c - Can't use global variables with gcc

Question

I have a file like this:

char* vidmem = (char*)0xb8000;

int main()
{
    vidmem[0] = 'x';
    vidmem[1] = 0x0f;
}

But when i compile it with gcc it behave like vidmem does not even exist. I have this problem with all variable declared outside of function. Maby this one can be declare inside main but other variables in other files linked to this can't be. This how I compile it:

gcc -c main.c -o main.obj -ffreestanding -fno-exceptions -m64

And this is how i assemble all the files:

gcc -m64 -Wl,--build-id=none -static -fno-asynchronous-unwind-tables -nostdlib -nodefaultlibs -lgcc main.obj [..] -T linker.ld -o out.bin

Why this happen? And how can i solve it?

EDIT:

I am making an OS and this is the code from the C kernel. I am using linux subsystem for windows to compile it and qemu for testing

Here is a link to all the code: http://www.mediafire.com/file/7x21lh4dnc93dz9/OS.7z/file

Answer 1

You can declare global variables but you have to initialize inside main like this: char* vidmem;

char* vidmem;

int main()
{
    vidmem = (char*)0xb8000;

    vidmem[0] = 'x';
    vidmem[1] = 0x0f;
}