problem: multiple files

[grandemahatma1]

hallo,
I'm pretty new to C and I'm having some problems while compiling multiple files; here what I do:

I've got 3 files:
1.cpp , 2.cpp , INIT.h

and these are the files:
1.cpp:

#include <INIT.h>
int main (int argc, char *argv[]) {
a=3;
printf("%d",a);
}


2.cpp:

#include <INIT.h>
void funzione(){
a=2;
}


INIT.h:

#ifndef HEADER
#define HEADER
#include <stdio.h>
int a;
#endif



Ok, now I compile them in this way:
g++ -c 1.cpp -I.
g++ -c 2.cpp -I.

but when I type:
g++ 1.o 2.o

it says:
2.o: (.bss+0x0): multiple definition of `a'
1.o: (.bss+0x0): first defined here
collect2: ld returned 1 exit status

why is it so?
thanks a lot for your help
Ale

[manontheedge]

you're declaring 'a' in the header file ... and then defining it in '1' AND '2'.

first, your file '2' isn't doing anything except trying to define what is being defined in file '1'

so, you are redefining 'a', which isn't allowed, you can only define that variable 'a' once.

by define, I mean, when you have a = 6, that's a definition ... you can't then say, a = 5 because it's already been defined somewhere else

[arnaudk]

by define, I mean, when you have a = 6, that's a definition ... you can't then say, a = 5 because it's already been defined somewhere else

That's not true. The following is perfectly valid:

int a;
int main()
{
  a=5;
  a=6; // changed my mind.
  return 0;
}
 

The problem grandemahatma1 is having is that a is declared twice within the same (global) scope. In this sense, the error message produced by the compiler is somewhat misleading but that's just a question of semantics.

The reason is that the inclusion guard can not work across translation units; INIT.h is included in 1.cpp when it is compiled, and again in 2.cpp when it is compiled since the two are preprocessed and compiled independently thus HEADER will be undefined in both cases when the header is included. After preprocessing, this leaves you with a declaration int a; in both 1.cpp and 2.cpp. Then you link the two together and wham, you get a multiple declaration error. To see what I mean, preprocess the code only using the -E option and you'll see the multiple declarations.

The solution is not to declare variables in header file but to declare "int a;" in one of the compilation units and declare "extern int a;" in the other compilation units to indicate that a has external linkage. Or, even better, not to use global variables at all.