netdev - TIOCOUTQ implementation for sockets vs. tty

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Message-ID: <87ft12ri0t.fsf@henneberg-systemdesign.com>
Date:   Wed, 10 Mar 2021 19:16:34 +0100
From:   Henneberg - Systemdesign <lists@...neberg-systemdesign.com>
To:     netdev@...r.kernel.org
Subject: TIOCOUTQ implementation for sockets vs. tty

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?

Regards
-Jochen