[PATCH v2 3/6] man/man7/unix.7: SIOCINQ: Document behavior for SOCK_SEQPACKET

["Günther Noack" <gnoack3000@gmail.com>]

Document the SIOCINQ behavior for SOCK_SEQPACKET,
which sums up the lengths of all messages in the receive queue.

Restructure sentence order, so that socket type differences are
discussed in on paragraph, and commonalities in the next.

For reference, the SIOCINQ lookup is implemented in
unix_inq_len() in linux.git net/unix/af_unix.c.

This function first rejects SIOCINQ for listening sockets with
-EINVAL, then does a case analysis for the three types of Unix Domain
Sockets:

For SOCK_STREAM, the result is:

	return READ_ONCE(unix_sk(sk)->inq_len);

For SOCK_SEQPACKET, the result is calculated using:

	skb_queue_walk(&sk->sk_receive_queue, skb)
        	amount += unix_skb_len(skb);

For SOCK_DGRAM, the result is:

	skb = skb_peek(&sk->sk_receive_queue);
	if (skb)
		amount = skb->len;

Cc: Kuniyuki Iwashima <kuniyu@google.com>
Assisted-by: LLM
Signed-off-by: Günther Noack <gnoack3000@gmail.com>
---

Notes:
    I double checked this all in the code, and quoted the code.  Both the
    commit message and commit body were substantially edited based on the
    initial LLM-based version.  (Nevertheless, I would have likely missed
    the inaccuracy if the LLM hadn't found it.)
    
    The LLM initially just slapped an additional sentence into the
    paragraph, but that was mixing the socket-type-specific concerns with
    the general description -- I split that paragraph apart so that we
    have one paragraph for the three socket types and one for the
    socket-type-independent description.

 man/man7/unix.7 | 15 ++++++++-------
 1 file changed, 8 insertions(+), 7 deletions(-)

diff --git a/man/man7/unix.7 b/man/man7/unix.7
index d0c1765dfd79..c370b021589a 100644
--- a/man/man7/unix.7
+++ b/man/man7/unix.7
@@ -605,7 +605,15 @@ .SS Ioctls
 .B SIOCINQ
 For
 .B SOCK_STREAM
+and
+.B SOCK_SEQPACKET
 sockets, this call returns the number of unread bytes in the receive buffer.
+For
+.B SOCK_DGRAM
+sockets,
+this call returns size of the next pending datagram,
+or 0 when no datagram is pending.
+.IP
 The socket must not be in LISTEN state, otherwise an error
 .RB ( EINVAL )
 is returned.
@@ -625,13 +633,6 @@ .SS Ioctls
 .\" That number is normally larger than the number of bytes of pending
 .\" output.  Since this info is, from userland's point of view, imprecise,
 .\" and it may well change, probably best not to document this now.
-For
-.B SOCK_DGRAM
-sockets,
-the returned value is the same as
-for Internet domain datagram sockets;
-see
-.BR udp (7).
 .SH ERRORS
 .TP
 .B EADDRINUSE
-- 
2.56.0