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From: daveb@geac.UUCP (Dave Collier-Brown)
Newsgroups: comp.lang.c
Subject: Re: Re: Arrays of Unknown Length in Structures (and sizeof)
Message-ID: <1609@geac.UUCP>
Date: Wed, 14-Oct-87 10:23:02 EDT
Article-I.D.: geac.1609
Posted: Wed Oct 14 10:23:02 1987
Date-Received: Thu, 15-Oct-87 19:47:09 EDT
References: <243@mit-prep.ARPA> <1044@ius1.cs.cmu.edu> <668@its63b.ed.ac.uk> <3453@diku.UUCP>
Reply-To: daveb@geac.UUCP (Dave Collier-Brown)
Organization: The little blue rock next to that twinkly star.
Lines: 44
Keywords: structs x.400 unix
Summary: examples, with context


  A bit of side-discussion on the question:  here's some examples from
real things which use both variable and fixed-size structs, and why.

  Fixed:
	This one's easy.  Unix wants to have the compiler generate
efficient code for stepping through small, possibly ordered, tables of
data. Therefore it defines p++ as "increment p by sizeof the things p
points to, and the code generated looks like:
	add	regP,size
	compare	regX,offset+regP
	...
when searching a table of elements of size "size" for something at
offset "offset" in each element.  For example, disk-cache buffer
headers. 

  variable:
	Not so easy.  The best example I've seen was declarations for
describing x.400-like hierarchical message structures...

   +--------------------------------------------------------------------+
   || tag || length || subtag | length | data | subtag | length | data ||
   +--------------------------------------------------------------------+
                     +------------------------+------------------------+
                        an embedded record         another one

   +--------------------------------------------------------------------+
                        the containing record.

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. 

 --dave (compilers are smawt, hawold) c-b
-- 
 David Collier-Brown.                 {mnetor|yetti|utgpu}!geac!daveb
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