Queue Management

xQueueCreate(uxQueueLength, uxItemSize) xQueueGenericCreate( ( uxQueueLength ), ( uxItemSize ), ( queueQUEUE_TYPE_BASE ) )

queue.h

Creates a new queue instance, and returns a handle by which the new queue can be referenced.

Internally, within the FreeRTOS implementation, queues use two blocks of memory. The first block is used to hold the queue’s data structures. The second block is used to hold items placed into the queue. If a queue is created using xQueueCreate() then both blocks of memory are automatically dynamically allocated inside the xQueueCreate() function. (see http://www.freertos.org/a00111.html). If a queue is created using xQueueCreateStatic() then the application writer must provide the memory that will get used by the queue. xQueueCreateStatic() therefore allows a queue to be created without using any dynamic memory allocation.

http://www.FreeRTOS.org/Embedded-RTOS-Queues.html

Example usage:

struct AMessage
{
   char ucMessageID;
   char ucData[ 20 ];
};

void vATask( void *pvParameters )
{
QueueHandle_t xQueue1, xQueue2;

   // Create a queue capable of containing 10 uint32_t values.
   xQueue1 = xQueueCreate( 10, sizeof( uint32_t ) );
   if( xQueue1 == 0 )
   {
    // Queue was not created and must not be used.
   }

   // Create a queue capable of containing 10 pointers to AMessage structures.
   // These should be passed by pointer as they contain a lot of data.
   xQueue2 = xQueueCreate( 10, sizeof( struct AMessage * ) );
   if( xQueue2 == 0 )
   {
    // Queue was not created and must not be used.
   }

   // ... Rest of task code.
}
Return
If the queue is successfully create then a handle to the newly created queue is returned. If the queue cannot be created then 0 is returned.
Parameters
  • uxQueueLength: The maximum number of items that the queue can contain.
  • uxItemSize: The number of bytes each item in the queue will require. Items are queued by copy, not by reference, so this is the number of bytes that will be copied for each posted item. Each item on the queue must be the same size.

xQueueCreateStatic(uxQueueLength, uxItemSize, pucQueueStorage, pxQueueBuffer) xQueueGenericCreateStatic( ( uxQueueLength ), ( uxItemSize ), ( pucQueueStorage ), ( pxQueueBuffer ), ( queueQUEUE_TYPE_BASE ) )

queue.h

Creates a new queue instance, and returns a handle by which the new queue can be referenced.

Internally, within the FreeRTOS implementation, queues use two blocks of memory. The first block is used to hold the queue’s data structures. The second block is used to hold items placed into the queue. If a queue is created using xQueueCreate() then both blocks of memory are automatically dynamically allocated inside the xQueueCreate() function. (see http://www.freertos.org/a00111.html). If a queue is created using xQueueCreateStatic() then the application writer must provide the memory that will get used by the queue. xQueueCreateStatic() therefore allows a queue to be created without using any dynamic memory allocation.

http://www.FreeRTOS.org/Embedded-RTOS-Queues.html

Example usage:

struct AMessage
{
   char ucMessageID;
   char ucData[ 20 ];
};

#define QUEUE_LENGTH 10
#define ITEM_SIZE sizeof( uint32_t )

// xQueueBuffer will hold the queue structure.
StaticQueue_t xQueueBuffer;

// ucQueueStorage will hold the items posted to the queue.  Must be at least
// [(queue length) * ( queue item size)] bytes long.
uint8_t ucQueueStorage[ QUEUE_LENGTH * ITEM_SIZE ];

void vATask( void *pvParameters )
{
QueueHandle_t xQueue1;

   // Create a queue capable of containing 10 uint32_t values.
   xQueue1 = xQueueCreate( QUEUE_LENGTH, // The number of items the queue can hold.
                        ITEM_SIZE     // The size of each item in the queue
                        &( ucQueueStorage[ 0 ] ), // The buffer that will hold the items in the queue.
                        &xQueueBuffer ); // The buffer that will hold the queue structure.

   // The queue is guaranteed to be created successfully as no dynamic memory
   // allocation is used.  Therefore xQueue1 is now a handle to a valid queue.

   // ... Rest of task code.
}
Return
If the queue is created then a handle to the created queue is returned. If pxQueueBuffer is NULL then NULL is returned.
Parameters
  • uxQueueLength: The maximum number of items that the queue can contain.
  • uxItemSize: The number of bytes each item in the queue will require. Items are queued by copy, not by reference, so this is the number of bytes that will be copied for each posted item. Each item on the queue must be the same size.
  • pucQueueStorageBuffer: If uxItemSize is not zero then pucQueueStorageBuffer must point to a uint8_t array that is at least large enough to hold the maximum number of items that can be in the queue at any one time - which is ( uxQueueLength * uxItemsSize ) bytes. If uxItemSize is zero then pucQueueStorageBuffer can be NULL.
  • pxQueueBuffer: Must point to a variable of type StaticQueue_t, which will be used to hold the queue’s data structure.

void vQueueDelete(QueueHandle_t xQueue)

queue.h

Delete a queue - freeing all the memory allocated for storing of items placed on the queue.

Parameters
  • xQueue: A handle to the queue to be deleted.

xQueueSend(xQueue, pvItemToQueue, xTicksToWait) xQueueGenericSend( ( xQueue ), ( pvItemToQueue ), ( xTicksToWait ), queueSEND_TO_BACK )

queue.h

This is a macro that calls xQueueGenericSend(). It is included for backward compatibility with versions of FreeRTOS.org that did not include the xQueueSendToFront() and xQueueSendToBack() macros. It is equivalent to xQueueSendToBack().

Post an item on a queue. The item is queued by copy, not by reference. This function must not be called from an interrupt service routine. See xQueueSendFromISR () for an alternative which may be used in an ISR.

Example usage:

struct AMessage
{
   char ucMessageID;
   char ucData[ 20 ];
} xMessage;

uint32_t ulVar = 10UL;

void vATask( void *pvParameters )
{
QueueHandle_t xQueue1, xQueue2;
struct AMessage *pxMessage;

   // Create a queue capable of containing 10 uint32_t values.
   xQueue1 = xQueueCreate( 10, sizeof( uint32_t ) );

   // Create a queue capable of containing 10 pointers to AMessage structures.
   // These should be passed by pointer as they contain a lot of data.
   xQueue2 = xQueueCreate( 10, sizeof( struct AMessage * ) );

   // ...

   if( xQueue1 != 0 )
   {
    // Send an uint32_t.  Wait for 10 ticks for space to become
    // available if necessary.
    if( xQueueSend( xQueue1, ( void * ) &ulVar, ( TickType_t ) 10 ) != pdPASS )
    {
        // Failed to post the message, even after 10 ticks.
    }
   }

   if( xQueue2 != 0 )
   {
    // Send a pointer to a struct AMessage object.  Don't block if the
    // queue is already full.
    pxMessage = & xMessage;
    xQueueSend( xQueue2, ( void * ) &pxMessage, ( TickType_t ) 0 );
   }

   // ... Rest of task code.
}
Return
pdTRUE if the item was successfully posted, otherwise errQUEUE_FULL.
Parameters
  • xQueue: The handle to the queue on which the item is to be posted.
  • pvItemToQueue: A pointer to the item that is to be placed on the queue. The size of the items the queue will hold was defined when the queue was created, so this many bytes will be copied from pvItemToQueue into the queue storage area.
  • xTicksToWait: The maximum amount of time the task should block waiting for space to become available on the queue, should it already be full. The call will return immediately if this is set to 0 and the queue is full. The time is defined in tick periods so the constant portTICK_PERIOD_MS should be used to convert to real time if this is required.

xQueueSendFromISR(xQueue, pvItemToQueue, pxHigherPriorityTaskWoken) xQueueGenericSendFromISR( ( xQueue ), ( pvItemToQueue ), ( pxHigherPriorityTaskWoken ), queueSEND_TO_BACK )

queue.h

This is a macro that calls xQueueGenericSendFromISR(). It is included for backward compatibility with versions of FreeRTOS.org that did not include the xQueueSendToBackFromISR() and xQueueSendToFrontFromISR() macros.

Post an item to the back of a queue. It is safe to use this function from within an interrupt service routine.

Items are queued by copy not reference so it is preferable to only queue small items, especially when called from an ISR. In most cases it would be preferable to store a pointer to the item being queued.

Example usage for buffered IO (where the ISR can obtain more than one value per call):

void vBufferISR( void )
{
char cIn;
BaseType_t xHigherPriorityTaskWoken;

   // We have not woken a task at the start of the ISR.
   xHigherPriorityTaskWoken = pdFALSE;

   // Loop until the buffer is empty.
   do
   {
    // Obtain a byte from the buffer.
    cIn = portINPUT_BYTE( RX_REGISTER_ADDRESS );

    // Post the byte.
    xQueueSendFromISR( xRxQueue, &cIn, &xHigherPriorityTaskWoken );

   } while( portINPUT_BYTE( BUFFER_COUNT ) );

   // Now the buffer is empty we can switch context if necessary.
   if( xHigherPriorityTaskWoken )
   {
    // Actual macro used here is port specific.
    portYIELD_FROM_ISR ();
   }
}
Return
pdTRUE if the data was successfully sent to the queue, otherwise errQUEUE_FULL.
Parameters
  • xQueue: The handle to the queue on which the item is to be posted.
  • pvItemToQueue: A pointer to the item that is to be placed on the queue. The size of the items the queue will hold was defined when the queue was created, so this many bytes will be copied from pvItemToQueue into the queue storage area.
  • pxHigherPriorityTaskWoken: xQueueSendFromISR() will set *pxHigherPriorityTaskWoken to pdTRUE if sending to the queue caused a task to unblock, and the unblocked task has a priority higher than the currently running task. If xQueueSendFromISR() sets this value to pdTRUE then a context switch should be requested before the interrupt is exited.

xQueueSendToBack(xQueue, pvItemToQueue, xTicksToWait) xQueueGenericSend( ( xQueue ), ( pvItemToQueue ), ( xTicksToWait ), queueSEND_TO_BACK )

queue.h

This is a macro that calls xQueueGenericSend().

Post an item to the back of a queue. The item is queued by copy, not by reference. This function must not be called from an interrupt service routine. See xQueueSendFromISR () for an alternative which may be used in an ISR.

Example usage:

struct AMessage
{
   char ucMessageID;
   char ucData[ 20 ];
} xMessage;

uint32_t ulVar = 10UL;

void vATask( void *pvParameters )
{
QueueHandle_t xQueue1, xQueue2;
struct AMessage *pxMessage;

   // Create a queue capable of containing 10 uint32_t values.
   xQueue1 = xQueueCreate( 10, sizeof( uint32_t ) );

   // Create a queue capable of containing 10 pointers to AMessage structures.
   // These should be passed by pointer as they contain a lot of data.
   xQueue2 = xQueueCreate( 10, sizeof( struct AMessage * ) );

   // ...

   if( xQueue1 != 0 )
   {
    // Send an uint32_t.  Wait for 10 ticks for space to become
    // available if necessary.
    if( xQueueSendToBack( xQueue1, ( void * ) &ulVar, ( TickType_t ) 10 ) != pdPASS )
    {
        // Failed to post the message, even after 10 ticks.
    }
   }

   if( xQueue2 != 0 )
   {
    // Send a pointer to a struct AMessage object.  Don't block if the
    // queue is already full.
    pxMessage = & xMessage;
    xQueueSendToBack( xQueue2, ( void * ) &pxMessage, ( TickType_t ) 0 );
   }

   // ... Rest of task code.
}
Return
pdTRUE if the item was successfully posted, otherwise errQUEUE_FULL.
Parameters
  • xQueue: The handle to the queue on which the item is to be posted.
  • pvItemToQueue: A pointer to the item that is to be placed on the queue. The size of the items the queue will hold was defined when the queue was created, so this many bytes will be copied from pvItemToQueue into the queue storage area.
  • xTicksToWait: The maximum amount of time the task should block waiting for space to become available on the queue, should it already be full. The call will return immediately if this is set to 0 and the queue is full. The time is defined in tick periods so the constant portTICK_PERIOD_MS should be used to convert to real time if this is required.

xQueueSendToBackFromISR(xQueue, pvItemToQueue, pxHigherPriorityTaskWoken) xQueueGenericSendFromISR( ( xQueue ), ( pvItemToQueue ), ( pxHigherPriorityTaskWoken ), queueSEND_TO_BACK )

queue.h

This is a macro that calls xQueueGenericSendFromISR().

Post an item to the back of a queue. It is safe to use this macro from within an interrupt service routine.

Items are queued by copy not reference so it is preferable to only queue small items, especially when called from an ISR. In most cases it would be preferable to store a pointer to the item being queued.

Example usage for buffered IO (where the ISR can obtain more than one value per call):

void vBufferISR( void )
{
char cIn;
BaseType_t xHigherPriorityTaskWoken;

   // We have not woken a task at the start of the ISR.
   xHigherPriorityTaskWoken = pdFALSE;

   // Loop until the buffer is empty.
   do
   {
    // Obtain a byte from the buffer.
    cIn = portINPUT_BYTE( RX_REGISTER_ADDRESS );

    // Post the byte.
    xQueueSendToBackFromISR( xRxQueue, &cIn, &xHigherPriorityTaskWoken );

   } while( portINPUT_BYTE( BUFFER_COUNT ) );

   // Now the buffer is empty we can switch context if necessary.
   if( xHigherPriorityTaskWoken )
   {
    taskYIELD ();
   }
}
Return
pdTRUE if the data was successfully sent to the queue, otherwise errQUEUE_FULL.
Parameters
  • xQueue: The handle to the queue on which the item is to be posted.
  • pvItemToQueue: A pointer to the item that is to be placed on the queue. The size of the items the queue will hold was defined when the queue was created, so this many bytes will be copied from pvItemToQueue into the queue storage area.
  • pxHigherPriorityTaskWoken: xQueueSendToBackFromISR() will set *pxHigherPriorityTaskWoken to pdTRUE if sending to the queue caused a task to unblock, and the unblocked task has a priority higher than the currently running task. If xQueueSendToBackFromISR() sets this value to pdTRUE then a context switch should be requested before the interrupt is exited.

xQueueSendToFront(xQueue, pvItemToQueue, xTicksToWait) xQueueGenericSend( ( xQueue ), ( pvItemToQueue ), ( xTicksToWait ), queueSEND_TO_FRONT )

queue.h

This is a macro that calls xQueueGenericSend().

Post an item to the front of a queue. The item is queued by copy, not by reference. This function must not be called from an interrupt service routine. See xQueueSendFromISR () for an alternative which may be used in an ISR.

Example usage:

struct AMessage
{
   char ucMessageID;
   char ucData[ 20 ];
} xMessage;

uint32_t ulVar = 10UL;

void vATask( void *pvParameters )
{
QueueHandle_t xQueue1, xQueue2;
struct AMessage *pxMessage;

   // Create a queue capable of containing 10 uint32_t values.
   xQueue1 = xQueueCreate( 10, sizeof( uint32_t ) );

   // Create a queue capable of containing 10 pointers to AMessage structures.
   // These should be passed by pointer as they contain a lot of data.
   xQueue2 = xQueueCreate( 10, sizeof( struct AMessage * ) );

   // ...

   if( xQueue1 != 0 )
   {
    // Send an uint32_t.  Wait for 10 ticks for space to become
    // available if necessary.
    if( xQueueSendToFront( xQueue1, ( void * ) &ulVar, ( TickType_t ) 10 ) != pdPASS )
    {
        // Failed to post the message, even after 10 ticks.
    }
   }

   if( xQueue2 != 0 )
   {
    // Send a pointer to a struct AMessage object.  Don't block if the
    // queue is already full.
    pxMessage = & xMessage;
    xQueueSendToFront( xQueue2, ( void * ) &pxMessage, ( TickType_t ) 0 );
   }

   // ... Rest of task code.
}
Return
pdTRUE if the item was successfully posted, otherwise errQUEUE_FULL.
Parameters
  • xQueue: The handle to the queue on which the item is to be posted.
  • pvItemToQueue: A pointer to the item that is to be placed on the queue. The size of the items the queue will hold was defined when the queue was created, so this many bytes will be copied from pvItemToQueue into the queue storage area.
  • xTicksToWait: The maximum amount of time the task should block waiting for space to become available on the queue, should it already be full. The call will return immediately if this is set to 0 and the queue is full. The time is defined in tick periods so the constant portTICK_PERIOD_MS should be used to convert to real time if this is required.

xQueueSendToFrontFromISR(xQueue, pvItemToQueue, pxHigherPriorityTaskWoken) xQueueGenericSendFromISR( ( xQueue ), ( pvItemToQueue ), ( pxHigherPriorityTaskWoken ), queueSEND_TO_FRONT )

queue.h

This is a macro that calls xQueueGenericSendFromISR().

Post an item to the front of a queue. It is safe to use this macro from within an interrupt service routine.

Items are queued by copy not reference so it is preferable to only queue small items, especially when called from an ISR. In most cases it would be preferable to store a pointer to the item being queued.

Example usage for buffered IO (where the ISR can obtain more than one value per call):

void vBufferISR( void )
{
char cIn;
BaseType_t xHigherPrioritTaskWoken;

   // We have not woken a task at the start of the ISR.
   xHigherPriorityTaskWoken = pdFALSE;

   // Loop until the buffer is empty.
   do
   {
    // Obtain a byte from the buffer.
    cIn = portINPUT_BYTE( RX_REGISTER_ADDRESS );

    // Post the byte.
    xQueueSendToFrontFromISR( xRxQueue, &cIn, &xHigherPriorityTaskWoken );

   } while( portINPUT_BYTE( BUFFER_COUNT ) );

   // Now the buffer is empty we can switch context if necessary.
   if( xHigherPriorityTaskWoken )
   {
    taskYIELD ();
   }
}
Return
pdTRUE if the data was successfully sent to the queue, otherwise errQUEUE_FULL.
Parameters
  • xQueue: The handle to the queue on which the item is to be posted.
  • pvItemToQueue: A pointer to the item that is to be placed on the queue. The size of the items the queue will hold was defined when the queue was created, so this many bytes will be copied from pvItemToQueue into the queue storage area.
  • pxHigherPriorityTaskWoken: xQueueSendToFrontFromISR() will set *pxHigherPriorityTaskWoken to pdTRUE if sending to the queue caused a task to unblock, and the unblocked task has a priority higher than the currently running task. If xQueueSendToFromFromISR() sets this value to pdTRUE then a context switch should be requested before the interrupt is exited.

xQueueReceive(xQueue, pvBuffer, xTicksToWait) xQueueGenericReceive( ( xQueue ), ( pvBuffer ), ( xTicksToWait ), pdFALSE )

queue.h

This is a macro that calls the xQueueGenericReceive() function.

Receive an item from a queue. The item is received by copy so a buffer of adequate size must be provided. The number of bytes copied into the buffer was defined when the queue was created.

Successfully received items are removed from the queue.

This function must not be used in an interrupt service routine. See xQueueReceiveFromISR for an alternative that can.

Example usage:

struct AMessage
{
   char ucMessageID;
   char ucData[ 20 ];
} xMessage;

QueueHandle_t xQueue;

// Task to create a queue and post a value.
void vATask( void *pvParameters )
{
struct AMessage *pxMessage;

   // Create a queue capable of containing 10 pointers to AMessage structures.
   // These should be passed by pointer as they contain a lot of data.
   xQueue = xQueueCreate( 10, sizeof( struct AMessage * ) );
   if( xQueue == 0 )
   {
    // Failed to create the queue.
   }

   // ...

   // Send a pointer to a struct AMessage object.  Don't block if the
   // queue is already full.
   pxMessage = & xMessage;
   xQueueSend( xQueue, ( void * ) &pxMessage, ( TickType_t ) 0 );

   // ... Rest of task code.
}

// Task to receive from the queue.
void vADifferentTask( void *pvParameters )
{
struct AMessage *pxRxedMessage;

   if( xQueue != 0 )
   {
    // Receive a message on the created queue.  Block for 10 ticks if a
    // message is not immediately available.
    if( xQueueReceive( xQueue, &( pxRxedMessage ), ( TickType_t ) 10 ) )
    {
        // pcRxedMessage now points to the struct AMessage variable posted
        // by vATask.
    }
   }

   // ... Rest of task code.
}
Return
pdTRUE if an item was successfully received from the queue, otherwise pdFALSE.
Parameters
  • xQueue: The handle to the queue from which the item is to be received.
  • pvBuffer: Pointer to the buffer into which the received item will be copied.
  • xTicksToWait: The maximum amount of time the task should block waiting for an item to receive should the queue be empty at the time of the call. xQueueReceive() will return immediately if xTicksToWait is zero and the queue is empty. The time is defined in tick periods so the constant portTICK_PERIOD_MS should be used to convert to real time if this is required.

BaseType_t xQueueReceiveFromISR(QueueHandle_t xQueue, void *const pvBuffer, BaseType_t *const pxHigherPriorityTaskWoken)

queue.h

Receive an item from a queue. It is safe to use this function from within an interrupt service routine.

Example usage:

QueueHandle_t xQueue;

// Function to create a queue and post some values.
void vAFunction( void *pvParameters )
{
char cValueToPost;
const TickType_t xTicksToWait = ( TickType_t )0xff;

   // Create a queue capable of containing 10 characters.
   xQueue = xQueueCreate( 10, sizeof( char ) );
   if( xQueue == 0 )
   {
    // Failed to create the queue.
   }

   // ...

   // Post some characters that will be used within an ISR.  If the queue
   // is full then this task will block for xTicksToWait ticks.
   cValueToPost = 'a';
   xQueueSend( xQueue, ( void * ) &cValueToPost, xTicksToWait );
   cValueToPost = 'b';
   xQueueSend( xQueue, ( void * ) &cValueToPost, xTicksToWait );

   // ... keep posting characters ... this task may block when the queue
   // becomes full.

   cValueToPost = 'c';
   xQueueSend( xQueue, ( void * ) &cValueToPost, xTicksToWait );
}

// ISR that outputs all the characters received on the queue.
void vISR_Routine( void )
{
BaseType_t xTaskWokenByReceive = pdFALSE;
char cRxedChar;

   while( xQueueReceiveFromISR( xQueue, ( void * ) &cRxedChar, &xTaskWokenByReceive) )
   {
    // A character was received.  Output the character now.
    vOutputCharacter( cRxedChar );

    // If removing the character from the queue woke the task that was
    // posting onto the queue cTaskWokenByReceive will have been set to
    // pdTRUE.  No matter how many times this loop iterates only one
    // task will be woken.
   }

   if( cTaskWokenByPost != ( char ) pdFALSE;
   {
    taskYIELD ();
   }
}
Return
pdTRUE if an item was successfully received from the queue, otherwise pdFALSE.
Parameters
  • xQueue: The handle to the queue from which the item is to be received.
  • pvBuffer: Pointer to the buffer into which the received item will be copied.
  • pxTaskWoken: A task may be blocked waiting for space to become available on the queue. If xQueueReceiveFromISR causes such a task to unblock *pxTaskWoken will get set to pdTRUE, otherwise *pxTaskWoken will remain unchanged.

UBaseType_t uxQueueMessagesWaiting(const QueueHandle_t xQueue)

queue.h

Return the number of messages stored in a queue.

Return
The number of messages available in the queue.
Parameters
  • xQueue: A handle to the queue being queried.

UBaseType_t uxQueueMessagesWaitingFromISR(const QueueHandle_t xQueue)

Utilities to query queues that are safe to use from an ISR.

These utilities should be used only from witin an ISR, or within a critical section.

UBaseType_t uxQueueSpacesAvailable(const QueueHandle_t xQueue)

queue.h

Return the number of free spaces available in a queue. This is equal to the number of items that can be sent to the queue before the queue becomes full if no items are removed.

Return
The number of spaces available in the queue.
Parameters
  • xQueue: A handle to the queue being queried.

xQueueReset(xQueue) xQueueGenericReset( xQueue, pdFALSE )

Reset a queue back to its original empty state.

The return value is now obsolete and is always set to pdPASS.

xQueuePeek(xQueue, pvBuffer, xTicksToWait) xQueueGenericReceive( ( xQueue ), ( pvBuffer ), ( xTicksToWait ), pdTRUE )

queue.h

This is a macro that calls the xQueueGenericReceive() function.

Receive an item from a queue without removing the item from the queue. The item is received by copy so a buffer of adequate size must be provided. The number of bytes copied into the buffer was defined when the queue was created.

Successfully received items remain on the queue so will be returned again by the next call, or a call to xQueueReceive().

This macro must not be used in an interrupt service routine. See xQueuePeekFromISR() for an alternative that can be called from an interrupt service routine.

Example usage:

struct AMessage
{
   char ucMessageID;
   char ucData[ 20 ];
} xMessage;

QueueHandle_t xQueue;

// Task to create a queue and post a value.
void vATask( void *pvParameters )
{
struct AMessage *pxMessage;

   // Create a queue capable of containing 10 pointers to AMessage structures.
   // These should be passed by pointer as they contain a lot of data.
   xQueue = xQueueCreate( 10, sizeof( struct AMessage * ) );
   if( xQueue == 0 )
   {
    // Failed to create the queue.
   }

   // ...

   // Send a pointer to a struct AMessage object.  Don't block if the
   // queue is already full.
   pxMessage = & xMessage;
   xQueueSend( xQueue, ( void * ) &pxMessage, ( TickType_t ) 0 );

   // ... Rest of task code.
}

// Task to peek the data from the queue.
void vADifferentTask( void *pvParameters )
{
struct AMessage *pxRxedMessage;

   if( xQueue != 0 )
   {
    // Peek a message on the created queue.  Block for 10 ticks if a
    // message is not immediately available.
    if( xQueuePeek( xQueue, &( pxRxedMessage ), ( TickType_t ) 10 ) )
    {
        // pcRxedMessage now points to the struct AMessage variable posted
        // by vATask, but the item still remains on the queue.
    }
   }

   // ... Rest of task code.
}
Return
pdTRUE if an item was successfully received from the queue, otherwise pdFALSE.
Parameters
  • xQueue: The handle to the queue from which the item is to be received.
  • pvBuffer: Pointer to the buffer into which the received item will be copied.
  • xTicksToWait: The maximum amount of time the task should block waiting for an item to receive should the queue be empty at the time of the call. The time is defined in tick periods so the constant portTICK_PERIOD_MS should be used to convert to real time if this is required. xQueuePeek() will return immediately if xTicksToWait is 0 and the queue is empty.

BaseType_t xQueuePeekFromISR(QueueHandle_t xQueue, void *const pvBuffer)

queue.h

A version of xQueuePeek() that can be called from an interrupt service routine (ISR).

Receive an item from a queue without removing the item from the queue. The item is received by copy so a buffer of adequate size must be provided. The number of bytes copied into the buffer was defined when the queue was created.

Successfully received items remain on the queue so will be returned again by the next call, or a call to xQueueReceive().

Return
pdTRUE if an item was successfully received from the queue, otherwise pdFALSE.
Parameters
  • xQueue: The handle to the queue from which the item is to be received.
  • pvBuffer: Pointer to the buffer into which the received item will be copied.

void vQueueAddToRegistry(QueueHandle_t xQueue, const char *pcName)

The registry is provided as a means for kernel aware debuggers to locate queues, semaphores and mutexes.

Call vQueueAddToRegistry() add a queue, semaphore or mutex handle to the registry if you want the handle to be available to a kernel aware debugger. If you are not using a kernel aware debugger then this function can be ignored.

configQUEUE_REGISTRY_SIZE defines the maximum number of handles the registry can hold. configQUEUE_REGISTRY_SIZE must be greater than 0 within FreeRTOSConfig.h for the registry to be available. Its value does not effect the number of queues, semaphores and mutexes that can be created - just the number that the registry can hold.

Parameters
  • xQueue: The handle of the queue being added to the registry. This is the handle returned by a call to xQueueCreate(). Semaphore and mutex handles can also be passed in here.
  • pcName: The name to be associated with the handle. This is the name that the kernel aware debugger will display. The queue registry only stores a pointer to the string - so the string must be persistent (global or preferably in ROM/Flash), not on the stack.

const char *pcQueueGetName(QueueHandle_t xQueue)

The queue registry is provided as a means for kernel aware debuggers to locate queues, semaphores and mutexes.

Call pcQueueGetName() to look up and return the name of a queue in the queue registry from the queue’s handle.

Return
If the queue is in the registry then a pointer to the name of the queue is returned. If the queue is not in the registry then NULL is returned.
Parameters
  • xQueue: The handle of the queue the name of which will be returned.

void vQueueUnregisterQueue(QueueHandle_t xQueue)

The registry is provided as a means for kernel aware debuggers to locate queues, semaphores and mutexes.

Call vQueueAddToRegistry() add a queue, semaphore or mutex handle to the registry if you want the handle to be available to a kernel aware debugger, and vQueueUnregisterQueue() to remove the queue, semaphore or mutex from the register. If you are not using a kernel aware debugger then this function can be ignored.

Parameters
  • xQueue: The handle of the queue being removed from the registry.

BaseType_t xQueueIsQueueEmptyFromISR(const QueueHandle_t xQueue)

Utilities to query queues that are safe to use from an ISR.

These utilities should be used only from witin an ISR, or within a critical section.

BaseType_t xQueueIsQueueFullFromISR(const QueueHandle_t xQueue)

Utilities to query queues that are safe to use from an ISR.

These utilities should be used only from witin an ISR, or within a critical section.

xQueueOverwrite(xQueue, pvItemToQueue) xQueueGenericSend( ( xQueue ), ( pvItemToQueue ), 0, queueOVERWRITE )

queue.h

Only for use with queues that have a length of one - so the queue is either empty or full.

Post an item on a queue. If the queue is already full then overwrite the value held in the queue. The item is queued by copy, not by reference.

This function must not be called from an interrupt service routine. See xQueueOverwriteFromISR () for an alternative which may be used in an ISR.

Example usage:

 void vFunction( void *pvParameters )
 {
 QueueHandle_t xQueue;
 uint32_t ulVarToSend, ulValReceived;

    // Create a queue to hold one uint32_t value.  It is strongly
    // recommended *not* to use xQueueOverwrite() on queues that can
    // contain more than one value, and doing so will trigger an assertion
    // if configASSERT() is defined.
    xQueue = xQueueCreate( 1, sizeof( uint32_t ) );

    // Write the value 10 to the queue using xQueueOverwrite().
    ulVarToSend = 10;
    xQueueOverwrite( xQueue, &ulVarToSend );

    // Peeking the queue should now return 10, but leave the value 10 in
    // the queue.  A block time of zero is used as it is known that the
    // queue holds a value.
    ulValReceived = 0;
    xQueuePeek( xQueue, &ulValReceived, 0 );

    if( ulValReceived != 10 )
    {
        // Error unless the item was removed by a different task.
    }

    // The queue is still full.  Use xQueueOverwrite() to overwrite the
    // value held in the queue with 100.
    ulVarToSend = 100;
    xQueueOverwrite( xQueue, &ulVarToSend );

    // This time read from the queue, leaving the queue empty once more.
    // A block time of 0 is used again.
    xQueueReceive( xQueue, &ulValReceived, 0 );

    // The value read should be the last value written, even though the
    // queue was already full when the value was written.
    if( ulValReceived != 100 )
    {
        // Error!
    }

    // ...
}
Return
xQueueOverwrite() is a macro that calls xQueueGenericSend(), and therefore has the same return values as xQueueSendToFront(). However, pdPASS is the only value that can be returned because xQueueOverwrite() will write to the queue even when the queue is already full.
Parameters
  • xQueue: The handle of the queue to which the data is being sent.
  • pvItemToQueue: A pointer to the item that is to be placed on the queue. The size of the items the queue will hold was defined when the queue was created, so this many bytes will be copied from pvItemToQueue into the queue storage area.

xQueueOverwriteFromISR(xQueue, pvItemToQueue, pxHigherPriorityTaskWoken) xQueueGenericSendFromISR( ( xQueue ), ( pvItemToQueue ), ( pxHigherPriorityTaskWoken ), queueOVERWRITE )

queue.h

A version of xQueueOverwrite() that can be used in an interrupt service routine (ISR).

Only for use with queues that can hold a single item - so the queue is either empty or full.

Post an item on a queue. If the queue is already full then overwrite the value held in the queue. The item is queued by copy, not by reference.

Example usage:

 QueueHandle_t xQueue;

 void vFunction( void *pvParameters )
 {
    // Create a queue to hold one uint32_t value.  It is strongly
    // recommended *not* to use xQueueOverwriteFromISR() on queues that can
    // contain more than one value, and doing so will trigger an assertion
    // if configASSERT() is defined.
    xQueue = xQueueCreate( 1, sizeof( uint32_t ) );
}

void vAnInterruptHandler( void )
{
// xHigherPriorityTaskWoken must be set to pdFALSE before it is used.
BaseType_t xHigherPriorityTaskWoken = pdFALSE;
uint32_t ulVarToSend, ulValReceived;

    // Write the value 10 to the queue using xQueueOverwriteFromISR().
    ulVarToSend = 10;
    xQueueOverwriteFromISR( xQueue, &ulVarToSend, &xHigherPriorityTaskWoken );

    // The queue is full, but calling xQueueOverwriteFromISR() again will still
    // pass because the value held in the queue will be overwritten with the
    // new value.
    ulVarToSend = 100;
    xQueueOverwriteFromISR( xQueue, &ulVarToSend, &xHigherPriorityTaskWoken );

    // Reading from the queue will now return 100.

    // ...

    if( xHigherPrioritytaskWoken == pdTRUE )
    {
        // Writing to the queue caused a task to unblock and the unblocked task
        // has a priority higher than or equal to the priority of the currently
        // executing task (the task this interrupt interrupted).  Perform a context
        // switch so this interrupt returns directly to the unblocked task.
        portYIELD_FROM_ISR(); // or portEND_SWITCHING_ISR() depending on the port.
    }
}
Return
xQueueOverwriteFromISR() is a macro that calls xQueueGenericSendFromISR(), and therefore has the same return values as xQueueSendToFrontFromISR(). However, pdPASS is the only value that can be returned because xQueueOverwriteFromISR() will write to the queue even when the queue is already full.
Parameters
  • xQueue: The handle to the queue on which the item is to be posted.
  • pvItemToQueue: A pointer to the item that is to be placed on the queue. The size of the items the queue will hold was defined when the queue was created, so this many bytes will be copied from pvItemToQueue into the queue storage area.
  • pxHigherPriorityTaskWoken: xQueueOverwriteFromISR() will set *pxHigherPriorityTaskWoken to pdTRUE if sending to the queue caused a task to unblock, and the unblocked task has a priority higher than the currently running task. If xQueueOverwriteFromISR() sets this value to pdTRUE then a context switch should be requested before the interrupt is exited.