extern int i. so is it a definition or a declaration?

I'm completely confused. the famous stupid example x/y: we have 2 cpp files the first file extern int x; int y = x+1;

Second file extern int y; int x = y+1;

It is clear that depending on which file is executed first, there will be a different result. In eckell, it is stated that for all static objects (probably meaning those allocated in a static memory area), the layout and loading mechanism guarantees that before performing a dynamic operation, initialization defined by the programmer, static data will be initialized with zeros. in this regard, there is a misunderstanding . Do I understand correctly that, for example, if file #1 is triggered first x is roughly speaking no declaration, but already a full-fledged definition, since it acquires both the address and the value zero ???

Author: Harry, 2020-12-24
Source

1 answers

For the source file, this is a declaration. It will record that there is a variable x with some address, which will be known later. It will become known later-when linking...

Here's what it looks like in VC++: 

extern int y;
int x = y + 1;

Here is the assembler: somewhere outside is declared y

EXTRN   ?y@@3HA:DWORD                   ; y

Inside, x is declared and defined - it has its own place in memory and a specific address:

_BSS    SEGMENT
?x@@3HA DD  01H DUP (?)             ; x
_BSS    ENDS

And the initialization code is added:

??__Ex@@YAXXZ PROC                  ; `dynamic initializer for 'x'', COMDAT
; File G:\Tmp\Test\x.cpp
; Line 2
    mov eax, DWORD PTR ?y@@3HA          ; y
    inc eax
    mov DWORD PTR ?x@@3HA, eax          ; x
    ret 0
??__Ex@@YAXXZ ENDP                  ; `dynamic initializer for 'x''

Which refers to an external a variable.

It works the same way in the second file - with the external x.

Well, the decoupling of addresses occurs already at the stage of linking (linking), and not compilation. Not to mention getting values by variables, which is obtained here only when executed...

 2
Author: Harry, 2020-12-24 09:47:59