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What's the problem?

What's the problem?

Posted Nov 15, 2010 11:48 UTC (Mon) by rlhamil (guest, #6472)
In reply to: What's the problem? by kevinm
Parent article: Ghosts of Unix Past: a historical search for design patterns

On some systems, pipe(fds) is equivalent to socketpair(AF_UNIX,SOCK_STREAM,0,fds),
in which case one could pass an fd (using the ugly and obscure semantics for doing so over
an AF_UNIX socket) over a pipe.

On some other systems, a pipe is STREAMS based, and STREAMS has its own mechanism
for passing fds over STREAMS pipes. Moreover, an anonymous STREAMS pipe can be given
a name in the filesystem namespace (something distinct from a regular named pipe), and
can have the connld module pushed onto it by the "server" end, in which case each client
opening the named object gets a private pipe to the server, and the server is notified
that it can receive a file descriptor for that. In turn, client and server could then pass other
file descriptors over the resulting private pipe.

(On Solaris, pipe() is STREAMS based; but one can write an LD_PRELOADable object
that redefines pipe() in terms of socketpair(), and most programs that don't specifically
depend on STREAMS pipe semantics won't know the difference.)

Unfortunately, STREAMS is far from universal. As a networking API, it's less popular than
sockets, and as a method of implementing a protocol stack, unless there are shortcuts between
for example IP and TCP, it's not efficient enough for fast (say 1Gb and faster) connections.
But for local use, it's still pretty flexible where available.

For performance, some systems do not implement pipes as either socketpair() or STREAMS.
(I just looked at Darwin 10.6.4; the pipe() implementation was changed away from
socketpair() allegedly for performance, and may not even be bidirectional anymore,
although a minimal few ioctls are still supported, but not fd passing.)

As for other abstractions not often thought of with a file descriptor, let me recall
Apollo Domain OS. Its display manager "transcript pads" IIRC had a presence in the
filesystem namespace. And although on one level they were like a terminal, on another,
although they were append-only, one could for all practical purposes seek backward into
them, equivalent to scrolling back. Moreover, certain graphics modes were permitted
within such a pad, and would actually be replayed when scrolled back to! In addition to that,
files in Domain OS were "typed": they had type handlers that could impose particular record
semantics, or even encapsulate version history functions (their optional DSEE did that,
and was the direct ancestor of ClearCase). More conventional interpretations were possible;
they'd always had type "uasc" (unstructured byte stream), although it had a hidden header
which threw off some block counts; a later "unstruct" type gave more accurate sematics of
a regular Unix file. They could also do some neat namespace tricks: some objects that
weren't strictly directories could nevertheless choose to deal with whatever came after them
in a pathname. So if one opened /path/to/magic/thingie/of/mine, it's possible that
/path/to/magic was in some sense a typed file rather than a system-supported directory,
but could choose to allow as valid that a residual path was passed to it, in which case
it would be implicitly handed thingie/of/mine as something it could use to determine the
initial state it was to provide to whatever opened it. _Very_ flexible! Only some of the
abstractions that Plan 9 (or the seldom-used HURD) promise came close to what
Domain OS could do. If I felt like adding something to my collection, a

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