了傳輸數(shù)據(jù)出現(xiàn)混淆的情況。通常設(shè)計(jì)是一個(gè)隊(duì)列被多個(gè)任務(wù)寫入數(shù)據(jù)，然后有一個(gè)任務(wù)讀取，暫時(shí)稱之為多寫一讀，反之，多讀一寫則較少遇到。
3.2 收發(fā)數(shù)據(jù)堵塞
當(dāng)某個(gè)任務(wù)試圖讀或者寫一個(gè)隊(duì)列時(shí)，其可以指定一個(gè)阻塞超時(shí)時(shí)間，
-
讀?。喝蝿?wù)讀取數(shù)據(jù)時(shí)，在設(shè)置堵塞超時(shí)時(shí)間內(nèi)，如果隊(duì)列為空，該任務(wù)將保持阻塞狀態(tài)以等待隊(duì)列數(shù)據(jù)有效。當(dāng)其它任務(wù)或中斷服務(wù)例程往其等待的隊(duì)列中寫入了數(shù)據(jù)，該任務(wù)將自動(dòng)由阻塞態(tài)轉(zhuǎn)移為就緒態(tài)。
-
寫入：如果隊(duì)列被多個(gè)任務(wù)寫入，那么將導(dǎo)致多個(gè)任務(wù)堵塞以等待隊(duì)列有效，當(dāng)隊(duì)列有效的時(shí)候，這些任務(wù)中的優(yōu)先級(jí)最高的任務(wù)優(yōu)先進(jìn)入就緒態(tài)。
4 常用函數(shù)
FreeRTOS的消息隊(duì)列常用接口都封裝在queue.h中，通過宏定義統(tǒng)一將接口函數(shù)的命名風(fēng)格整理得十分統(tǒng)一；具體如下圖所示；
這里有兩種需要注意；
- 任務(wù)與任務(wù)之間同步：例如圖中①處的API適用于任務(wù)間同步；
xQueueSendToFrontxQueueSendToFrontxQueueSendxQueueOverwrite
- 任務(wù)與中斷之間同步：圖中②處的API適用于任務(wù)于中斷間同步，
xxxISR()后綴的函數(shù)都是FreeRTOS中保證了線程安全的；xQueueSendToFrontFromISRxQueueSendToBackFromISRxQueueOverwriteFromISRxQueueSendFromISR
4.1 創(chuàng)建隊(duì)列
- QueueHandle_t
QueueHandle_t是一個(gè)void類型的指針變量，定義在queue.h中，具體如下；
/**
* Type by which queues are referenced. For example, a call to xQueueCreate()
* returns an QueueHandle_t variable that can then be used as a parameter to
* xQueueSend(), xQueueReceive(), etc.
*/
typedef void * QueueHandle_t;基本上每個(gè)隊(duì)列函數(shù)都會(huì)使用這個(gè)變量，這里我們統(tǒng)一稱為隊(duì)列的句柄；
- xQueueCreate這個(gè)函數(shù)可以創(chuàng)建一個(gè)隊(duì)列，創(chuàng)建成功則會(huì)返回一個(gè)隊(duì)列句柄，如果創(chuàng)建失敗則返回
NULL，其函數(shù)原型是xQueueGenericCreate，具體如下所示；
#define xQueueCreate( uxQueueLength, uxItemSize ) \
xQueueGenericCreate( ( uxQueueLength ), ( uxItemSize ), ( queueQUEUE_TYPE_BASE ) )xQueueCreate如下所示；
QueueHandle_t xQueueCreate(
UBaseType_t uxQueueLength,
UBaseType_t uxItemSize
);
4.2 發(fā)送數(shù)據(jù)
下面都是從任務(wù)發(fā)送數(shù)據(jù)到隊(duì)列，
- xQueueSendToFront將數(shù)據(jù)發(fā)送至隊(duì)首，函數(shù)聲明如下；
BaseType_t xQueueSendToBack(
QueueHandle_t xQueue,
const void *pvItemToQueue,
TickType_t xTicksToWait
); - xQueueSendToBack將數(shù)據(jù)發(fā)送至隊(duì)尾，函數(shù)聲明如下；
BaseType_t xQueueSendToFront(
QueueHandle_t xQueue,
const void * pvItemToQueue,
TickType_t xTicksToWait
);- xQueueSend與
xQueueSendToBack入隊(duì)順序相同，函數(shù)聲明如下所示；
BaseType_t xQueueSend(
QueueHandle_t xQueue,
const void * pvItemToQueue,
TickType_t xTicksToWait
);具體的參數(shù)描述如下：
xTicksToWait設(shè)為0 ，且隊(duì)列已滿，則xQueueSendToFront()與xQueueSendToBack()均會(huì)立即返回。阻塞時(shí)間是以系統(tǒng)心跳周期為單位的，所以絕對(duì)時(shí)間取決于系統(tǒng)心跳頻率。常量 portTICK_RATE_MS 可以用來把心跳時(shí)間單位轉(zhuǎn)換為毫秒時(shí)間單位。xTicksToWait設(shè)置為portMAX_DELAY， 并且在FreeRTOSConig.h 中設(shè)定INCLUDE_vTaskSuspend為 1，那么阻塞等待將沒有超時(shí)限制。
4.3 接收數(shù)據(jù)
- xQueueReceivexQueueReceive()用于從隊(duì)列中接收(讀取）數(shù)據(jù)單元。接收到的單元同時(shí)會(huì)從隊(duì)列中刪除。函數(shù)聲明如下；
BaseType_t xQueueReceive(
QueueHandle_t xQueue,
void *pvBuffer,
TickType_t xTicksToWait
); - xQueuePeekxQueuePeek()也是從從隊(duì)列中接收數(shù)據(jù)單元，不同的是并不從隊(duì)列中刪出接收到的單元。 xQueuePeek()從隊(duì)列首接收到數(shù)據(jù)后，不會(huì)修改隊(duì)列中的數(shù)據(jù)，也不會(huì)改變數(shù)據(jù)在隊(duì)列中的存儲(chǔ)序順。函數(shù)聲明如下；
BaseType_t xQueuePeek(
QueueHandle_t xQueue,
void * const pvBuffer,
TickType_t xTicksToWait
);具體的參數(shù)描述如下：
- uxQueueSpacesAvailableuxQueueSpacesAvailable()用于查詢隊(duì)列中可用的空閑空間數(shù)量；函數(shù)聲明如下；
UBaseType_t uxQueueSpacesAvailable( const QueueHandle_t xQueue );
- uxQueueMessagesWaitinguxQueueMessagesWaiting()用于查詢隊(duì)列中當(dāng)前有效數(shù)據(jù)單元個(gè)數(shù)；函數(shù)聲明如下；
UBaseType_t uxQueueMessagesWaiting( const QueueHandle_t xQueue );
4.4 刪除隊(duì)列
- vQueueDelete用來刪除一個(gè)隊(duì)列，直接傳入已創(chuàng)建的隊(duì)列句柄即可，函數(shù)聲明如下；
void vQueueDelete( QueueHandle_t xQueue );5 舉例
多個(gè)任務(wù)寫入一個(gè)任務(wù)讀取的時(shí)候應(yīng)該怎么做呢？如下圖所示2；
這里有三個(gè)任務(wù)，所以為了搞清楚數(shù)據(jù)來自哪個(gè)任務(wù)，因此將數(shù)據(jù)單元封裝起來，使用iMeaning表示數(shù)據(jù)單元的源頭，當(dāng)然這里還是比較簡(jiǎn)單的應(yīng)用。
6 總結(jié)
本文介紹了FreeRTOS的消息隊(duì)列比價(jià)常用的方法，當(dāng)然是相對(duì)簡(jiǎn)單的，側(cè)重在了解概念上，需要實(shí)際的應(yīng)用從而加深理解，更加詳細(xì)已經(jīng)靈活的應(yīng)用可以參考FreeRTOS作者撰寫的Mastering_the_FreeRTOS_Real_Time_Kernel-A_Hands-On_Tutorial_Guide。