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xQueuePeek() 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.
}
Syntax
Implemented in ![]()
queue.c:1636
Return value
pdTRUE if an item was successfully received from the queue, otherwise pdFALSE.
Related Functions
Found 77 other functions taking a ![]()
QueueDefinition
argument:
Function
Description
Put a Message to a Queue.
Release a Semaphore token
Get a Message or Wait for a Message from a Queue.
Wait until a Semaphore token becomes available
Release a Mutex that was obtained by osMutexWait
Wait until a Mutex becomes available
Delete a queue - freeing all the memory allocated for storing of items placed on the queue.
It is preferred that the macros xQueueSendFromISR(), xQueueSendToFrontFromISR() and xQueueSendToBackFromISR() be used in place of calling this function directly. xQueueGiveFromISR() is an equivalent for use by semaphores that don't actually copy any data. Post an item on 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 xHigherPriorityTaskWokenByPost;
// We have not woken a task at the start of the ISR.
xHigherPriorityTaskWokenByPost = pdFALSE;
// Loop until the buffer is empty.
do
{
// Obtain a byte from the buffer.
cIn = portINPUT_BYTE( RX_REGISTER_ADDRESS );
// Post each byte.
xQueueGenericSendFromISR( xRxQueue, &cIn, &xHigherPriorityTaskWokenByPost, queueSEND_TO_BACK );
} while( portINPUT_BYTE( BUFFER_COUNT ) );
// Now the buffer is empty we can switch context if necessary. Note that the
// name of the yield function required is port specific.
if( xHigherPriorityTaskWokenByPost )
{
portYIELD_FROM_ISR();
}
}
It is preferred that the macros xQueueSend(), xQueueSendToFront() and xQueueSendToBack() are used in place of calling this function directly. 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( xQueueGenericSend( xQueue1, ( void * ) &ulVar, ( TickType_t ) 10, queueSEND_TO_BACK ) != 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;
xQueueGenericSend( xQueue2, ( void * ) &pxMessage, ( TickType_t ) 0, queueSEND_TO_BACK );
}
// ... Rest of task code.
}
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.
}
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.
}
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 the number of messages stored in a queue.
Delete a Semaphore
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.
Delete a Mutex
Delete a Message Queue
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().
Returns the current count value of a counting semaphore
Get the number of messaged stored in a queue.
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 the number of messages stored in a queue.
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.
Delete a queue - freeing all the memory allocated for storing of items placed on the queue.
Receive an item from a queue without removing the item from the queue.
Get the available space in a message queue.
Release a Recursive Mutex
Release a Recursive Mutex