Forcing compiler to make arithmetic calculations during preprocess

Question

Compiler: MPLABX IDE V5.30 Operating system: Windows 10

What i'm trying to do is define some constant values (to make future changes easier) and create some other constants via arithmetic operations during preprocessing. Then use those constants during runtime.

Here is an example version of my intentions;

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

#define foo 100 //in case you change FOO in circuit, change this too!
#define bar (sqrt(foo))

int main(void) {
    if ( bar > user_input)
    {
    do();
    }
}

The problem is, I thought, since the input was a constant value, defined things would be calculated by the compiler and bar would be replaced by 10 instead of (sqrt(foo)). But when i compile it, data and program size dramatically changed. When i disassembly it, there are tons of instructions instead of simply putting directly a number there.

Then i followed a suggestion from another question's answer and place a const squareroot() function and const int declaration but the compiler gave an alert like; main.c:50:38: error: initializer element is not a compile-time constant

here is the second try;
#include <stdio.h>
#include <squareroot.h>

#define foo 100 //in case you change FOO in circuit, change this too!
const int bar=squareroot(foo);

int main(void) {
    if ( bar > user_input)
    {
    do();
    }
}

const int squareroot(const int input)
{
do()
}

How can i express myself to my compiler to make it understand some of the lines in my code are constant whatever happens during runtime, so that it can do the aritmetic instead of just simply passing the token/text to the function body?

Answer 1

#define is a plaintext replacement and nothing more. All of the #define transformations occur in the preprocessing translation phase which is before any analysis of expressions and so on.

The list of which expressions the compiler must support in a constant expression can be found in section 6.6 of the current C Standard, for a summary see here. Calling a function is not included.

(Of course, individual compilers may offer features not required by the Standard).

If you must use a compiler which does not support calling the floating point function sqrt in a constant expression then your options include:

• Hardcode your constants, runing another preprocessing phase to set them up if desirable.
• Have a global variable which you initialize at the start of the main function .