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From: eggert@twinsun.com (Paul Eggert)
Newsgroups: gnu.gcc
Subject: Re: IEEE FLP question: Use of NAN and Inf
Message-ID: <8912200113.AA07280@rise.twinsun.com>
Date: 20 Dec 89 01:13:52 GMT
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phd_ivo@gsbacd.uchicago.edu writes:

	Moreover, can someone please tell me how to define
	the IEEE double's of +Inf, -Inf, and NaN, so that
	I can assign and test for them?

Here's how.

	#define Inf (1e300 * 1e300)
	#define NaN (Inf - Inf)

`-Inf' gets you minus infinity.

There are lots of NaNs; the above method gives you only one of them.
Use isnan(X) to test whether X is a NaN.
If your vendor doesn't supply isnan() (sigh), this should work:

	int isnan(x) double x; {return x!=x;}

Warning: GCC 1.36 is buggy in this area; 1.37 will have some fixes.

Eventually GCC should have full IEEE support rather than relying on vendors.