Blocking on Multiple RTOS Objects
Introduction to Queue Sets
Queue sets are a FreeRTOS feature that
enables an RTOS task to block (pend) when receiving from multiple queues and/or
semaphores at the same time.
Queues and semaphores are grouped into sets, then, instead of blocking on an
individual queue or semaphore, a task instead blocks on the set.
Note: While it is sometimes necessary to block (pend) on more than one queue if you
are integrating FreeRTOS with third party of legacy code, designs that are free
from such restrictions can normally achieve the same functionality in a more
efficient way using the alternative design pattern that is
documented at the bottom of this page.
Using Queue Sets
Queue sets are used in a similar way to the select() API function, and related
functions, that are part of the standard Berkeley sockets networking API.
Queue sets can contain queues and semaphores, which together are known as
queue set members. API function parameters and return values that can take either
a queue handle or a semaphore handle use the QueueSetMemberHandle_t type.
Variables of type QueueHandle_t and SemaphoreHandle_t can normally be implicitly
converted to an QueueSetMemberHandle_t parameter or return value without compiler
warnings being generated (explicit casting to and from the QueueSetMemberHandle_t
type is not normally required).
Creating a queue set
Before a queue set can be used it must be created using the
xQueueCreateSet() API function.
Once created the queue set is referenced by a variable of type
Adding a member to a queue set
The xQueueAddToSet() API function
is used to add a queue or semaphore to a queue set.
Blocking (pending) on a queue set
The xQueueSelectFromSet() API function is
used to test whether any of the set members are ready for
reading - where reading means 'receiving' when the member is a queue, and 'taking'
when the member is a semaphore.
Just like when using the xQueueReceive()
and xSemaphoreTake() API functions, xQueueSelectFromSet()
allows the calling task to optionally block until a member of the queue set is ready
NULL is returned if a call to xQueueSelectFromSet() times out. Otherwise
xQueueSelectFromSet() returns the handle of the queue set member that is ready
for reading, allowing the calling task to immediately call xQueueReceive() or
xSemaphoreTake() (on a queue handle or semaphore handle respectively) with the guarantee
that the operation will succeed.
Source Code Examples
The xQueueCreateSet() API function documentation
page includes a source code example.
The standard demo/test file called QueueSet.c (located in the
FreeRTOS/Demo/Common/Minimal/ directory of the main FreeRTOS zip file
download) contains a comprehensive example.
Unless there is a specific integration issue
that necessitates blocking on multiple queues, the same functionality can normally
be achieved with a lower code size, RAM size, and run time overhead using a single
queue. The FreeRTOS-Plus-UDP implementation provides a convenient example of how
this is done, and is described in the following sub-sections.
UDP/IP Stack: Problem Definition
The task that manages the FreeRTOS-Plus-UDP stack is event driven. There are multiple
event sources. Some events do not have any data associated with them. Some events have
a variable amount of data associated with them. Events include:
An application task sending a packet. Packets can contain a large
and variable amount of data.
The Ethernet hardware receiving a frame. Frames contain large and
variable amounts of data.
The Ethernet hardware completing the transmission of a frame, freeing
network and DMA buffers.
Various software timers, including the ARP timer. Timer events are
not associated with any data.
UDP/IP Stack: Solution
The UDP/IP stack could use a different queue for each event source, then
use a queue set to block on all the queues at once. Instead, the UDP/IP stack:
Defines a structure that contains a member to hold the event
type, and another member to hold the data (or a pointer to the data)
that is associated with the event.
Uses a single queue that is created to hold the defined structure. Each
event source posts to the same queue.
The structure definition is shown below.
typedef struct IP_TASK_COMMANDS
eIPEvent_t eEventType; /* Tells the receiving task what the event is. */
void *pvData; /* Holds or points to any data associated with the event. */
Examples of how this structure is used:
When the ARP timer expires it sends an event to the queue with eEventType
set to eARPTimerEvent (an enumerated type). ARP timer events are not
associated with any data so pvData is not set.
When the Ethernet driver receives a frame it sends an event to the
queue with eEventType set to
eEthernetRxEvent, and pvData set to point to the frame buffer.
The UDP/IP task processes events using a simple loop:
/* The variable used to receive from the queue. */
for( ;; )
/* Wait until there is something to do. */
xQueueReceive( xNetworkEventQueue, &xReceivedEvent, portMAX_DELAY );
/* Perform a different action for each event type. */
switch( xReceivedEvent.eEventType )
case eNetworkDownEvent :
case eEthernetRxEvent :
prvProcessEthernetFrame( xReceivedEvent.pvData );
case eARPTimerEvent :
case eStackTxEvent :
prvProcessGeneratedPacket( xReceivedEvent.pvData );
/* Should not get here. */
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