Re: TIOCOUTQ implementation for sockets vs. tty

Willy Tarreau [off-list ref] writes:

Hi,

On Wed, Mar 10, 2021 at 07:16:34PM +0100, Henneberg - Systemdesign wrote:

quoted

Hi,

I have a question regarding the implementation of ioctl TIOCOUTQ for
various sockets compared to the tty implementation.

For several sockets, e. g. AF_BLUETOOTH it is done like this

af_bluetooth.c:
case TIOCOUTQ:
	if (sk->sk_state == BT_LISTEN)
		return -EINVAL;

	amount = sk->sk_sndbuf - sk_wmem_alloc_get(sk);
	if (amount < 0)
		amount = 0;
	err = put_user(amount, (int __user *)arg);
	break;

so the ioctl returns the available space in the send queue if I
understand the code correctly (this is also what I observed from tests).

The tty does this:

n_tty.c:
case TIOCOUTQ:
	return put_user(tty_chars_in_buffer(tty), (int __user *) arg);

so it returns the used space in the send queue. This is also what I
would expect from the manpage description.

Is this mismatch intentional?

At least both man pages (tty_ioctl and tcp(7)) mention that TIOCOUTQ
should return the number of byte in queue.

What I suspect for sockets is that sk_sndbuf grows with allocations
and that sk_wmem_alloc_get() in fact returns the number of unused
allocations thus the difference would be the amount queued. But I
could be wrong and I would tend to read the code the same way as you
did.

I am quite sure that my assumption is right. When I call the ioctl right
before and after writing data to an AF_BLUETOOTH socket the returned
value decreases where it should increase.

-Jochen

Willy

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