C: enum VS #define for mathematical constants?

Question

I'm wondering what would be the best way to store math constants that are used throughout an entire program?

#define PI 3.14159265
#define SPEEDOFLIGHT 2.99792458e8

or

enum constants { PI = 3.14159265; SPEEDOFLIGHT = 2.99792458e8; }

Thanks

Answer 1

Do not use const variables for this! In the C language, a const qualified variable is not a constant in the sense of constant expression, so it cannot be used in initializing a static/global variable. This has major practical consequences; for instance, the following will not work:

static const double powers_of_pi[] = { 1, PI, PI*PI, PI*PI*PI, PI*PI*PI*PI, };

The proper solution is #define. It's probably best to use the l suffix so that they'll have type long double, and include sufficiently many decimal places that the values will be correct for long double types up to 128-bit. Then you can use them wherever any floating point type is expected; C will silently convert them down to lower precision as needed.