freertos操作系统_仓储系统_学ucos还是free rtos(8)

图3.4：系统节拍计时器下的FreeRTOS就绪列表全貌

pxReadyTasksLists[]列出了在vTaskSwitchContext()中已经操纵完成的内容，是如何使用pxIndex的一个很好的例子。让我们假设我们仅有一个优先级，优先级0，并且有三个任务在此优先级上。这与我们之前看到的基本就绪列表图相，但这一次我们将包括所有的数据结构和字段。

就如你在图3.3中所见，pxCurrentTCB显示我们当前正在运行任务B。下一个时刻，vTaskSwitchContext()运行，它调用listGET_OWNER_OF_NEXT_ENTRY()载入下一个任务来运行。如图3.4所示，这个函数使用pxIndex->pxNext找出下一个任务是任务C，并且pxIndex指向任务C的列表元素，同时pxCurrentTCB指向任务C的TCB。

请注意，每个struct xlistitem对象实际上都是来自相关TCB的xGenericListItem对象。

3.6. 队列

FreeRTOS允许任务使用队列来互相间通信和同步。中断服务程序（ISRs）同样使用队列来通信和同步。

基本队列数据结构如下：

typedef struct QueueDefinition

{

signed char *pcHead; /* Points to the beginning of the queue

storage area. */

signed char *pcTail; /* Points to the byte at the end of the

queue storage area. One more byte is

allocated than necessary to store the

queue items; this is used as a marker. */

signed char *pcWriteTo; /* Points to the free next place in the

storage area. */

signed char *pcReadFrom; /* Points to the last place that a queued

item was read from. */

xList xTasksWaitingToSend; /* List of tasks that are blocked waiting

to post onto this queue. Stored in

priority order. */

xList xTasksWaitingToReceive; /* List of tasks that are blocked waiting

to read from this queue. Stored in

priority order. */

volatile unsigned portBASE_TYPE uxMessagesWaiting; /* The number of items currently

in the queue. */

unsigned portBASE_TYPE uxLength; /* The length of the queue

defined as the number of

items it will hold, not the

number of bytes. */

unsigned portBASE_TYPE uxItemSize; /* The size of each items that

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