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From: kwh@bentley.UUCP (KW Heuer)
Newsgroups: net.lang,net.lang.c
Subject: Re: C builtin functions
Message-ID: <712@bentley.UUCP>
Date: Tue, 15-Apr-86 17:23:43 EST
Article-I.D.: bentley.712
Posted: Tue Apr 15 17:23:43 1986
Date-Received: Fri, 18-Apr-86 04:16:23 EST
References: <4017@pur-ee.UUCP>
Organization: AT&T Bell Laboratories, Liberty Corner
Lines: 26
Xref: watmath net.lang:2357 net.lang.c:8589

In article <4017@pur-ee.UUCP> pur-ee!pasm (PASM Parallel Processing Laboratory) writes:
>	but you can just as correctly write:
>		sizeof int

I've seen compilers that will accept this, but I believe K&R says the
parens are necessary when the argument is a datatype (to avoid the
ambiguity of e.g. "sizeof char * - 1").

>	What the heck is a compile-time function?  Real useful -
>	functions that return constants.  Come on now.

I can think of two cases, off the top of my head, where a compile-time
function makes sense:

	double log10(x) double x; { return log(x)/log(10.0); }
	char buf[max(XSIZE,YSIZE)];

It's more efficient for log(10.0) to be evaluated at compile-time,
and it's essential for the compiler to know the constant result of
max().  In practice, of course, these are handled by a preprocessor
constant (for the specific value LOG10 or its reciprocal) and a
macro implementation of max (which unfortunately can't also be used
as a function in general -- this is a potential argument in favor
of builtins?).

Karl W. Z. Heuer (ihnp4!bentley!kwh), The Walking Lint