How to generate NaN, -Infinity and +Infinity in AN

问题:

I use ANSI C89 (not C++), and I want to generate NaN, -Infinity and +Infinity.

Is there any standard way (eg. standard macro)? Or is there any platform and compiler independent way to generate these numbers?

float f = 0.0 / 0.0; // Is f ALWAYS in any platform is NaN?

回答1:

There is in C99, but not in previous standards AFAIK.

In C99, you'll have NAN and INFINITY macros.

From "Mathematics <math.h>" (§7.12) section

The macro INFINITY expands to a constant expression of type float representing positive or unsigned infinity, if available; ...

If you're stuck with ANSI C89, you're out of luck. See C-FAQ 14.9.

回答2:

I don't know if this is standard or portable, but here's a start:

jcomeau@intrepid:/tmp$ cat test.c; make test; ./test
#include <stdio.h>
int main() {
 printf("%f\n", 1.0 / 0);
 printf("%f\n", -1.0 / 0);
 printf("%f\n", 0.0 / 0);
 return 0;
}
cc     test.c   -o test
test.c: In function ‘main’:
test.c:3: warning: division by zero
test.c:4: warning: division by zero
test.c:5: warning: division by zero
inf
-inf
-nan

Strangely enough, I can't get positive NaN using this naive approach.

---

Also see this: http://www.gnu.org/s/hello/manual/libc/Infinity-and-NaN.html

回答3:

If you use an old compiler where INFINITY does not exists you can also use the macro HUGE_VAL instead, also defined in the <math.h> library.

HUGE_VAL should be available in C89/C90 standard (ISO/IEC 9899:1990).

References: http://en.cppreference.com/w/c/numeric/math/HUGE_VAL