Xref: utzoo comp.std.c:374 comp.lang.c:12593
Path: utzoo!attcan!uunet!island!walker
From: walker@island.uu.net (Richard Walker)
Newsgroups: comp.std.c,comp.lang.c
Subject: Re: Variable length arg lists for macros
Message-ID: <362@island.uu.net>
Date: 14 Sep 88 21:49:33 GMT
References: <438@ucsvc.unimelb.edu.au> <1036@cbnews.ATT.COM> <228@itivax.UUCP>
Reply-To: walker@island.uu.net (Richard Walker)
Organization: Island Graphics, Marin County, California
Lines: 33

In article <228@itivax.UUCP> scs@itivax.UUCP (Steve C. Simmons) writes:
>In article <1036@cbnews.ATT.COM> lvc@cbnews.ATT.COM (Lawrence V. Cipriani) writes:
>-In article <438@ucsvc.unimelb.edu.au>, u5565522@ucsvc.unimelb.edu.au (David Clunie) writes:
>-
>-	#ifdef TRACE
>-	#define trace(anything)	anything
>-	#else
>-	#define trace(anything)
>-	#endif
>-
>	if ( x == m )
>		trace( printf( "the consequences\n) ; )
>	froob( some function ) ;
>
>This produced rather different programs depending on the definition of
>'trace'!

The problem illustrated above is caused by the bad usage of the macro. The
macro call should be formed as a statement to avoid the flow control problem.
In other words, the errors above are caused by the missing ';' at the end
of the trace macro call.

Especially, macros which expand into code with flow control
should be formed such that the semicolon cleanly terminates
the flow, e.g.:

#define COMPLEX_MACRO(foo) (if(SOME_TEST(foo)) {statements;}else)

Then the correct usage of COMPLEX_MACRO would be:
COMPLEX_MACRO(foo);
which also handles the case where COMPLEX_MACRO expands into nothing.

cat <flames_about_crufty_usage_of_code_in_macros >/dev/null