[TUHS] Re {TUHS} Synchronous vs Asynchronous IO in Unix

Clem Cole clemc at ccc.com
Sat Sep 26 09:16:41 AEST 2015

Sadly I have heard a number of stories/expereiences like this.   That's why
the original Posix.4 specification had a new API: asynchronous system traps
(AST) similar to what most other real time systems have had such as RSX,
VMS or VxWorks for that matter and true async I/O calls. The idea was
instead of trying to "fix" signals or read/write, put a new (optional) API
in that had the proper semantics for a real IPC (like being queued, having
priority etc...).    The AST call was later removed and z queued hacked was
splitter into signals although async I/O did stay.   I was sad to see AST
go.  I always felt it was better to not "fix" it, but rather offer and
optional way that me the needs of people that wanted it.

FWIW:  I've implemented AST a couple of times in my career into UNIX
systems.   Very handy why you really want those semantics.


On Fri, Sep 25, 2015 at 6:05 PM, Dave Horsfall <dave at horsfall.org> wrote:

> On Mon, 21 Sep 2015, Doug McIlroy wrote:
> > Signal() was there first and foremost to support SIGKILL; it did not
> > purport to provide a sound basis for asynchronous IPC.
> Yeah, brother.  In a previous life, I got burned very badly when I relied
> upon signals Doing the Right Thing [tm].
> I think it was dump/restor, on a BSDi box; the damned thing communicated
> betwixt its child processes with signals, and I lost every backup tape
> without realising it.
> --
> Dave Horsfall DTM (VK2KFU)  "Those who don't understand security will
> suffer."
>               Ph'nglui mglw'nafh Cthulhu R'lyeh wgah'nagl fhtagn!
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