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Question

I have a variable defined in one of my files, It may be manipulated by the code in the file itself, but it is always a constant value to any external file.

How do i declare the variable as constant without raising errors on assigning this variable a value inside the file it is defined in while allowing compiler to optimize it's read as if it is a constant in those external units ?

Answer 1

A rvalue can't be modified. Use an accessor function to access it guarantees you only offer an rvalue, eg

static int value;

extern int getconst();

int getconst() {
  return value;
}

This makes:

getconst() = -1; // Compiler error

Alternatively you can expose your value via a const pointer to a const int :

#include <stdio.h>

static int value = -1;

extern const int * const public_non_modifiable;
const int * const public_non_modifiable = &value;


int main() {
  printf("%d\n", *public_non_modifiable); // fine
  *public_non_modifiable = 0; // compiler error
  return 0;
}

Answer 2

Declaring in the file that modifies the virables as follows:

int e = 0, e_ant = 0, adj;

and declaring in the other files in this way:

extern const int e, e_ant, adj;

works for me. I am using a dsPIC with MPLAB.

The first file computes the value of all the variables. A second file only shows those values in a LCD Display, with a totally forbiden writing of this variables.