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From: ast@cs.vu.nl (Andy Tanenbaum)
Newsgroups: comp.os.minix
Subject: Re: Minix Library Reentrancy
Message-ID: <752@ast.cs.vu.nl>
Date: 24 May 88 21:36:41 GMT
References: <433@moncsbruce.oz> <1532@runx.ips.oz>
Reply-To: ast@cs.vu.nl (Andy Tanenbaum)
Organization: VU Informatica, Amsterdam
Lines: 19

In article <1532@runx.ips.oz> brucee@runx.ips.oz (Bruce Evans) writes:
>
>It is impossible to pass messages on the stack to the kernel, since the
>kernel explicitly checks (in proc.c/mini_send()) that all user messages
>are in the DATA segment (as opposed to the STACK segment).
>
>This seems like an oversight, since stack and data are much the same in C.
>Of course, an extra check to allow the stack segment will mainly slow
>down the kernel. Is there anything more to it than this?
In an early version of MINIX, around V1.-15 or so, you could have messages
in either stack or data space.  This version also swapped, had shared text,
sticky bits, and proper message queues instead of Ada-rendezvous like
message passing as is currently there.  Unfortunately, this version was not
real speedy, so a few features got lopped off, including messages in stack
space.  This feature is rarely needed, and does have a substantial performance
penalty, as the kernel must check which segment the message is in, and if the
entire message is within the segment, etc.

Andy Tanenbaum (ast@cs.vu.nl)