"multiple definition of value" when compiling C program with uninitialized global in g++ but not gcc

Question

I'm trying to understand the usage of extern and global variable declaration in header files so I came up with the following test program written in C.

The main.c file

//main.c
#include "global.h"
#include <stdio.h>

int nExternValue = 6;

int main(int argc, char* argv[])
{
    printf("%d \n", nValue);
    printf("%d \n", nExternValue);

    AddToValue();

    printf("%d \n", nValue);
    printf("%d \n", nExternValue);
}

The global.h file

#ifndef _GLOBAL_H
#define _GLOBAL_H

//WRONG! do not declare a variable here
int nValue;

//OK! extern variable makes it a global accessable variable
extern int nExternValue;

//OK! function prototype can be placed in h file
int AddToValue();

#endif

and a AddValue.c file which implements the AddToValue function.

#include "global.h"

int AddToValue() {
    nValue++;
    nExternValue++;
}

I compiled the app using gcc, AND ran it:

$ gcc main.c AddValue.c -o test
$./test
0 
6 
1 
7 

I complied the app using g++ and got the following linker error:

$ g++ main.c AddValue.c -o test
/tmp/ccFyGDYM.o:(.bss+0x0): multiple definition of `nValue'
/tmp/cc3ixXdu.o:(.bss+0x0): first defined here
collect2: ld returned 1 exit status

Why doesn't the gcc linker produce an error? I though the nValue variable would be declared multiple times and that would produce an error!

$ gcc --version
gcc (Ubuntu/Linaro 4.6.3-1ubuntu5) 4.6.3
Copyright (C) 2011 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

$ g++ --version
g++ (Ubuntu/Linaro 4.6.3-1ubuntu5) 4.6.3
Copyright (C) 2011 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

Answer 1

C and C++ are different languages. Case in point, the above program is a valid C program but in ill-formed C++ program. You have violated C++'s one definition rule. There is no corresponding rule in C.

When compiling with gcc, you are compiling the above text as a C program. When compiling with g++, you are compiling the above text as a C++ program.

Answer 2

When compiled with gcc, uninitialized global variable (such as nValue) will be treated as a common symbol. The same common symbol occurred in different compilation unit will be merged during link time. If compiled with g++ (which means that the source program will be treated as C++ program), uninitialized global variable are implicitly initialized with a default value 0. Since global.h is included in multiple source files, the compiler will consider the symbol nValue defined multiple times.

Please also have a look at this post: Why uninitialized global variable is weak symbol?