What is the standard to declare constant variables in ANSI C?

const in C is very different to const in C++.

In C it means that the object won't be modified through that identifier:

int a = 42;
const int *b = &a;

*b = 12; /* invalid, the contents of `b` are const */
a = 12; /* ok, even though *b changed */

Also, unlike C++, const objects cannot be used, for instance, in switch labels:

const int k = 0;
switch (x) {
    case k: break; /* invalid use of const object */
}

So ... it really depends on what you need.

Your options are

• #define: really const but uses the preprocessor
• const: not really const
• enum: limited to int

larger example

#define CONST 42
const int konst = 42;
enum /*unnamed*/ { fixed = 42 };

printf("%d %d %d\n", CONST, konst, fixed);

/* &CONST makes no sense */
&konst; /* can be used */
/* &fixed makes no sense */