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From: ado@elsie.UUCP
Newsgroups: comp.lang.c
Subject: Re: #if inside #define -- easy to do, but is it a good idea?
Message-ID: <7510@elsie.UUCP>
Date: Sat, 3-Oct-87 13:37:06 EDT
Article-I.D.: elsie.7510
Posted: Sat Oct  3 13:37:06 1987
Date-Received: Sun, 4-Oct-87 02:44:58 EDT
References: <157@decvax.UUCP> <29339@sun.uucp> <10898@orchid.waterloo.edu>
Organization: NIH-LEC, Bethesda, MD
Lines: 43
Summary: existing techniques

< > > From time immemorial, C has forbidden # control lines inside a
< > > macro definition.
< ...here's a real example where it would be useful...
<     #if !defined(compiler_A)
<     #if !defined(compiler_B)
<     #if !defined(compiler_C)
<     ^%$%^#^%$&^% syntax error since we don't know this compiler.
< 
<     #else  /* compiler_C */
<     #   define ARGSUSED /*ARGSUSED*/
<     #endif /* compiler_C */
< 
<     #else  /* compiler_B */
<     #   define ARGSUSED #argsused
<     #endif /* compiler_B */
< 
<     #else  /* compiler_A */
<     #   define ARGSUSED #pragma argsused
<     #endif /* compiler_A */
< 
< goes into a header file, and a program can simply use the manifest
< ARGSUSED
< in the appropriate places.

Rather than allowing # directives in macros, it suffices to do macro
substitution in directives (as is done by the 4.3BSD C compiler and by the
System V Release 2 compiler, but not by the DECUS C preprocessor).  The
above code then becomes

	#ifdef compiler_A
	#define USEDARGS	pragma argsused
	#elif compiler_B
	#define USEDARGS	argsused
	#elif compiler_C
	#define USEDARGS	/*ARGUSED*/
	#else
		!Funky!Stuff!
	#endif
and a program can simply use
	#USEDARGS
in the appropriate places.
-- 
ado@vax2.nlm.nih.gov	ADO, VAX, and NIH are trademarks of Ampex and DEC.