Linker Symbol resolution in C/C++

[VanKha]

Hi everyone ,
I've just read a book on Computer Systems.(Computer Systems:A programmer's perspective ) . On the part of "Linking" , the author talks about strong/weak symbol and as an example for the rule "Given a strong symbol and weak symbols , choose the strong one" , he gives the following C modules

/* foo5.c */
#include<stdio.h>
void f();
int x=15213,y=15212;
int main()
{
    f();
    printf("%d %d",x,y);
    return 0;
}
 
/* bar5.c */
double x;
void f()
{ 
   x=-0.0;
}
 

And points out that the symbol in foo5.c will be chosen.And the program wil run without errors but has unexpected result (y being overwritten with the **** value ? ) . That's right ! But only if I compile with gcc ( my OS is Fedora 5 ) ! With g++ it will generate the multiple definition error ! (Of course , with g++ , I've change stdio->iostream, etc...).
So is it because the C++ compiler/linker is "smarter" ? So , what's the theories and rules added in c++ linker (if any ?)
One more question , I still dont understand this : if the symbol for x in foo5.c has been chosen , what causes y to be overwritten ? The author of the book said about int-4 byte and double-8 byte ,... But once x has been chosen as int , why double is concerned here ? I mean , why not the linker just throw away other definitions and treat x as int always from that time on ?
Replies appreciated ;

[weaknessforcats]

This won't even compile as C code because there are two global variables named x.

[VanKha]

Have you tried it ? I compiled it successfully !
It has to do with strong symbol / weak symbol resolution as I've said .
A lot of documents pop out on this topic even if you use Google .
Here is one picked randomly : http://www.andrew.cmu.edu/user/ngm/1...nk-answers.pdf
( Read from "Linking Summary" page 6 )
And there's a chapter in the book I mentioned ...
I'm telling this in case you think I'm playing a joke :D !

[weaknessforcats]

I tried it. There are two global variables named x and it dies in the link.

If you have a compiler that accepts this, then get a better compiler.

[VanKha]

But...What about the the references I mentioned ?
Btw,I use gcc(again,my OS is Linux Fedora 5)

[sicarie]

sicarie@hostname:~/bin$ gcc foo5.c
foo5.c:12: error: conflicting types for ‘x’
foo5.c:3: error: previous definition of ‘x’ was here
sicarie@hostname:~/bin$ gcc -v
gcc version 4.1.2 20061115 (prerelease) (Debian 4.1.1-21)

Fedora's current version now is 8, I believe.

http://fedoraproject.org/get-fedora