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From: torek@elf.ee.lbl.gov (Chris Torek)
Newsgroups: comp.std.c
Subject: Re: so how do I do it? (was Re: call to revolt)
Message-ID: <14739@dog.ee.lbl.gov>
Date: 27 Jun 91 13:49:17 GMT
References: <rabson.677868220@physics.ubc.ca> <m0jspJY-0001ieC@shiva.reed.edu> <1991Jun27.115736.18417@tkou02.enet.dec.com>
Reply-To: torek@elf.ee.lbl.gov (Chris Torek)
Organization: Lawrence Berkeley Laboratory, Berkeley
Lines: 47
X-Local-Date: Thu, 27 Jun 91 06:49:17 PDT

In article <1991Jun27.115736.18417@tkou02.enet.dec.com>
diamond@jit533.enet@tkou02.enet.dec.com (Norman Diamond) writes:
>Huh?  Oh, you mean that the buffer was laid out by some other entity
>than your C program.  Yes, C (along with every other language except Ada!)
>fails to provide for manipulations of such data.

Not strictly true:  Mesa (for instance) allows record specifiers that
include actual bit or byte addresses.  In Mesa, though, it was all
or nothing: either you said

	type foo = record [ blah, blah, blah ];

and got arbitrary compiler-munching (including order), or you gave it
the `machine-dependent' clause and got absolute control.

There are a bunch of different concepts that different languages do
or do not allow you to express.  One is:

	This is exactly how the bits work.  Change none of them.
	[Example: Mesa `machine-dependent' records]

which is good for talking to devices or network byte streams.  Another
is:

	This is the data I need carried about.  Optimize for time.
	[Example: Pascal records]

Still another is:

	This is the data I need carried about.  Optimize for space.
	[Example: Pascal packed records]

C takes a somewhat wishy-washy approach and tries to do everything at
once: `struct's are not reorderable, but are not necessarily packed.
As it turns out, though, even Pascal's packed/notpacked approach is
insufficient in some cases: you might want to say `pack this as much as
is convenient, but not so much as to cause order of magnitude
slowdowns'.  Some Pascal compilers treat

	type seven = 0..127;
	var foo : packed array [0..16777215] of sevenbits;

as arrays of bytes, some as arrays of seven-bit fields; the latter
is sometimes severely slower, but does save 2 megabytes.
-- 
In-Real-Life: Chris Torek, Lawrence Berkeley Lab CSE/EE (+1 415 486 5427)
Berkeley, CA		Domain:	torek@ee.lbl.gov