Thread: No error from multiple definitions of global variables

I was reading this thread (which discusses how you can't declare global variables in header files, etc) and thought I'd try it out. I seem to remember reading something about how const global variables in C++ were automatically static, that is, limited to the scope of one file, which would explain Elysia's last post.

Evidently I remembered correctly on that point.

Anyway. I came across a very strange phenomenon in my experimenting, which I eventually narrowed down to this:

Code:

$ ./globalx
$ cat globalx.c
int x;

$ cat globalxmain.c
int x;

int main() {
    return 0;
}
$ gcc -c globalxmain.c
$ gcc -c globalx.c
$ gcc globalx.o globalxmain.o -o globalx
$ ./globalx
$

Why does this work? How can a global variable, declared in two separate source files, be legal C? (It's not legal C++, by the way.)

I thought global variables could only be declared once, like functions, or the linker would complain . . . .

What am I missing here?

[edit] Mm'kay, if I initialize x in both places I get the expected linker error.

Code:

$ gcc globalx.c globalxmain.c -o globalx
/tmp/ccvX2E7d.o:(.data+0x0): multiple definition of `x'
/tmp/ccMaJSet.o:(.data+0x0): first defined here
collect2: ld returned 1 exit status
$

I guess un-initialized global variable declarations can be repeated or something like that? . . . . [/edit]

Yes, I was aware that you could declare global variables several times in one file, but I didn't think it applied to different source files. It looks like I was wrong.

Edit: And just for citizen, I compiled that with gcc -Wall -ansi -pedantic.

So did I, but just forgot to mention it.

As tabstop points out, all the tentative definitions are taken together (in the absense of an actual definition) to mean (in the singular)
int x = 0;

But if there is an actual definition in the code, say
int x = 1;
then all the tentative definitions are ignored for the purposes of allocating and initialising some memory.

Just another one of those little wrinkles which separate C and C++
http://david.tribble.com/text/cdiffs.htm#C99-odr