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Path: utzoo!linus!genrad!mit-eddi!mit-vax!eagle!harpo!seismo!rlgvax!cvl!umcp-cs!chris
From: chris@umcp-cs.UUCP
Newsgroups: net.lang.c
Subject: Re: type casting problem
Message-ID: <139@umcp-cs.UUCP>
Date: Sat, 11-Jun-83 00:27:13 EDT
Article-I.D.: umcp-cs.139
Posted: Sat Jun 11 00:27:13 1983
Date-Received: Sat, 11-Jun-83 11:08:50 EDT
References: decvax.112, <1607@yale-com.UUCP>
Organization: Univ. of Maryland, Computer Science Dept.
Lines: 32

If you're willing to pick up a bunch of other cruft you can
#include <sys/param.h> (at least on x.yBSD) and get the constant
NBBY, number of bits per byte.  You also get NBPW, number of bytes
per word.  You unfortuantely get all sorts of uninteresting stuff
as well.

As for generating extract-macros, getting masks is easy:

#define	UNSIGNEDCHAR(c)		((c)&((1<<NBBY)-1))

The shift factor is just NBBY:

#define	MIDDLEBYTE(w)		UNSIGNEDCHAR((w)>>NBBY)

How about an include file called <machdep.h>, containing lots of
things like the above, and/or the version of C being run (say
perhaps #define MANY_UNSIGNEDS means the compiler understands
unsigned long and unsigned char)?  Possibly these might just be
macros, or even typedefs as in <sys/types.h>:

	<machdep.h> on a Vax running 4BSD
	typedef unsigned char u_char;
	#define U_CHAR(c)	(c)

	<machdep.h> on an 11/45 running V7
	typedef char u_char;
	#define U_CHAR(c)	((c)&0377)

and so on.  At least the ``fundamental constants'' (NBBY, NBPW)
should be available someplace other than <sys/param.h>.

			- Chris ({allegra,seismo}!umcp-cs!chris)