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From: minow@decvax.UUCP (Martin Minow)
Newsgroups: comp.lang.c
Subject: Re: #define CTRL(x) followup (it can be done in ANSI C)
Message-ID: <103@decvax.UUCP>
Date: Mon, 24-Nov-86 19:58:14 EST
Article-I.D.: decvax.103
Posted: Mon Nov 24 19:58:14 1986
Date-Received: Mon, 24-Nov-86 23:54:06 EST
References: <7136@elsie.UUCP> <779@valid.UUCP> <5396@brl-smoke.ARPA> <560@mcgill-vision.UUCP>
Reply-To: minow@decvax.UUCP (Martin Minow)
Organization: Ultrix Eng. Group - Merrimack, NH
Lines: 40

According to what I have heard, the *latest* version of the C spec.
(i.e., the one "available for public comment") will permit CTRL(X)
as follows:

You must provide a header file such as

	#define __A	001
	#define __B	002
	 (etc.)
	#define CTRL(X) __##X

Now, writing CTRL(A) expands to __A which is rescanned, recognized as
a macro, and expanded as 001.  There are a few control characters
that can't be expanded this way, such as <CTRL/@> (NUL).

It should be pointed out that a better approach to this specific problem
would be
	#define NUL	'\000'
	#define SOH	'\001'
	 (etc.)
	#define DEL	'\177'

Then, the control characters will be functionally described.

As I understand the spec, Reiser constructions map into ANSI C as follows:

		Reiser				ANSI
token pasting:	#define foo(a,b) a/**/b		a##b
string repl.:	#define foo(a) "xxx a yyy"	"xxx #a yyy"
char repl.:	#define foo(a) 'a'		Not provided to my knowledge.

It should be pointed out that the Reiser constructions are not provided
in many non-Unix compilers, nor have they been well-documented and
formally specified.

Disclaimer: I am speaking for myself; and am neither a spokesman for
the ANSI committee nor for Dec.

Martin Minow
decvax!minow