c & c++ default global variable linkage, multiple declaration & definition problem

For example:

code1.c / .cpp

int a;

// ... and so on

code2.c / .cpp

int a;

int main(void) {
    return 0;
}

go to compile:

$gcc code1.c code2.c      # this is fine
$

$g++ code1.cpp code2.cpp  # this is dead
/tmp/ccLY66HQ.o:(.bss+0x0): multiple definition of `a'
/tmp/ccnIOmPC.o:(.bss+0x0): first defined here
collect2: ld returned 1 exit status

Is there any global variable linkage difference between C & C++?