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From: cchase@ee.cornell.edu (Craig M. Chase)
Newsgroups: comp.lang.c
Subject: Two variables for the price of one
Keywords: C Preprocessor
Message-ID: <1990Aug26.144403.17638@batcomputer.tn.cornell.edu>
Date: 26 Aug 90 14:44:03 GMT
Sender: rtaylor@batcomputer.tn.cornell.edu (Russ Taylor)
Organization: Cornell Dept. of Electrical Engineering
Lines: 42

Hi, I'm working on a a project involving the direct manipulation of 
some hardware registers in some custom circuitry.  For reasons that 
I won't go into here, we've got a bunch of registers that work as
pairs.  What I mean is that (unless something goes drastically 
wrong) these registers will always have the save value.  When I 
write to one, I really want to write to both.

So... my question is, does anybody know any neat tricks in the 
C preprocessor to define a single name that will work with both
registers.  As a seed suggestion, let me tell you what I tried:

#define fancy_reg reg1 = reg2

So, if I do
   fancy_reg = 0xFFFF;


the preprocessor generates:
   reg1 = reg2 = 0xFFFF;

which is great.  

Problem is, if I try:
   if (fancy_reg == 0xFFFF)
I get:
   if (reg1 = reg2 == 0xFFFF)
and that doesn't do the right thing (== binds tighter than =, right?)

The only solution to this that I've seen is to always use parenthesis
around fancy_reg when I want to read the register pair, and avoid
the parenthesis when I want to write them.  This solution is hardly
preferable to manipulating the two registers independently.

Any suggestions?

Craig Chase

-- 
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Craig Chase            |         Cornell University
cchase@ee.cornell.edu  | Department of Electrical Engineering
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