How to declare a global const variable and initialize it with a function in C?

Question

I would like to declare a constant global variable and initialize it with a function. The problem is, I can't use my function to initialize it because a function call is not a compiler-time constant, but I also can't split the declaration and the initialization as it is a const variable.

For example, the following codes are invalid.

#include <stdio.h>

int my_function(void){
    return 42;
}

const int const_variable = my_function();//invalid: my_function() is not
                                         //a compiler-time constant

/*main() here*/

#include <stdio.h>

const int const_variable;
const_variable = 10;//invalid: can't modify a const-qualified type (const int)

/*main() here*/

I looked for some answers. I saw that some people suggested the use of pointers to modify the value of a const variable after its declaration, but this could cause severe readability, portability and debugging problems.

Is there any simple, portable way to solve my problem?

What you're trying to do is simply not valid C. Variables in the global scope (outside any functions) simply can't be initialized at run-time. — Some programmer dude, Mar 24 '22 at 10:36
The only thing you can do with a const variable is initializing it upon declaration with a constant value like this: `const int const_variable = 10;` — Jabberwocky, Mar 24 '22 at 10:37
Ok… Thank you for your comments! I'll try to find another way to do what I want. — Hobbes, Mar 24 '22 at 10:41
Hobbes, what is the real function calculating? Certainly not just 42. Post the code of that function. Perhaps there is a way to do that at compile time. — chux - Reinstate Monica, Mar 24 '22 at 10:55
It is a SDL2 function, SDL_GetScancodeFromName(). I'll try and get the source — Hobbes, Mar 24 '22 at 18:18

score 2 · Accepted Answer · answered Mar 24 '22 at 10:41

How to declare a global const variable and initialize it with a function in C?

There is no way. It is not possible.

Is there any simple, portable way to solve my problem?

No.

You can use a macro #define MY_FUNCTION() (42).

You can write a separate program, that uses that C function (or is written in a completely different language), that generates C source code with that constant and that generated source code is then compiled with your project.

Or you can switch to a different programming language with more features, for example Rust, D, C++.

Not sure if the macro approach is helpful.... – Jabberwocky Mar 24 '22 at 10:45 — Jabberwocky, Mar 24 '22 at 10:45

How to declare a global const variable and initialize it with a function in C?

1 Answers1