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From: crowl@cs.rochester.edu (Lawrence Crowl)
Newsgroups: comp.lang.c
Subject: Re: Re: Arrays of Unknown Length in Structures (and sizeof)
Message-ID: <3260@sol.ARPA>
Date: Thu, 15-Oct-87 13:14:41 EDT
Article-I.D.: sol.3260
Posted: Thu Oct 15 13:14:41 1987
Date-Received: Sat, 17-Oct-87 08:48:32 EDT
References: <243@mit-prep.ARPA> <1044@ius1.cs.cmu.edu> <668@its63b.ed.ac.uk> <3453@diku.UUCP> <1609@geac.UUCP>
Reply-To: crowl@cs.rochester.edu (Lawrence Crowl)
Organization: U of Rochester, CS Dept, Rochester, NY
Lines: 21
Keywords: structs x.400 unix

In article <1609@geac.UUCP> daveb@geac.UUCP (Dave Collier-Brown) writes:
>Most of the other examples of variable-sized structs were attempts to save a
>pointer-indirection instruction.  Since indirections are usually cheap (even
>under GCOS), avoidance of them is *not* a win if the price is hard-to-maintain
>variable-size struct-munging code.  And on many machines (Vax, HW-BULL,
>Motorola), the cost of an indirection in space and time works out to less than
>the extra calculations for doing anything more than allocating and discarding
>the variable-size structs.

Variable-size structs are needed in shared memory environments were a data
structure can be mapped at many different addresses.  For example, consider a
shared-memory implementation of a message buffer.  It has a fixed size header
followed by a variable number of message slots.  Because the message buffer may
reside at different addresses, the header cannot contain a pointer to the
variable number of slots.  This could also be solved with relative pointers.
To directly support such structures, at least one of the two methods must be
provided.
-- 
  Lawrence Crowl		716-275-9499	University of Rochester
		      crowl@cs.rochester.edu	Computer Science Department
...!{allegra,decvax,rutgers}!rochester!crowl	Rochester, New York,  14627