Path: utzoo!utgpu!water!watmath!clyde!att-cb!att-ih!pacbell!ames!mailrus!umix!uunet!mcvax!botter!star!jos
From: jos@cs.vu.nl (Jos Warmer)
Newsgroups: comp.software-eng
Subject: Re: Question Re: Configuration Management
Message-ID: <667@kajak.cs.vu.nl>
Date: 8 Mar 88 11:51:16 GMT
References: <497@aimt.UUCP> <5257@well.UUCP> <2984@metavax.UUCP> <2985@metavax.UUCP> <34359UH2@PSUVM>
Reply-To: jos@cs.vu.nl (Jos Warmer)
Organization: VU Informatica, Amsterdam
Lines: 39

In article <34359UH2@PSUVM> UH2@PSUVM.BITNET (Lee Sailer) writes:
>How does this "smart linker" business tie into the "shared libraries"
>in Unix V.3.  As I understand it, (1) when I need a module, the whole
>library is loaded, but (2) when another program needs a module from
>the library, it shares the core image that is already in memory.
>
>So, for example, at any moment, there is only one copy of all the stdio
>(that's standard input-output in Unix-speak) stuff in memory at any given
>moment, and all programs that need it share.  (This also makes the
>executables smaller and saves disk space and load time.)
>
>                                     Just asking,       Lee

Shared libraries do have the advantage of making the
executables smaller.  There is however also a disadvantage
which we stumbled across last week.

We have a program installed under VMS at a client of us.
The program was tested and worked perfectly well for several months.
Then all of a sudden it started to produce erroneous output,
and we got an alarming telephone call.

What happened ?

The day things went wrong, they had a new version of the VMS
C compiler installed, including of course the (shared) libraries.
The errors in the unchanged working program emerged from an
error in the new libraries.

Moral:

    Replacing shared libraries with new versions could make
    lots of working programs crash.  Because those libraries are
    instantaneously used in almost ALL running programs, one
    error in such a library can be fatal for the complete system.

	Jos Warmer,
	VU Informatica Amsterdam
	jos@cs.vu.nl