C static struct declaration directive

Question

I'm trying to declare a static C struct. The declaration of the struct is as follows:

typedef struct { int a; int b; int c }dummy_struct;

However when making the definition of the struct, I want to store a value in c that is calculated using the values in a and b and another constant value.

My initial idea was to store it this way:

static dummy_struct dummy = { .a = 5, .b = 10, .c = CALC_VALUE(0.5, a, b) }

And then I would define CALC_VALUE as a preprocessor so that I could run some calulations on a, b and the constant value like so:

#define CALC_VALUE(constant, a, b) (constant * (a/2) * (b*3))

But my compiler complains every time I do this so I'm resorting to the following method for the declaration:

static dummy_struct dummy = { .a = 5, .b = 10, .c = CALC_VALUE(0.5, 5, 10) }

Is there a cleaner way to do this without using #defines for 'a' and 'b'

Why not just store 0.5 in the struct, and then do the calculation where you need the value? — Joseph Sible-Reinstate Monica
– Joseph Sible-Reinstate Monica, Commented Oct 13, 2020 at 18:44
Structure members are not variables, they can't be accessed without referring to the variable containing the struct. — Barmar
– Barmar, Commented Oct 13, 2020 at 18:47
And you can only use literals and constant expressions when initializing a static variable. — Barmar
– Barmar, Commented Oct 13, 2020 at 18:48
@JosephSible-ReinstateMonica The problem is not storing 0.5, it is about being able to access the value in a and b from within the struct declaration. I don't mind using a #define for 0.5, but I want to be able to access a and b as they are both already defined for the struct member. So I was wondering if there is a way to access the value stored in the local member. If you look at my second code snippet, I want to be able to do something like that. — rorschach
– rorschach, Commented Oct 13, 2020 at 18:48
@Barmar Ok got it. That is the limitation I'm trying to get past I guess. — rorschach
– rorschach, Commented Oct 13, 2020 at 18:49

Barmar · Accepted Answer · 2020-10-13 18:52:49Z

Use a macro to generate the entire initial value of the struct.

#define CALC_VALUE(constant, a_val, b_val) { \ .a = (a_val), \ .b = (b_val), \ .c = ((constant) * ((a_val)/2) * ((b_val)*3)) \ }

Then you can do:

static dummy_struct dummy = CALC_VALUE(0.5, 5, 10);

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