Const int to int doesnt work properly - c

Question

If I convert const int to int inside a void it works.
But when I create an extern const int it doesn't.
void ReadFromEpprom()
{
int Start = 343;
const int End=Start; //This works
}
Example 2
Header File
extern const int End;
Source file
const int End;
void ReadFromEpprom()
{
int Start = 343;
End=Start; //This doesn't work
}
In second situation I get error:
(364) attempt to modify object qualified const
How can I solve this?
Should I make it with another way?

Answer 1

When you declare a constant variable this means the variable will not change.So it makes sense that constant variables must be initialized immediately.You are doing this in your first example which is correct.In your second example you have a constant variable and then you are trying to modify it's value.This is incorrect since the variable is already constant.

Answer 2

The extern here is a red herring.
If you use const int End then you need to initialise End at the point of this declaration. That's because it's const.
So const int End = Start; works fine, but const int End; is not syntactically viable.
In C you can't arrange things so you have a const at global scope and a function that sets the value at run-time. But what you could do is embed the value in a function (as as static), and call that function to initialise and subsequently retrieve the value.

Answer 3

Initialiazation versus assignment:
A const object can be initialized (given a value at the declaration), but not assigned (given a value later).
This is initialization, and "works". It is initialization the local variable End that exist inside ReadFromEpprom().
void ReadFromEpprom()
{
...
const int End=Start; //This works
End=Start; attempts to assigned End that exist at file scope, outside of ReadFromEpprom(). const objects cannot be assigned.
const int End;
void ReadFromEpprom()
{
...
End=Start; //This doesn't work
}
How can I solve this?
Let external code to read localEnd via a function ReadEnd(), yet allow local code to write localEnd.
static int localEnd;
int ReadEnd() {
return localEnd;
}
void ReadFromEpprom() {
int Start = 343;
localEnd = Start;
}

Answer 4

I agree with all the other answers.
It seems that you want to initialize a const value with a value that will be determined at run time, which is impossible.
What you can do is two workarounds.
Remove const. Add a comment near its definition that the variable is set only once in the very beginning.
Use a function instead of a const int variable; implement the "run code only once" idea inside this function.
Header file:
int CalcEnd();
Source file:
int CalcEnd()
{
static int init_done = 0;
static int result;
if (!init_done)
{
result = 343; // or any complex calculation you need to do
init_done = 1;
}
return result;
}
Note how the function uses static variables and logic to do initialization only once.
If you use the function idea, and don't want to remember typing the parentheses in the function call (CalcEnd()), you can define a macro:
#define End MyCalcEnd()
This will make it appear as if End were const int variable, when actually it's a function call. This usage of macros is controversial, use it only if you are sure it will not lead to confusion later.