SysV IPC: eyesore or misunderstood art?

[Ed Gould]

>>AT&T added a new object space (IPC objects) for shared memory, message
>>queues, and semaphores.  Presumably they had a reason not to want to use the
>>existing object space of file descriptors.  I can imagine ...

>Naah.  They just didn't have the imagination to do it right.  The IPC
>namespace business is an abomination.  ...
>To my mind, the SysV IPC stuff fairly screams:  "glued on to the system
>at the last minute as an afterthought."

The history of Unix development and support gives some clues to
understanding some of the (mis)features in System V.  While AT&T was
prohibited, by a consent decree signed between AT&T and the Justice
Department, from being in the computer business, there was a group with
in the (then) Bell System called the "Unix Support Group" (USG).  USG's
job was to support Unix for the Bell Operating Companies and other
internal Bell System uses.  They were not trying to develop
general-purpose systems, but to meet the specific needs of their
users.

System V shared memory is an example of something that was developed
for some particular Bell need.  In fact, there were two completely
different shared memory schemes at one time - one on PDP11s and one on
VAXen - that were designed for entirely different purposes.  Needless
to say, they were completely incompatible with each other.  Current
System V shared memory - if I'm not mistaken - is essentially the VAX
version.

It wasn't "glued on ... at the last minute," but neither was it
designed originally as a general-purpose mechanism.  It served some
specific need within Bell before there were other available models from
which to choose.

-- 
Ed Gould                    mt Xinu, 2560 Ninth St., Berkeley, CA  94710  USA
ed at mtxinu.COM		    +1 415 644 0146

"I'll fight them as a woman, not a lady.  I'll fight them as an engineer."