c - 'Initializer not constant' on global variable?

Question

So I get the 'initializer element not constant' error when compiling the following code:

#include <stdlib.h>
#include <stdio.h>
#include <math.h>

float wl = 2.0f;
float k = 2.0f * (float) M_PI / wl;

int main ()
{
     //Do stuff
}

If I move "float k" inside the main method, there's no errors, but this isn't an option for me, because I NEED float k to be a global variable. Even if I change it to this:

const float wl = 2.0f;
const float k = 2.0f * (float) M_PI / wl;

the error still happens. How do I fix this?

Answer 1

According to C99 standard:

§6.7.8 Initialization

4. All the expressions in an initializer for an object that has static storage duration shall be constant expressions or string literals.

Using const doesn't help here because, in C, const variables are not really const. Check out this post for more details.

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To work out, you can make wl constant by using preprocessor:

#define wl 2.0f

By doing this, 2.0f * (float) M_PI / wl can be a compile time constant.