Quality RTOS & Embedded Software

 Real time embedded FreeRTOS RSS feed 
Quick Start Supported MCUs PDF Books Trace Tools Ecosystem TCP & FAT Training

[Queue Management]

queue. h
BaseType_t xQueueReceiveFromISR ( QueueHandle_t xQueue, void *pvBuffer, BaseType_t *pxHigherPriorityTaskWoken );

Receive an item from a queue. It is safe to use this function from within 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.
pxHigherPriorityTaskWoken A task may be blocked waiting for space to become available on the queue. If xQueueReceiveFromISR causes such a task to unblock *pxHigherPriorityTaskWoken will get set to pdTRUE, otherwise *pxHigherPriorityTaskWoken will remain unchanged.

From FreeRTOS V7.3.0 pxHigherPriorityTaskWoken is an optional parameter and can be set to NULL.

pdTRUE if an item was successfully received from the queue, otherwise pdFALSE.
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 xTaskWokenByReceive will have been set to
        pdTRUE.  No matter how many times this loop iterates only one
        task will be woken. */

    if( xTaskWokenByReceive != pdFALSE )
        /* We should switch context so the ISR returns to a different task.
        NOTE:  How this is done depends on the port you are using.  Check
        the documentation and examples for your port. */
        taskYIELD ();

[ Back to the top ]    [ About FreeRTOS ]    [ Sitemap ]    [ ]

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

Latest News

FreeRTOS kernel V10 is available for immediate download. Now MIT licensed.

FreeRTOS Partners

ARM Connected RTOS partner for all ARM microcontroller cores

IAR Partner

Microchip Premier RTOS Partner

RTOS partner of NXP for all NXP ARM microcontrollers

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