C/C++ global vs static global

Question

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Static vs global

I'm confused about the differences between global and static global variables. If static means that this variable is global only for the same file then why in two different files same name cause a name collisions?

Can someone explain this?

Answer 1

Global variables (not static) are there when you create the .o file available to the linker for use in other files. Therefore, if you have two files like this, you get name collision on a:

a.c:

#include <stdio.h>

int a;

int compute(void);

int main()
{
    a = 1;
    printf("%d %d\n", a, compute());
    return 0;
}

b.c:

int a;

int compute(void)
{
    a = 0;
    return a;
}

because the linker doesn't know which of the global as to use.

However, when you define static globals, you are telling the compiler to keep the variable only for that file and don't let the linker know about it. So if you add static (in the definition of a) to the two sample codes I wrote, you won't get name collisions simply because the linker doesn't even know there is an a in either of the files:

a.c:

#include <stdio.h>

static int a;

int compute(void);

int main()
{
    a = 1;
    printf("%d %d\n", a, compute());
    return 0;
}

b.c:

static int a;

int compute(void)
{
    a = 0;
    return a;
}

This means that each file works with its own a without knowing about the other ones.

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As a side note, it's ok to have one of them static and the other not as long as they are in different files. If two declarations are in the same file (read translation unit), one static and one extern, see this answer.