# xQueueReceive holds one item sometimes **URL:** \ **Category:** Kernel\ **Created:** [June 10, 2021, 1:30pm UTC](https://forums.freertos.org/t/xqueuereceive-holds-one-item-sometimes/12824 "2021-06-10T13:30:57Z")\ **Posts on this page:** 10\ **Page:** 1
**Author:** ![alex\_temp](https://avatars.discourse-cdn.com/v4/letter/a/ad7895/32.png) [@alex\_temp](https://forums.freertos.org/u/alex_temp)\ **Post date:** [June 10, 2021, 1:30pm UTC](https://forums.freertos.org/t/xqueuereceive-holds-one-item-sometimes/12824/1 "2021-06-10T13:30:58Z")
I have UART transmittion, i use queue to send data from UART IRQ with xQueueSendFromISR and task with xQueueReceive to receive data. Data is struct with char \*data; size\_t len; only. And this works fine for 90% time, but some times xQueueReceive don’t want to receive item from queue, it like hold it inside, but when next UART payload is sended from IRQ, Iam receive the old “holded” data first, and after that present data comes. Does anyone can help me with this strange problem. ---
**Author:** ![richard-damon](https://d3iykfbgfh8zhg.cloudfront.net/user_avatar/forums.freertos.org/richard-damon/32/38_2.png) [@richard-damon](https://forums.freertos.org/u/richard-damon)\ **Post date:** [June 10, 2021, 1:41pm UTC](https://forums.freertos.org/t/xqueuereceive-holds-one-item-sometimes/12824/2 "2021-06-10T13:41:08Z")
First question, do you have confit ASSERT defined and stack overflow checking enabled to catch configuration errors. Second, does the ISR properly use the was woken flag from toxQueueSendFromISR call?. ---
**Author:** ![alex\_temp](https://avatars.discourse-cdn.com/v4/letter/a/ad7895/32.png) [@alex\_temp](https://forums.freertos.org/u/alex_temp)\ **Post date:** [June 10, 2021, 1:52pm UTC](https://forums.freertos.org/t/xqueuereceive-holds-one-item-sometimes/12824/3 "2021-06-10T13:52:44Z")
No, configASSERT off, and configCHECK\_FOR\_STACK\_OVERFLOW off too. I check returning value from xQueueSendFromISR and its good. ---
**Author:** ![richard-damon](https://d3iykfbgfh8zhg.cloudfront.net/user_avatar/forums.freertos.org/richard-damon/32/38_2.png) [@richard-damon](https://forums.freertos.org/u/richard-damon)\ **Post date:** [June 10, 2021, 2:00pm UTC](https://forums.freertos.org/t/xqueuereceive-holds-one-item-sometimes/12824/4 "2021-06-10T14:00:13Z")
Not the return value, but the xHigherPriorityTaskWasWoken parameter, if it sets that you need to invoke the scheduler at the end of you ISR or the task that was woken might not get run for a while. ---
**Author:** ![rtel](https://d3iykfbgfh8zhg.cloudfront.net/user_avatar/forums.freertos.org/rtel/32/31_2.png) [@rtel](https://forums.freertos.org/u/rtel)\ **Post date:** [June 10, 2021, 2:03pm UTC](https://forums.freertos.org/t/xqueuereceive-holds-one-item-sometimes/12824/5 "2021-06-10T14:03:55Z")
Always recommend to make use of the facilities available to help debugging, like configASSERT and stack overflow checking. ---
**Author:** ![RAc](https://d3iykfbgfh8zhg.cloudfront.net/user_avatar/forums.freertos.org/rac/32/1389_2.png) [@RAc](https://forums.freertos.org/u/RAc)\ **Post date:** [June 10, 2021, 2:10pm UTC](https://forums.freertos.org/t/xqueuereceive-holds-one-item-sometimes/12824/6 "2021-06-10T14:10:57Z")
Hi Alex, it would be nice if you provided some more useful info voluntarily… What is the call you use to create the queue with? In particular what queue depth? Can it simply be the case that the task that does the queue receive is starved out by higher priority threads and thus doesn’t have a chance to unqueue the items timely? Remember that the very purpose of a multi entry queue is like a shock absorber; if the characters come in faster than they can be processed, the queue buffers them. That is all defined and desired behavior. Sorry if this should be a shot in the dark, but your problem description is farily generic, so we have to make wild guesses until we get more info. You might want to trace the flow of events with counters throughout your system to get a better idea of the timing involved. Using tracealyzer is also a good idea. ---
**Author:** ![alex\_temp](https://avatars.discourse-cdn.com/v4/letter/a/ad7895/32.png) [@alex\_temp](https://forums.freertos.org/u/alex_temp)\ **Post date:** [June 10, 2021, 3:14pm UTC](https://forums.freertos.org/t/xqueuereceive-holds-one-item-sometimes/12824/7 "2021-06-10T15:14:15Z")
I rty to give you pieces of code from which you must understand the logic of work. ```auto struct { char *data; size_t len; } recv_t; uart3RxQueue = xQueueCreate(15, sizeof(recv_t)); Uart3RxQueueParserHandle = sys_thread_new("Uart3RxQueueParser", Uart3RxQueueParser, NULL, 1000, 7); ``` //RX TASK ```auto void Uart3RxQueueParser(){ recv_t recv_buff2; BaseType_t QueueGood; while(1){ ack_cnt2++; do QueueGood = xQueueReceive(uart3RxQueue, &recv_buff2, portMS_TO_TASK_TICKS(5000));//portMAX_DELAY); //when issue happens i get here but dont receive data while(QueueGood != pdTRUE); if (QueueGood != pdTRUE) { uartRecvQueueErr++; } receiving_cnt++; parse_received(&recv_buff2); free(recv_buff2.data); ... } } ``` //UART IRQ ```auto volatile recv_t recv_buff; volatile char uartRxStaticBuff[UART3_RXBUF_SIZE]; volatile uint32_t uartRxChar=0; void UART3_IRQHandler(void) { uartRxStaticBuff[uartRxChar] = (char)UART_REG; uartRxChar++; ... if (uartRxChar == UART3_RXBUF_SIZE-1) { recv_buff.len = UART3_RXBUF_SIZE; uartRxChar=0; uartQueueSended++; recv_buff.data = (char*)malloc(recv_buff.len); memcpy(recv_buff.data,&uartRxStaticBuff,recv_buff.len); QueueGood = xQueueSendFromISR(uart3RxQueue, &recv_buff, &xHigherPriorityTaskWoken); uartQueueSended2++; if (QueueGood != pdTRUE) { error_rxQueue_cnt++; } portYIELD_FROM_ISR(xHigherPriorityTaskWoken); } ... } ``` so most of time uartQueueSended,uartQueueSended2,receiving\_cnt are the same, but after issue happens receiving\_cnt becomes smaller by 1 ---
**Author:** ![richard-damon](https://d3iykfbgfh8zhg.cloudfront.net/user_avatar/forums.freertos.org/richard-damon/32/38_2.png) [@richard-damon](https://forums.freertos.org/u/richard-damon)\ **Post date:** [June 10, 2021, 3:23pm UTC](https://forums.freertos.org/t/xqueuereceive-holds-one-item-sometimes/12824/8 "2021-06-10T15:23:13Z")
Are you using a version of malloc that is ISR safe? It normally isn’t ---
**Author:** ![alex\_temp](https://avatars.discourse-cdn.com/v4/letter/a/ad7895/32.png) [@alex\_temp](https://forums.freertos.org/u/alex_temp)\ **Post date:** [June 13, 2021, 3:29pm UTC](https://forums.freertos.org/t/xqueuereceive-holds-one-item-sometimes/12824/9 "2021-06-13T15:29:11Z")
its generic malloc, but before i used fixed array for recv\_buff.data and don’t call malloc in irq, but result stay same. Also i forgot to tell that i use LPC1788 with FreeRTOS V8.2.0. I hardly know FreeRTOS, so i think may be important to mention this defines(because its related to IRQ): configPRIO\_BITS = 5 IRQ\_PRI\_RTOS\_KERNEL = 31 IRQ\_PRI\_MAX\_SYSCALL\_RTOS = 15 configKERNEL\_INTERRUPT\_PRIORITY = (IRQ\_PRI\_RTOS\_KERNEL \<\< (8 - configPRIO\_BITS)) configMAX\_SYSCALL\_INTERRUPT\_PRIORITY = (IRQ\_PRI\_MAX\_SYSCALL\_RTOS \<\< (8 - configPRIO\_BITS)) IRQ\_PRI\_UART3 = 4 ---
**Author:** ![richard-damon](https://d3iykfbgfh8zhg.cloudfront.net/user_avatar/forums.freertos.org/richard-damon/32/38_2.png) [@richard-damon](https://forums.freertos.org/u/richard-damon)\ **Post date:** [June 13, 2021, 7:34pm UTC](https://forums.freertos.org/t/xqueuereceive-holds-one-item-sometimes/12824/10 "2021-06-13T19:34:45Z")
Your use of malloc there likely risks corrupting the heap, as it is almost certainly not save to use in an ISR then. The code looks ok, I presume that error\_rxQueue\_cnt stays 0, as if a xQueueSendFromISR fails you will increment it.