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From: arnold@gatech.UUCP (Arnold Robbins)
Newsgroups: net.lang.c
Subject: Re: C-Debug package ... (a la "context-independent macros")
Message-ID: <11839@gatech.UUCP>
Date: Fri, 1-Feb-85 12:12:18 EST
Article-I.D.: gatech.11839
Posted: Fri Feb  1 12:12:18 1985
Date-Received: Sat, 2-Feb-85 12:46:56 EST
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Organization: Firefighters Extraordinaire, Inc.
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Okay, folks, we've discussed this one before.  The macro

#define assert(x) if (!(x)) die_miserably("x bombed\n")

doesn't work because of problems with a missing else.  It was suggested

#define assert(x) if (!(x)) die_miserably("x bombed\n") else /* no ; */

but if the real semicolon is left off by accident, the next statement is eaten.
Now, putting braces around it also won't work:

#define assert(x) { if (!(x)) die_miserably("x bombed\n"); }

because of this:

	if (something)
		assert (something_else);/* <--- this ; will bomb cc */
	else
		another_thing_altogether();
	
So, as has been mentioned before, the two ways to do this are

#define assert(x)  do { if (!(x)) die_miserably("x bombed\n"); } while (0)

which will get executed exactly once, and protects the macro syntactically.
This also should not pose a code generation/efficiency problem.  The other
way is this:

#define assert(x)	if(x) ; else die_miserably ("x bombed\n")

which will also work. This topic has gone around (several times) before.
I am hoping that this note will settle it, so that we can discuss something
else.  Thanks,
-- 
Arnold Robbins
CSNET:	arnold@gatech	ARPA:	arnold%gatech.csnet@csnet-relay.arpa
UUCP:	{ akgua, allegra, hplabs, ihnp4, seismo, ut-sally }!gatech!arnold

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