Multiple thread acess same uorb toptic

1

When I read the code,I find there are some thread which access the same uorb topic,
that is not advertised and subscribed with the function orb_advertise_multi and orb_subscribe_multi.
So I read the the uorb modules,the read function of uorb topic is:

ssize_t
uORB::DeviceNode::read(struct file *filp, char *buffer, size_t buflen)
{
SubscriberData *sd = (SubscriberData *)filp_to_sd(filp);

/* if the object has not been written yet, return zero */
if (_data == nullptr) {
	return 0;
}

/* if the caller's buffer is the wrong size, that's an error */
if (buflen != _meta->o_size) {
	return -EIO;
}

/*
 * Perform an atomic copy & state update
 */
irqstate_t flags = px4_enter_critical_section();

if (_generation > sd->generation + _queue_size) {
	/* Reader is too far behind: some messages are lost */
	_lost_messages += _generation - (sd->generation + _queue_size);
	sd->generation = _generation - _queue_size;
}

if (_generation == sd->generation && sd->generation > 0) {
	/* The subscriber already read the latest message, but nothing new was published yet.
	 * Return the previous message
	 */
	--sd->generation;
}

/* if the caller doesn't want the data, don't give it to them */
if (nullptr != buffer) {
	memcpy(buffer, _data + (_meta->o_size * (sd->generation % _queue_size)), _meta->o_size);
}

if (sd->generation < _generation) {
	++sd->generation;
}

/* set priority */
sd->set_priority(_priority);

/*
 * Clear the flag that indicates that an update has been reported, as
 * we have just collected it.
 */
sd->set_update_reported(false);

px4_leave_critical_section(flags);

return _meta->o_size;

}

As the description of above code,Multiple thread can read the same topic even if I don’t use orb_advertise_multi and orb_subscribe_multi. Am I correct？

2

The multi versions are for multiple instances of the same topic. For example all gyros publish sensor_gyro, but then there’s instance 0, 1, 2, etc.

3

Hi ,dagar!
I know what you say.Your example for gyros publish with instance 0, 1, 2 exactly are not the same device node.
What I say is that you can use single version advertise and use single subscribe in multiple thread to get the newest data.

4

Yes multiple readers of the same advertised topic is not a problem. Each is its own subscription. My point was that multi is a separate topic.