global variables - Why are we not allowed to have assignment statements in the file scope in C?

Question

Why are we only allowed to declare and define variables in global section?Why not include assignment in global section? Example:

#include<stdio.h>
int a;
a=5;//Valid because its similar to int a=5; Therefore a initialiser to a Tentative definition
a=8;//Invalid because We can have only one initialiser for a tentative definition
void main(){
 ...
}

Why do we need this? What would be the consequences if we were allowed to have more than one initializer to a tentative definition

My next question is why only constant initializer elements are allowed?

#include<stdio.h> 
int i=5;
int j=i+5;//[Error] initializer element is not constant
void main(){
 ...
}

Similarly what would be the consequences we face if this rule was not present?

Please note my question is not exactly why this happens? I'm trying to figure why these restrictions were given in the first place.

Answer 1

We are not allowed to use assignment in file scope because program execution starts from main. Compiler creates _start function which is executed first and then jump to main is made from there. When main returns, control goes back to _start which is having proper exit procedure to terminate program. So anything which is written outside the functions is only meant for the initializations which will be done compile time

Initialization is different from declaration and assignment. When we initialize variable compiler will make such arrangement that when program execution starts, its value will be what we have initialized. But when we declare a variable, it will be having default initial value specified by its scope. Assignment is done at runtime and not at compile time