extern declaration and global scope

Question

I'm wondering why is this code compiles and run. I thought that if a variable is declared as static (in global scope) it will be accessible only within the file it is declared.

functions.h

static int x = 10;

main.c

#include <stdio.h>
#include "functions.h"

extern int x;

int main()
{
   printf("%d", x);
}

Answer 1

The preprocessor takes the text in functions.h and copies it as is into main.c After preprocessing (and before compilation) your main.c looks as follows:

#include <stdio.h>
static int x = 10;

extern int x;

int main()
{
   printf("%d", x);
}

You will have linker problems if functions.h is included into a second source file, and you try to link both object files into one executable.

Answer 2

Technically, it is indeed declared within the main.c, as this includes the functions.h. If it was a sparate compilation module, you'd be right.

But I'd have suspected that within the same compilation unit extern and staticwould collide with each other. At least it would be worth a warning.

Answer 3

when you are including functions.h in main.c , you are actually copy content of function.h in main.c so your final code become something like :

#include <stdio.h>
static int x = 10;

extern int x;

int main()
{
   printf("%d", x);
}

So your extern line is redundant. you can achieve what you want by this remove #include "functions.h" from main.c

1. compile function.h using g++ -c function.h
2. compile main.c using g++ -c main.c
3. then build g++ function.o main.o -o out third line would not compile because of static int .