Zephyr Ring Buffer: why is no wrap-around happening?

RE: Zephyr Ring Buffer: why is no wrap-around happening?

nik2k — Fri, 28 Feb 2025 08:16:31 GMT

Hi, this seems like a way more comprehensive approach than the simple scan + dump routine that I had in mind. I ditched the ring buffer idea and am using a k_fifo to send the devices from the ble-scan-callback to the main thread. I'll try putting the table logic in main. Thanks a lot for taking the time to write such an informative response!

RE: Zephyr Ring Buffer: why is no wrap-around happening?

nik2k — Fri, 28 Feb 2025 08:09:18 GMT

Hi, thanks for the clarification. I definitely misunderstood the intended use case of the ring buffer API. I studied the code you linked a little more and am now using a ringbuffer somewhere else in my code (for offloading UART data processing).

RE: Zephyr Ring Buffer: why is no wrap-around happening?

Susheel Nuguru — Wed, 26 Feb 2025 14:07:21 GMT

I think you misunderstood, or the documentation is not clear enough.

The ring buffer supports wraparound but only if the space is free when wrapping around.

At this line in ring_buf_item_put, there is an explicit check to see if the new item is going to a free space or not. In other words, overwriting is not allowed.

RE: Zephyr Ring Buffer: why is no wrap-around happening?

Jakob Ruhe — Wed, 26 Feb 2025 09:29:34 GMT

The function `ring_buf_put()` returns the number of bytes written to the ringbuffer. In your case the ringbuffer gets full after the first 2 writes and therefore you will not be able to write anything more to it before you remove something from it.

A side note: You can build and run the above example on your machine instead by building for the `native_sim` board. It is very powerful.

Noe, I am not sure exactly what you want to accomplish but to me it sounds that a ringbuffer is not ideal for your use case.

If you want to keep track of some devices, perhaps count number of packets from them or whatever, then just use a table. I shared a code snippet for that in this thread:

Scan does not shows short/long names for HR peripheral(s) - Nordic Q&A - Nordic DevZone - Nordic DevZone

The code snippet looks like this:

#define NAME_LEN 30

typedef struct {
  bool used;
  bt_addr_le_t address;
  char name[NAME_LEN];
  int64_t last_seen;
} device_t;

static device_t s_devices[50];

device_t *find_device(const bt_addr_le_t *address)
{
	for (ssize_t i = 0; i < ARRAY_SIZE(s_devices); i++) {
		if (s_devices[i].used && bt_addr_le_eq(&s_devices[i].address, address)) {
			return &s_devices[i];
		}
	}
	return NULL;
}

device_t *add_device(const bt_addr_le_t *address)
{
	for (ssize_t i = 0; i < ARRAY_SIZE(s_devices); i++) {
		if (!s_devices[i].used) {
			s_devices[i].used = true;
			s_devices[i].address = *address;
			s_devices[i].last_seen = k_uptime_get();
			return &s_devices[i];
		}
	}
	return NULL;
}

I leave as an exercise to you to write a function that goes through the table and sets `used` to `false` if a device has not been heard for a while.

Good luck!