Embedded Files
1. Introduction
- A queue is a First In First Out (FIFO) buffer.
- In FreeRTOS:
+ Queue is used to stored data/message that is exchanged among tasks.
+ Data will be copied (not reference) to queue.
+ Access by Multiple Tasks
+ Blocking on Queue Reads (a task moves to Blocked state to wait for data)
+ Blocking on Queue Writes (a task moves to Blocked state to wait until free space is available on the queue)
+ Blocking on Multiple Queues (a task moves to Blocked state to wait for data on a group of queues).
2. Demo
In this demo, we will create 3 tasks, 2 sending tasks and 1 receiving task. Sending task will send data that contains sender id and its counter.
/* structure that hold data*/typedef struct{ int sender; int counter;}Data;/* this variable hold queue handle */xQueueHandle xQueue;void setup() { Serial.begin(112500); /* create the queue which size can contains 5 elements of Data */ xQueue = xQueueCreate(5, sizeof(Data)); xTaskCreate( sendTask1, /* Task function. */ "sendTask1", /* name of task. */ 10000, /* Stack size of task */ NULL, /* parameter of the task */ 2, /* priority of the task */ NULL); /* Task handle to keep track of created task */ xTaskCreate( sendTask2, /* Task function. */ "sendTask2", /* name of task. */ 10000, /* Stack size of task */ NULL, /* parameter of the task */ 2, /* priority of the task */ NULL); /* Task handle to keep track of created task */ xTaskCreate( receiveTask, /* Task function. */ "receiveTask", /* name of task. */ 10000, /* Stack size of task */ NULL, /* parameter of the task */ 1, /* priority of the task */ NULL); /* Task handle to keep track of created task */}void loop() {}void sendTask1( void * parameter ){ /* keep the status of sending data */ BaseType_t xStatus; /* time to block the task until the queue has free space */ const TickType_t xTicksToWait = pdMS_TO_TICKS(100); /* create data to send */ Data data; /* sender 1 has id is 1 */ data.sender = 1; data.counter = 1; for(;;){ Serial.println("sendTask1 is sending data"); /* send data to front of the queue */ xStatus = xQueueSendToFront( xQueue, &data, xTicksToWait ); /* check whether sending is ok or not */ if( xStatus == pdPASS ) { /* increase counter of sender 1 */ data.counter = data.counter + 1; } /* we delay here so that receiveTask has chance to receive data */ delay(1000); } vTaskDelete( NULL );}/* this task is similar to sendTask1 */void sendTask2( void * parameter ){ BaseType_t xStatus; const TickType_t xTicksToWait = pdMS_TO_TICKS(100); Data data; data.sender = 2; data.counter = 1; for(;;){ Serial.println("sendTask2 is sending data"); xStatus = xQueueSendToFront( xQueue, &data, xTicksToWait ); if( xStatus == pdPASS ) { data.counter = data.counter + 1; } delay(1000); } vTaskDelete( NULL );}void receiveTask( void * parameter ){ /* keep the status of receiving data */ BaseType_t xStatus; /* time to block the task until data is available */ const TickType_t xTicksToWait = pdMS_TO_TICKS(100); Data data; for(;;){ /* receive data from the queue */ xStatus = xQueueReceive( xQueue, &data, xTicksToWait ); /* check whether receiving is ok or not */ if(xStatus == pdPASS){ /* print the data to terminal */ Serial.print("receiveTask got data: "); Serial.print("sender = "); Serial.print(data.sender); Serial.print(" counter = "); Serial.println(data.counter); } } vTaskDelete( NULL );}
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