Global Section Rules In C [duplicate] - c

Question

This question already has answers here:
Why are we not allowed to have assignment statements in the file scope in C?
(2 answers)
Closed 3 years ago.
I have some questions about global section rules in C, I'll state my question in cases.
Case 1: Why is this not allowed, says redefinition.
int a = 4;
a = 5;
int main()
{
printf("Hello World");
return 0;
}
Case 2: Why is this not allowed.
int a;
int b;
a = b;
int main()
{
printf("Hello World");
return 0;
}
Case 3: Why is this not allowed.
int a;
a = 4;
a = 5;//if this is removed, it works fine
int main()
{
printf("Hello World");
return 0;
}
Case 4: Why is this allowed, but test is not printed.
int test(void)
{printf("test");}
test();
int main()
{
printf("Hello World");
return 0;
}
Thanks.

Answer 1

case 1:
because you're doing assignment outside of function.
case 2:
because you're doing assignment outside of function.
case 3:
because you're doing assignment outside of function.
The first assignment is considered as initialization that's why it's fine.
case 4:
because you're just declaring test() function again with different signature if the default is not int.
and, You're not calling test() from the main function.
int test(void)
{
printf("test");
return 0; // it can cause a warning if missing
}
int main()
{
printf("Hello World");
test();
return 0;
}

Answer 2

The "global" section is actually compiled into a segment in the binary executable that is then loaded into memory when you program runs.
The exact details of how it works depend on the specific OS you build for, but basically, the only thing that you can do in the global section is declare or define initialized or uninitialized variables.
In your case 1 you fist define an initialized global variable a, but then you try to assign a new value to it.
This is actual code that needs to run after the program is loaded, so it has to be inside a function!
Your case 2 is almost same as one: you define 2 uninitialized global variables, but then you add code to manipulate them.
This code has to be inside a function!
case 3 is more interesting:
If you try to compile it, you should get warnings like this:
warning: data definition has no type or storage class a = 4;
warning: type defaults to ‘int’ in declaration of ‘a’ [-Wimplicit-int]
The compiler thinks you are defining a variable without a type, but with a value, so it assigns a default type of int to it.
Notice this will also work:
int a;
int a = 4;
And will remove the warnings.
The compiler will treat the first line as declaration only (an intent to have a variable named a, but not the actual variable) and the second line will be the actual definition, creating the variable and initializing it with value 4.
Keep in mind, that in C, initializing a variable, and assigning value to it are two slightly different things with different rules.
Finally case 4 is very simple:
"test" is not printed because you never call the function test().
int test(void)
{printf("test");}
test();
This code is same as this code:
int test(void);
int main() {
...
}
int test(void)
{
printf("test");
}
What you did with the last line in you example is simply declare a function.
You told the compiler that somewhere in your code exists a function named test that expects no parameters and returns an integer value. (if you do not specify return type for function C default to int!)
Anything you write int the global scope the compiler will try to interpret as a declaration (you tell the compiler something exists) or a definition (you tell the compiler what something is).
If it can not interpret your statement this way you will get a compilation error.
You can not run code from global scope in C.