multiple definition in g++? - c

Question

The code is as follows:
global.h
#ifndef GLOBAL_H
#define GLOBAL_H
#include <stdio.h>
int test;
void test_fun(void);
#endif
global.c
#include "global.h"
void test_fun()
{
printf("%d\n", test);
}
main.c
#include "global.h"
int main(void)
{
test_fun();
test = 1;
printf("%d\n", test);
}
Makefile using gcc compiler
main: main.o global.o
gcc -o main main.o global.o
main.o: main.c global.h
gcc -c main.c
global.o: global.c global.h
gcc -c global.c
clean:
rm -f global.o main.o main
This works well.
However, when I change my code to C++, as follows:
global.h
#ifndef GLOBAL_H
#define GLOBAL_H
#include <iostream>
int test;
void test_fun(void);
#endif
global.cpp
#include "global.h"
void test_fun()
{
cout << test
}
main.cpp
#include "global.h"
int main(void)
{
test_fun();
test = 1;
std::cout << test;
}
Makefile using g++ compiler
main: main.o global.o
g++ -o main main.o global.o
main.o: main.cpp global.h
g++ main.cpp
global.o: global.cpp global.h
g++ global.cpp
clean:
rm -f global.o main.o main
The code above throws the output:
global.o:(.bss+0x0): multiple definition of `test'
What makes the different here?

Answer 1

You've int test; in a header which is included in 2 TUs, hence the error. Both the translation units main.c (or .cpp depending upon the compiler used) and global.c have global.h included, which leads to two definitions of the same variable in two object files, thus the linker error.
Pass test as an arguement to test_fun, thereby avoiding the usage of a global.
If you absolutely have to share the variable between the TUs, then remove int test; from global.h and in main.cpp do
int test;
and in global.cpp do
extern int test;
As an aside, since it's a global variable, test would be initialized to 0 and hence in main when you test_fun();, it should print 0 and then after setting it to 1, it'll print 1.
It's illegal in both C and C++ from a language standpoint, but as for why it works with a C compilers (like GCC) is because they implement a common extension, a legacy cruft.

Answer 2

... You are using a different programming language