FreeROTS队列的简易分析
- 架构：Cortex-M3
- 版本：FreeRTOS V9.0.0
- 前言：很多时候，我们需要任务与任务、中断与任务通信，FreeRTOS提供了队列来实现这些操作。
1.队列结构体
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;
| 成员 | 作用 |
|---|---|
| pcHead | 指向队列项的头 |
| pcTail | 指向队列项的尾 |
| pcWriteTo | 指向空闲的队列项 |
| pcReadFrom | 指向出队的最后一项 |
| xTasksWaitingToSend | 等待发送队列链表 |
| xTasksWaitingToReceive | 等待接收队列链表 |
| uxMessagesWaiting | 当前队列中存在队列项数目 |
| uxLength | 队列项的数目 |
| uxItemSize | 一个队列项的大小 |
| cRxLock | 当队列上锁后，存储的是队列接收的列表项数目 |
| cTxLock | 当队列上锁后，存储的是发送到队列的列表项数目 |
2.创建队列
具体函数为xQueueGenericCreate
QueueHandle_t xQueueGenericCreate( const UBaseType_t uxQueueLength, const UBaseType_t uxItemSize, const uint8_t ucQueueType )
{
Queue_t *pxNewQueue;
size_t xQueueSizeInBytes;
uint8_t *pucQueueStorage;
configASSERT( uxQueueLength > ( UBaseType_t ) 0 );
if( uxItemSize == ( UBaseType_t ) 0 )
{
/* There is not going to be a queue storage area. */
xQueueSizeInBytes = ( size_t ) 0;
}
else
{
/* Allocate enough space to hold the maximum number of items that
can be in the queue at any time. */
xQueueSizeInBytes = ( size_t ) ( uxQueueLength * uxItemSize ); /*lint !e961 MISRA exception as the casts are only redundant for some ports. */
}
pxNewQueue = ( Queue_t * ) pvPortMalloc( sizeof( Queue_t ) + xQueueSizeInBytes );
if( pxNewQueue != NULL )
{
/* Jump past the queue structure to find the location of the queue
storage area. */
pucQueueStorage = ( ( uint8_t * ) pxNewQueue ) + sizeof( Queue_t );
#if( configSUPPORT_STATIC_ALLOCATION == 1 )
{
/* Queues can be created either statically or dynamically, so
note this task was created dynamically in case it is later
deleted. */
pxNewQueue->ucStaticallyAllocated = pdFALSE;
}
#endif /* configSUPPORT_STATIC_ALLOCATION */
prvInitialiseNewQueue( uxQueueLength, uxItemSize, pucQueueStorage, ucQueueType, pxNewQueue );
}
return pxNewQueue;
}
创建函数要传二个参数：
- 队列项的数目
- 每个队列项的大小
根据传进来的这两个参数，计算出要申请的内存大小uxQueueLength * uxItemSize ，还要加上队列结构体Queue_t的大小。然后调用初始化函数：prvInitialiseNewQueue
static void prvInitialiseNewQueue( const UBaseType_t uxQueueLength, const UBaseType_t uxItemSize, uint8_t *pucQueueStorage, const uint8_t ucQueueType, Queue_t *pxNewQueue )
{
/* Remove compiler warnings about unused parameters should
configUSE_TRACE_FACILITY not be set to 1. */
( void ) ucQueueType;
if( uxItemSize == ( UBaseType_t ) 0 )
{
/* No RAM was allocated for the queue storage area, but PC head cannot
be set to NULL because NULL is used as a key to say the queue is used as
a mutex. Therefore just set pcHead to point to the queue as a benign
value that is known to be within the memory map. */
pxNewQueue->pcHead = ( int8_t * ) pxNewQueue;
}
else
{
/* Set the head to the start of the queue storage area. */
pxNewQueue->pcHead = ( int8_t * ) pucQueueStorage;
}
/* Initialise the queue members as described where the queue type is
defined. */
pxNewQueue->uxLength = uxQueueLength;
pxNewQueue->uxItemSize = uxItemSize;
( void ) xQueueGenericReset( pxNewQueue, pdTRUE );
#if ( configUSE_TRACE_FACILITY == 1 )
{
pxNewQueue->ucQueueType = ucQueueType;
}
#endif /* configUSE_TRACE_FACILITY */
#if( configUSE_QUEUE_SETS == 1 )
{
pxNewQueue->pxQueueSetContainer = NULL;
}
#endif /* configUSE_QUEUE_SETS */
traceQUEUE_CREATE( pxNewQueue );
}
经过初始化后，队列结构体如下：
3.队列发送函数
| 函数名 | 作用 |
|---|---|
| xQueueSend() | 发送信消息到队列尾（向后入队） |
| xQueueSendToBack() | 同上 |
| xQueueSendToFront() | 发送消息到队列头（向前入队） |
| xQueueOverwrite() | 以覆写的方式，发送消息到队列 |
| xQueueSendFromISR() | 用于中断服务函数，发送信消息到队列尾（向后入队） |
| xQueueSendToBackFromISR() | 同上 |
| xQueueSendToFrontFromISR() | 用于中断服务函数，发送消息到队列头（向前入队） |
| xQueueOverwriteFromISR() | 用于中断服务函数，以覆写的方式，发送消息到队列 |
除了FromISR类型的，其它四个函数要传入三个参数：
- 队列句柄
- 等收缓冲区
- 当队列满时的等待超时时间，如果是0，没有收到数据就会立即退出，不等待。
前四个都是一个函数的宏定义：xQueueGenericSend
具体分析代码：
BaseType_t xQueueGenericSend( QueueHandle_t xQueue, const void * const pvItemToQueue, TickType_t xTicksToWait, const BaseType_t xCopyPosition )
{
BaseType_t xEntryTimeSet = pdFALSE, xYieldRequired;
TimeOut_t xTimeOut;
Queue_t * const pxQueue = ( Queue_t * ) xQueue;
configASSERT( pxQueue );
configASSERT( !( ( pvItemToQueue == NULL ) && ( pxQueue->uxItemSize != ( UBaseType_t ) 0U ) ) );
configASSERT( !( ( xCopyPosition == queueOVERWRITE ) && ( pxQueue->uxLength != 1 ) ) );
#if ( ( INCLUDE_xTaskGetSchedulerState == 1 ) || ( configUSE_TIMERS == 1 ) )
{
configASSERT( !( ( xTaskGetSchedulerState() == taskSCHEDULER_SUSPENDED ) && ( xTicksToWait != 0 ) ) );
}
#endif
/* This function relaxes the coding standard somewhat to allow return
statements within the function itself. This is done in the interest
of execution time efficiency. */
for( ;; )
{
taskENTER_CRITICAL();
{
/* Is there room on the queue now? The running task must be the
highest priority task wanting to access the queue. If the head item
in the queue is to be overwritten then it does not matter if the
queue is full. */
if( ( pxQueue->uxMessagesWaiting < pxQueue->uxLength ) || ( xCopyPosition == queueOVERWRITE ) )
{
traceQUEUE_SEND( pxQueue );
xYieldRequired = prvCopyDataToQueue( pxQueue, pvItemToQueue, xCopyPosition );
#if ( configUSE_QUEUE_SETS == 1 )
{
if( pxQueue->pxQueueSetContainer != NULL )
{
if( prvNotifyQueueSetContainer( pxQueue, xCopyPosition ) != pdFALSE )
{
/* The queue is a member of a queue set, and posting
to the queue set caused a higher priority task to
unblock. A context switch is required. */
queueYIELD_IF_USING_PREEMPTION();
}
else
{
mtCOVERAGE_TEST_MARKER();
}
}
else
{
/* If there was a task waiting for data to arrive on the
queue then unblock it now. */
if( listLIST_IS_EMPTY( &( pxQueue->xTasksWaitingToReceive ) ) == pdFALSE )
{
if( xTaskRemoveFromEventList( &( pxQueue->xTasksWaitingToReceive ) ) != pdFALSE )
{
/* The unblocked task has a priority higher than
our own so yield immediately. Yes it is ok to
do this from within the critical section - the
kernel takes care of that. */
queueYIELD_IF_USING_PREEMPTION();
}
else
{
mtCOVERAGE_TEST_MARKER();
}
}
else if( xYieldRequired != pdFALSE )
{
/* This path is a special case that will only get
executed if the task was holding multiple mutexes
and the mutexes were given back in an order that is
different to that in which they were taken. */
queueYIELD_IF_USING_PREEMPTION();
}
else
{
mtCOVERAGE_TEST_MARKER();
}
}
}
#else /* configUSE_QUEUE_SETS */
{
/* If there was a task waiting for data to arrive on the
queue then unblock it now. */
if( listLIST_IS_EMPTY( &( pxQueue->xTasksWaitingToReceive ) ) == pdFALSE )
{
if( xTaskRemoveFromEventList( &( pxQueue->xTasksWaitingToReceive ) ) != pdFALSE )
{
/* The unblocked task has a priority higher than
our own so yield immediately. Yes it is ok to do
this from within the critical section - the kernel
takes care of that. */
queueYIELD_IF_USING_PREEMPTION();
}
else
{
mtCOVERAGE_TEST_MARKER();
}
}
else if( xYieldRequired != pdFALSE )
{
/* This path is a special case that will only get
executed if the task was holding multiple mutexes and
the mutexes were given back in an order that is
different to that in which they were taken. */
queueYIELD_IF_USING_PREEMPTION();
}
else
{
mtCOVERAGE_TEST_MARKER();
}
}
#endif /* configUSE_QUEUE_SETS */
taskEXIT_CRITICAL();
return pdPASS;
}
else
{
if( xTicksToWait == ( TickType_t ) 0 )
{
/* The queue was full and no block time is specified (or
the block time has expired) so leave now. */
taskEXIT_CRITICAL();
/* Return to the original privilege level before exiting
the function. */
traceQUEUE_SEND_FAILED( pxQueue );
return errQUEUE_FULL;
}
else if( xEntryTimeSet == pdFALSE )
{
/* The queue was full and a block time was specified so
configure the timeout structure. */
vTaskSetTimeOutState( &xTimeOut );
xEntryTimeSet = pdTRUE;
}
else
{
/* Entry time was already set. */
mtCOVERAGE_TEST_MARKER();
}
}
}
taskEXIT_CRITICAL();
/* Interrupts and other tasks can send to and receive from the queue
now the critical section has been exited. */
vTaskSuspendAll();
prvLockQueue( pxQueue );
/* Update the timeout state to see if it has expired yet. */
if( xTaskCheckForTimeOut( &xTimeOut, &xTicksToWait ) == pdFALSE )
{
if( prvIsQueueFull( pxQueue ) != pdFALSE )
{
traceBLOCKING_ON_QUEUE_SEND( pxQueue );
vTaskPlaceOnEventList( &( pxQueue->xTasksWaitingToSend ), xTicksToWait );
/* Unlocking the queue means queue events can effect the
event list. It is possible that interrupts occurring now
remove this task from the event list again - but as the
scheduler is suspended the task will go onto the pending
ready last instead of the actual ready list. */
prvUnlockQueue( pxQueue );
/* Resuming the scheduler will move tasks from the pending
ready list into the ready list - so it is feasible that this
task is already in a ready list before it yields - in which
case the yield will not cause a context switch unless there
is also a higher priority task in the pending ready list. */
if( xTaskResumeAll() == pdFALSE )
{
portYIELD_WITHIN_API();
}
}
else
{
/* Try again. */
prvUnlockQueue( pxQueue );
( void ) xTaskResumeAll();
}
}
else
{
/* The timeout has expired. */
prvUnlockQueue( pxQueue );
( void ) xTaskResumeAll();
traceQUEUE_SEND_FAILED( pxQueue );
return errQUEUE_FULL;
}
}
}
- 首先检查队列是不是满了，如果没有满或者覆写方式，就会进入
prvCopyDataToQueue来把消息入队
具体分析：
static BaseType_t prvCopyDataToQueue( Queue_t * const pxQueue, const void *pvItemToQueue, const BaseType_t xPosition )
{
BaseType_t xReturn = pdFALSE;
UBaseType_t uxMessagesWaiting;
/* This function is called from a critical section. */
uxMessagesWaiting = pxQueue->uxMessagesWaiting;
if( pxQueue->uxItemSize == ( UBaseType_t ) 0 )
{
#if ( configUSE_MUTEXES == 1 )
{
if( pxQueue->uxQueueType == queueQUEUE_IS_MUTEX )
{
/* The mutex is no longer being held. */
xReturn = xTaskPriorityDisinherit( ( void * ) pxQueue->pxMutexHolder );
pxQueue->pxMutexHolder = NULL;
}
else
{
mtCOVERAGE_TEST_MARKER();
}
}
#endif /* configUSE_MUTEXES */
}
else if( xPosition == queueSEND_TO_BACK )
{
( void ) memcpy( ( void * ) pxQueue->pcWriteTo, pvItemToQueue, ( size_t ) pxQueue->uxItemSize ); /*lint !e961 !e418 MISRA exception as the casts are only redundant for some ports, plus previous logic ensures a null pointer can only be passed to memcpy() if the copy size is 0. */
pxQueue->pcWriteTo += pxQueue->uxItemSize;
if( pxQueue->pcWriteTo >= pxQueue->pcTail ) /*lint !e946 MISRA exception justified as comparison of pointers is the cleanest solution. */
{
pxQueue->pcWriteTo = pxQueue->pcHead;
}
else
{
mtCOVERAGE_TEST_MARKER();
}
}
else
{
( void ) memcpy( ( void * ) pxQueue->u.pcReadFrom, pvItemToQueue, ( size_t ) pxQueue->uxItemSize ); /*lint !e961 MISRA exception as the casts are only redundant for some ports. */
pxQueue->u.pcReadFrom -= pxQueue->uxItemSize;
if( pxQueue->u.pcReadFrom < pxQueue->pcHead ) /*lint !e946 MISRA exception justified as comparison of pointers is the cleanest solution. */
{
pxQueue->u.pcReadFrom = ( pxQueue->pcTail - pxQueue->uxItemSize );
}
else
{
mtCOVERAGE_TEST_MARKER();
}
if( xPosition == queueOVERWRITE )
{
if( uxMessagesWaiting > ( UBaseType_t ) 0 )
{
/* An item is not being added but overwritten, so subtract
one from the recorded number of items in the queue so when
one is added again below the number of recorded items remains
correct. */
--uxMessagesWaiting;
}
else
{
mtCOVERAGE_TEST_MARKER();
}
}
else
{
mtCOVERAGE_TEST_MARKER();
}
}
pxQueue->uxMessagesWaiting = uxMessagesWaiting + 1;
return xReturn;
}
我用图解的方式更好理解向前入队和向后入队的区别:
- 当入队完成后，
uxMessagesWaiting会加1，然后检查xTasksWaitingToReceive是否有其它任务在阻塞等待接收队列消息，如果有任务等待接收，就会把那个任务从xTasksWaitingToReceive、xEventListItem、pxDelayedTaskList上删除，把任务加入到prvAddTaskToReadyList，如果阻塞的任务优先级大于当前任务，就立即切换任务。 - 当入队完成后，
xTasksWaitingToReceive上没有任务等待接收，就立即切换任务。 - 如果当前队列的消息已经满了，设置的超时时间为0，那么就立即退出临界区，返回errQUEUE_FULL
- 如果当前队列的消息已经满了，设置的超时时间不为0，退出临界区，挂起所有任务，上锁队列，如果还没超时，就把当前任务，挂载到
xEventListItem和DelayedList上，解锁队列，唤醒全部任务，并且切换任务。如果已经超时了，解锁队列，唤醒全部任务，返回errQUEUE_FULL。
4.队列接收函数
| 函数名 | 作用 |
|---|---|
| xQueueReceive() | 从队列中读队列项，并且读取后会删除队列项 |
| xQueuePeek() | 从队列中读队列项，并且读取后不会删除队列项 |
| xQueueReceiveFromISR() | 用于中断服务函数，从队列中读队列项，并且读取后会删除队列项 |
| xQueuePeekFromISR() | 用于中断服务函数，从队列中读队列项，并且读取后不会删除队列项 |
接收函数要传入三个参数
- 队列句柄
- 等收缓冲区
- 等待超时时间，如果是0，没有收到数据就会立即退出，不等待。
具体函数如下
#define xQueueReceive( xQueue, pvBuffer, xTicksToWait ) xQueueGenericReceive( ( xQueue ), ( pvBuffer ), ( xTicksToWait ), pdFALSE )
事实上调用的是xQueueGenericReceive
BaseType_t xQueueGenericReceive( QueueHandle_t xQueue, void * const pvBuffer, TickType_t xTicksToWait, const BaseType_t xJustPeeking )
{
BaseType_t xEntryTimeSet = pdFALSE;
TimeOut_t xTimeOut;
int8_t *pcOriginalReadPosition;
Queue_t * const pxQueue = ( Queue_t * ) xQueue;
configASSERT( pxQueue );
configASSERT( !( ( pvBuffer == NULL ) && ( pxQueue->uxItemSize != ( UBaseType_t ) 0U ) ) );
#if ( ( INCLUDE_xTaskGetSchedulerState == 1 ) || ( configUSE_TIMERS == 1 ) )
{
configASSERT( !( ( xTaskGetSchedulerState() == taskSCHEDULER_SUSPENDED ) && ( xTicksToWait != 0 ) ) );
}
#endif
/* This function relaxes the coding standard somewhat to allow return
statements within the function itself. This is done in the interest
of execution time efficiency. */
for( ;; )
{
taskENTER_CRITICAL();
{
const UBaseType_t uxMessagesWaiting = pxQueue->uxMessagesWaiting;
/* Is there data in the queue now? To be running the calling task
must be the highest priority task wanting to access the queue. */
if( uxMessagesWaiting > ( UBaseType_t ) 0 )
{
/* Remember the read position in case the queue is only being
peeked. */
pcOriginalReadPosition = pxQueue->u.pcReadFrom;
prvCopyDataFromQueue( pxQueue, pvBuffer );
if( xJustPeeking == pdFALSE )
{
traceQUEUE_RECEIVE( pxQueue );
/* Actually removing data, not just peeking. */
pxQueue->uxMessagesWaiting = uxMessagesWaiting - 1;
#if ( configUSE_MUTEXES == 1 )
{
if( pxQueue->uxQueueType == queueQUEUE_IS_MUTEX )
{
/* Record the information required to implement
priority inheritance should it become necessary. */
pxQueue->pxMutexHolder = ( int8_t * ) pvTaskIncrementMutexHeldCount(); /*lint !e961 Cast is not redundant as TaskHandle_t is a typedef. */
}
else
{
mtCOVERAGE_TEST_MARKER();
}
}
#endif /* configUSE_MUTEXES */
if( listLIST_IS_EMPTY( &( pxQueue->xTasksWaitingToSend ) ) == pdFALSE )
{
if( xTaskRemoveFromEventList( &( pxQueue->xTasksWaitingToSend ) ) != pdFALSE )
{
queueYIELD_IF_USING_PREEMPTION();
}
else
{
mtCOVERAGE_TEST_MARKER();
}
}
else
{
mtCOVERAGE_TEST_MARKER();
}
}
else
{
traceQUEUE_PEEK( pxQueue );
/* The data is not being removed, so reset the read
pointer. */
pxQueue->u.pcReadFrom = pcOriginalReadPosition;
/* The data is being left in the queue, so see if there are
any other tasks waiting for the data. */
if( listLIST_IS_EMPTY( &( pxQueue->xTasksWaitingToReceive ) ) == pdFALSE )
{
if( xTaskRemoveFromEventList( &( pxQueue->xTasksWaitingToReceive ) ) != pdFALSE )
{
/* The task waiting has a higher priority than this task. */
queueYIELD_IF_USING_PREEMPTION();
}
else
{
mtCOVERAGE_TEST_MARKER();
}
}
else
{
mtCOVERAGE_TEST_MARKER();
}
}
taskEXIT_CRITICAL();
return pdPASS;
}
else
{
if( xTicksToWait == ( TickType_t ) 0 )
{
/* The queue was empty and no block time is specified (or
the block time has expired) so leave now. */
taskEXIT_CRITICAL();
traceQUEUE_RECEIVE_FAILED( pxQueue );
return errQUEUE_EMPTY;
}
else if( xEntryTimeSet == pdFALSE )
{
/* The queue was empty and a block time was specified so
configure the timeout structure. */
vTaskSetTimeOutState( &xTimeOut );
xEntryTimeSet = pdTRUE;
}
else
{
/* Entry time was already set. */
mtCOVERAGE_TEST_MARKER();
}
}
}
taskEXIT_CRITICAL();
/* Interrupts and other tasks can send to and receive from the queue
now the critical section has been exited. */
vTaskSuspendAll();
prvLockQueue( pxQueue );
/* Update the timeout state to see if it has expired yet. */
if( xTaskCheckForTimeOut( &xTimeOut, &xTicksToWait ) == pdFALSE )
{
if( prvIsQueueEmpty( pxQueue ) != pdFALSE )
{
traceBLOCKING_ON_QUEUE_RECEIVE( pxQueue );
#if ( configUSE_MUTEXES == 1 )
{
if( pxQueue->uxQueueType == queueQUEUE_IS_MUTEX )
{
taskENTER_CRITICAL();
{
vTaskPriorityInherit( ( void * ) pxQueue->pxMutexHolder );
}
taskEXIT_CRITICAL();
}
else
{
mtCOVERAGE_TEST_MARKER();
}
}
#endif
vTaskPlaceOnEventList( &( pxQueue->xTasksWaitingToReceive ), xTicksToWait );
prvUnlockQueue( pxQueue );
if( xTaskResumeAll() == pdFALSE )
{
portYIELD_WITHIN_API();
}
else
{
mtCOVERAGE_TEST_MARKER();
}
}
else
{
/* Try again. */
prvUnlockQueue( pxQueue );
( void ) xTaskResumeAll();
}
}
else
{
prvUnlockQueue( pxQueue );
( void ) xTaskResumeAll();
if( prvIsQueueEmpty( pxQueue ) != pdFALSE )
{
traceQUEUE_RECEIVE_FAILED( pxQueue );
return errQUEUE_EMPTY;
}
else
{
mtCOVERAGE_TEST_MARKER();
}
}
}
}
如果当前有消息uxMessagesWaiting > 0，那么就调用 prvCopyDataFromQueue( pxQueue, pvBuffer )就把消息赋值。
``prvCopyDataFromQueue`具体函数如下：
static void prvCopyDataFromQueue( Queue_t * const pxQueue, void * const pvBuffer )
{
if( pxQueue->uxItemSize != ( UBaseType_t ) 0 )
{
pxQueue->u.pcReadFrom += pxQueue->uxItemSize;
if( pxQueue->u.pcReadFrom >= pxQueue->pcTail ) /*lint !e946 MISRA exception justified as use of the relational operator is the cleanest solutions. */
{
pxQueue->u.pcReadFrom = pxQueue->pcHead;
}
else
{
mtCOVERAGE_TEST_MARKER();
}
( void ) memcpy( ( void * ) pvBuffer, ( void * ) pxQueue->u.pcReadFrom, ( size_t ) pxQueue->uxItemSize ); /*lint !e961 !e418 MISRA exception as the casts are only redundant for some ports. Also previous logic ensures a null pointer can only be passed to memcpy() when the count is 0. */
}
}
我来图解这个函数容易理解：
1.出队完成后，uxMessagesWaiting就会减1，然后查看是否还有其它任务在阻塞等待接收，如果有其它阻塞任务等待接收就会把任务从xTasksWaitingToSend、xEventListItem和pxDelayedTaskList上移除，如果调度器没有挂起，就挂载到ReadyList，如果调度器挂起了，就以尾插的方式把任务挂载到xPendingReadyList。最后，如果阻塞任务优先级大于当前任务，则立马切换任务。
2.如果当前没有消息uxMessagesWaiting < 0，如果设置的超时时间为0，就立马返回errQUEUE_EMPTY，
如果超时时间不为0，设置当前进入时间，挂起任务和上锁队列。检查是否达到超时时间，如果已经超时了，就解锁队列，唤醒所有任务，并立马返回errQUEUE_EMPTY。如果还没有超时，并且队列为空，就会把当前任务挂载到pxEventList、DelayedList上，解锁队列，唤醒所有任务，并切换任务。