Why does xQueueReceive throw an unhandled exception (LoadProhibited)? - embedded

Question

I'm working on an esp32 FreeRTOS application with two tasks. Its purpose is to take UART messages received from a peripheral device and transmit them via mqtt to a central broker.
The first task reads input from Serial1, processes the contents into a message structure, and adds it to a FreeRTOS queue:
typedef struct {
int length;
char buffer[AZ_EL_MAX_MESSAGE_LENGTH];
} tag_message_t;
void uart_read_task(void * pvParameters){
BaseType_t xStatus;
tag_message_t tag_message;
while(true) {
while(Serial1.available())
{
first_char = Serial1.read();
if (first_char == '+') // Indicates the beginning of a message
{
for(int i = 0; i < AZ_EL_MAX_MESSAGE_LENGTH; i++)
{
message_buffer[i] = Serial1.read();
if (message_buffer[i] == '\n') // End of message received
{
ESP_LOGV(TAG, "Message found: %s", message_buffer);
strncpy(tag_message.buffer, message_buffer, i + 1);
tag_message.length = i + 1;
xStatus = xQueueSend(xMessagesToSendQueue, (void*) &tag_message, 0);
if (xStatus != pdPASS)
ESP_LOGW(TAG, "Failed to queue message.");
break;
}
}
}
}
vTaskDelay(pdMS_TO_TICKS(20)); // Wait the minimum BLE advertisement period for messages to come in, i.e. 20ms
}
}
The main loop() (which is technically the second FreeRTOS task) then attempts to receive from that queue and transmit over MQTT to a local broker:
void setup()
{
Serial.begin(115200);
// Configure and start WiFi
configure_network();
connect_network();
// Configure the MQTT connection
configure_mqtt_client();
// Configure and create the inter-task queues
xMessagesToSendQueue = xQueueCreate(100, sizeof(tag_message_t));
if (xMessagesToSendQueue == NULL) {
ESP_LOGE(TAG, "Unable to create messaging queue. Will not create UART handling message queue.");
delay(10000);
esp_restart();
} else {
ESP_LOGI(TAG, "Messaging queue generated");
configure_uart();
xTaskCreate(uart_read_task, "UART_Processing", 20000, NULL, 1, NULL);
}
}
void loop()
{
const TickType_t xTicksToWait = pdMS_TO_TICKS(100); // milliseconds to wait
tag_message_t received_message;
if (network_connected) {
connect_mqtt_client();
while(mqtt_client.connected())
{
mqtt_client.loop();
// Process messages on the xMessagesToSendQueue
if (xMessagesToSendQueue != NULL)
{
ESP_LOGI(TAG, "Processing message");
if (xQueueReceive(xMessagesToSendQueue, &received_message, xTicksToWait) == pdPASS)
{
ESP_LOGD(TAG, "Received message, transmitting.");
if(!mqtt_client.publish("aoa", received_message.buffer, received_message.length));
ESP_LOGW(TAG, "Failed transmission.");
}
else
{
vTaskDelay(pdMS_TO_TICKS(50));
}
}
else
{
ESP_LOGE(TAG, "Messages queue is null.");
}
}
} else {
ESP_LOGE(TAG, "WARNING Device not connected to the network. Reconnecting.");
connect_network();
}
delay(5000);
}
I've verified that the MQTT broker works, that it connects to WiFi, and it can properly read messages from Serial1. HOWEVER, the xQueueReceive() call in loop() throws a LoadProhibited exception every time it's called.
Can anyone tell me what I'm getting wrong here?

Answer 1

All, thank you for your help. It turns out this wasn't a FreeRTOS issue. After a little research (i.e. reading this and watching a more experienced engineer explain things: [link]https://hackaday.com/2017/08/17/secret-serial-port-for-arduinoesp32/) it turns out ESP32 Serial1 pins are connected to flash memory.
Every time I tried Serial1.read() or Serial1.readBytesUntil() the ESP32 crashed. Turns out reading the flash is taboo?
I replaced '''Serial1.read()''' with '''Serial2.read()''' and others. That fixed everything. Now I'm off to optimizing my queues!

Answer 2

You are trying to receive an address on the Queue without casting it.
2 solutions: You either declare received_message as a Pointer:
tag_message_t* received_message;
...
received_message = new tag_message_t;
if (xQueueReceive(xMessagesToSendQueue, received_message, xTicksToWait) == pdPASS)
and don't forget to delete it right after usage.
Or you can cast it after receiving :
if (xQueueReceive(xMessagesToSendQueue, &received_message, xTicksToWait) == pdPASS)
{
received_message = *(static_cast<tag_message_t*>(&received_message));
ESP_LOGD(TAG, "Received message, transmitting.");
if(!mqtt_client.publish("aoa", received_message.buffer, received_message.length));
ESP_LOGW(TAG, "Failed transmission.");
}
or any other sort of dereferencing you might wanna try.
There is also the possibility xQueueReceive is already doing that! So let's be sure of what is going on by adding this:
ESP_LOGD(TAG, "Received message, transmitting. %s", received_message.buffer);
Right after you get the message.
I don't think tag_message is being deleted inside the task, so if the struct is still valid and present, if you properly parse/cast its address, you should be able to acquire the message without any issues.