Question

When I write the following program:

file 1:

#include <stdio.h>    
int global;    
void print_global1() {
        printf("%p\n", &global);
}

file 2:

#include <stdio.h>
char global;    
void print_global2() {
        printf("%p\n", &global);
}

file 3:

void print_global1();
void print_global2();
int main()
{
        print_global1();
        print_global2();

        return 0;
}

output:

$ ./a.out
0x804a01c
0x804a01c

Here is my question:

• Why are the linker implementing “int global” and “char global” as the same global variable:
• How come the compiler does not complain (not the smallest warning with -Wall -Wextra -ansi …)
• How are the size of the global variable managed (the size of int and char are different)

PS: The second question is architecture/compiler related, so lets take the gcc or Visual C++ (for C) with the int size as 32 bits

EDIT: THIS IS NOT A QUESTION FOR C++ BUT for C!

I use gcc version 4.4.1 and on Ubuntu 9.10, Here is the compilation console output:

$ ls
global_data1.c  global_data2.c  global_data.c

$ gcc -Wall -Wextra -ansi global_data*.c
$ ./a.out
0x804a01c
0x804a01c
or 
$ gcc -Wall -Wextra -ansi -c global_data*.c
$ gcc -Wall -Wextra -ansi global_data*.o
$ ./a.out
0x804a01c
0x804a01c

Answer 1

gcc does not report any error/warnings. But g++ does.

EDIT:

Looks like C allows tentative definitions for a variable.

In your case both the global definitions are tentative and in that case the first one seen by the linker is chosen.

Change your file2 to:

char global = 1; // no more tentative...but explicit.

Now if you compile like before, the tentative def in file1 will be ignored.

Make both the def explicit by:

int global = 1; // in file1

char global = 1; // in file2

now neither can be ignored and we get the multiple def error.