[Queue Management]

BaseType_t xQueuePeek( QueueHandle_t xQueue, void *pvBuffer, TickType_t xTicksToWait );

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.

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. This must be at least large enough to hold the size of the queue item defined when the queue was created.
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.

If INCLUDE_vTaskSuspend is set to ‘1’ then specifying the block time as portMAX_DELAY will cause the task to block indefinitely (without a timeout).

pdTRUE if an item was successfully received (peeked) from the queue, otherwise pdFALSE.
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.

Copyright (C) Amazon Web Services, Inc. or its affiliates. All rights reserved.

Latest News

FreeRTOS v10.2.1 is available for immediate download. MIT licensed, includes 64-bit RISC-V, NXP Cortex-M33 demo & Nuvoton Cortex-M23 demo & STM32H745 dual core (AMP) demo.

New Updates to MIT licensed IoT MQTT and POSIX libraries are now available. New library IoT HTTPS is available.  Includes demo projects.

View a recording of the “OTA Update Security and Reliability” webinar, presented by TI and AWS.


FreeRTOS and other embedded software careers at AWS.

FreeRTOS Partners
ARM Connected RTOS partner for all ARM microcontroller cores
Espressif ESP32
IAR Partner
Microchip Premier RTOS Partner
STMicro RTOS partner supporting ARM7, ARM Cortex-M3, ARM Cortex-M4 and ARM Cortex-M0
Texas Instruments MCU Developer Network RTOS partner for ARM and MSP430 microcontrollers
OpenRTOS and SafeRTOS
Xilinx Microblaze and Zynq partner