Re: Pseudoterminal terminology in POSIX

["Michael Kerrisk (man-pages)" <mtk.lists@gmail.com>]

Hi Zack,

On 8/10/20 8:10 PM, Zack Weinberg wrote:
> On Mon, Aug 10, 2020 at 9:21 AM Joerg Schilling
> <Joerg.Schilling@fokus.fraunhofer.de> wrote:
>> Larry Dwyer via austin-group-l at The Open Group <austin-group-l@opengroup.org> wrote:
>>
>>> How about the "control" side and the "terminal" side (of the paired
>>> device files)?
>>
>> The Solaris man pty page since a really long time has this:
>>
>>     By default, 48 pseudo-terminal pairs are configured as  follows:
>>
>>        /dev/pty[p-r][0-9a-f] controller devices
>>        /dev/tty[p-r][0-9a-f] slave devices
>>
>> so I would be OK with "controller" side and "terminal" side.
> 
> (libc-alpha, Michael - sorry about not responding to any of this
> thread last week, my actual job has had me swamped.  I still mean to
> give a whack at revising the glibc manual with this terminology but I
> won't be able to get to it until *next* week at the earliest.)
> 
> I like "terminal side" for the tty[p-r][0-9a-f] | pts/[0-9]+ devices,
> but "control(ler) side" still gives the wrong impression IMNSHO.  The
> pty[p-r][0-9a-f] | ptmx devices don't exert any actual control over
> anything.  They are just the other side of a bidirectional
> communication channel.  It's not like USB, where the "master" side is
> the only one that can initiate a data transfer.

Yes, but on the other hand, the program on the master side is often
providing a some kind of "driving" functionality to operate the
program on the salve slide. So the term "control" here doesn't seem
completely out of place. And Joerg's observation that "controller"
is existing terminology in at least one implementation is an 
interesting data point.

> The relationship between "real" terminals and "pseudo" terminals is
> very much like the relationship between remote network sockets and
> loopback sockets.

Well, maybe, but...

> Data received from, or written to, a "real"
> terminal is transferred over a hardware communications channel from/to
> an external device, such as an RS232 serial line or a
> directly-attached console.  With a "pseudo" terminal, on the other
> hand, the data is transferred over a software queue from/to another
> program running on the same computer (e.g. sshd, screen, xterm).  

... the analogy is not obvious (it was only clear to me after
you elaborated it).

> So I
> think an inside/outside metaphor is more appropriate: how about
> "outside", "exterior", or "external" device for the pty[p-r][0-9a-f] |
> ptmx devices ?

We can certainly add it to the list of candidates, but there
are others proposals that feel better to me.

I'll let the conversation run a bit longer, and then try to
summarize the list of proposals we have so far.

Thanks,

Michael