Re: [PATCH 8/8] man/man7/unix.7: ERRORS: Document EACCES, EAGAIN, EMSGSIZE, ENOSPC, and EPERM

[Kuniyuki Iwashima <kuniyu@google.com>]

On Wed, 21 Oct 2026 23:08:00 +0200, Günther Noack wrote:
> These are all returned directly from linux.git net/unix/af_unix.c:
> 
> EACCES
> 	unix_find_bsd() checks path_permission(&path, MAY_WRITE) on the
> 	socket file of the remote address.  The rule was already described
> 	in prose, but the error was not listed.
> 
> EAGAIN
> 	unix_stream_connect() returns -EAGAIN when the listener's backlog
> 	is full and the socket is nonblocking (there is no EINPROGRESS for
> 	AF_UNIX).  unix_dgram_sendmsg() returns -EAGAIN when the receiver's
> 	queue is full and the send would block.
> 
> EMSGSIZE
> 	unix_dgram_sendmsg() fails if 'len > sk->sk_sndbuf - 32'.  This is
> 	the limit described in "Sockets API".
> 
> ENOSPC
> 	unix_autobind() fails when all 2^20 autobind names are taken.
> 
> EPERM
> 	unix_dgram_connect() and unix_dgram_sendmsg() fail if
> 	!unix_may_send(), that is, if the target datagram socket is
> 	connected to a socket other than the caller's.
> 
> 	This is not something that the remote end normally does,
> 	but it's a possible corner case.
> 
> Cc: Kuniyuki Iwashima <kuniyu@google.com>
> Assisted-by: LLM
> Signed-off-by: Günther Noack <gnoack3000@gmail.com>
> ---
> 
> Notes:
>     Verified in code.  Kept LLM wording, which seemed OK.
>     
>     Remark:
>     
>     The case where the remote end is connected to a different socket is
>     obscure, but still possible with Unix Domain Sockets, when a program
>     first bind(2)s to a name, and then connect(2)s to another name.  Then
>     that socket can be discovered by another process through the bound
>     name, but sending a dgram fails if the sender is not the one that that
>     socket has connected to.
> 
>  man/man7/unix.7 | 32 ++++++++++++++++++++++++++++++++
>  1 file changed, 32 insertions(+)
> 
> diff --git a/man/man7/unix.7 b/man/man7/unix.7
> index 054f1af75dd5..79198445c581 100644
> --- a/man/man7/unix.7
> +++ b/man/man7/unix.7
> @@ -655,10 +655,25 @@ defined in
>  .\" and it may well change, probably best not to document this now.
>  .SH ERRORS
>  .TP
> +.B EACCES
> +The caller does not have write permission on the socket file

If we align with other doc, "write permission is denied..."

> +specified as the remote address to
> +.BR connect (2)
> +or
> +.BR sendto (2)
> +(see "Pathname socket ownership and permissions", above).
> +.TP
>  .B EADDRINUSE
>  The specified local address is already in use or the filesystem socket
>  object already exists.

s/filesystem socket object/file/

Two sockets can have the same address once one's file
is removed even without closing the socket.


>  .TP
> +.B EAGAIN
> +The socket is nonblocking, and either
> +.BR connect (2)
> +was called on a stream or sequenced-packet socket
> +and the backlog of the listening socket is full,
> +or a datagram was sent to a socket whose receive queue is full.
> +.TP
>  .B EBADF
>  This error can occur for
>  .BR sendmsg (2)
> @@ -694,6 +709,11 @@ invalid state for the applied operation.
>  called on an already connected socket or a target address was
>  specified on a connected socket.
>  .TP
> +.B EMSGSIZE
> +The datagram to be sent is larger than the limit imposed by the
> +.B SO_SNDBUF
> +socket option (see "Sockets API", above).
> +.TP
>  .B ENFILE
>  The system-wide limit on the total number of open files has been reached.
>  .TP
> @@ -705,6 +725,11 @@ did not exist.
>  .B ENOMEM
>  Out of memory.
>  .TP
> +.B ENOSPC
> +The socket was to be autobound,
> +but all autobind addresses are in use
> +(see "Autobind feature", above).
> +.TP
>  .B ENOTCONN
>  Socket operation needs a target address, but the socket is not connected.
>  .TP
> @@ -716,6 +741,13 @@ use the out-of-band data option.
>  The sender passed invalid credentials in the
>  .IR "struct ucred" .
>  .TP
> +.B EPERM
> +.BR connect (2)
> +or
> +.BR sendto (2)
> +specified a datagram socket as the remote address,
> +but that socket is connected to a different socket.
> +.TP
>  .B EPIPE
>  Remote socket was closed on a stream socket.
>  If enabled, a
> -- 
> 2.56.0
>