Hi,

I have two source files call them a.c & b.c and a header file c.h.
In c.h I declare functions and I have a variable defined like "int my_var". (also tried "int my_var = 0") that should be accessible by both c files. In a.c & b.c I "import" the variable with "extern in my_var".
It compiles fine but upon linking I get errors like:
src.tcpip.proprietary.ftpclient.a.o:(.bss+0x1010): first defined here
src.tcpip.proprietary.ftpclient.b.o:(.bss+0x28): multiple definition of `my_var'
What do I have wrong here?

On Friday, May 28, 2021 at 9:00:59 PM UTC-7, stdcerr wrote:
> Hi,
>
> I have two source files call them a.c & b.c and a header file c.h.
> In c.h I declare functions and I have a variable defined like "int my_var". (also tried "int my_var = 0") that should be accessible by both c files. In a.c & b.c I "import" the variable with "extern in my_var".
> It compiles fine but upon linking I get errors like:
> src.tcpip.proprietary.ftpclient.a.o:(.bss+0x1010): first defined here
> src.tcpip.proprietary.ftpclient.b.o:(.bss+0x28): multiple definition of `my_var'
> What do I have wrong here?
I followed up and built the following test application:
A.c
#include "C.h"

int main(void){
int a = 0;
printf("A.c a %d\n",a);
Myexp();
printf("A.c a %d\n",a);
return 0;
B.c

#include "C.h"

int Myexp(void){
extern int a;
a=100;
printf("B.c a %d\n",a);
}
C.h
#ifndef __C_H__
#define __C_H__
#include <stdio.h>
int a;
int Myexp(void);
#endif

And compilation would give me:
gcc -o Myexp A.c B.c
/usr/bin/ld: /tmp/cczogQht.o:(.bss+0x0): multiple definition of `a'; /tmp/ccVRKYfb.o:(.bss+0x0): first defined here

stdcerr <ron.eggler@gmail.com> writes:

> I have two source files call them a.c & b.c and a header file c.h.
> In c.h I declare functions and I have a variable defined like "int
> my_var". (also tried "int my_var = 0") that should be accessible by
> both c files. In a.c & b.c I "import" the variable with "extern in
> my_var".
> It compiles fine but upon linking I get errors like:
> src.tcpip.proprietary.ftpclient.a.o:(.bss+0x1010): first defined here
> src.tcpip.proprietary.ftpclient.b.o:(.bss+0x28): multiple definition of `my_var'
> What do I have wrong here?

In .h files, shared variables should be declared using 'extern'.

Such variables then need to be defined in one, and only
one, .c file.

Example:

c.h:
#ifndef HAVE_c_h_HEADER
#define HAVE_c_h_HEADER

extern int shared_variable;

#endif

a.c:
#include "c.h"

...

b.c:
#include "c.h"

...

c.c:
#include "c.h"

int shared_variable; // no 'extern' => definition

Make sense?

On 5/29/21 12:00 AM, stdcerr wrote:
>
> Hi,
>
> I have two source files call them a.c & b.c and a header file c.h.
> In c.h I declare functions and I have a variable defined like "int my_var". (also tried "int my_var = 0") that should be accessible by both c files. In a.c & b.c I "import" the variable with "extern in my_var".
> It compiles fine but upon linking I get errors like:
> src.tcpip.proprietary.ftpclient.a.o:(.bss+0x1010): first defined here
> src.tcpip.proprietary.ftpclient.b.o:(.bss+0x28): multiple definition of `my_var'
> What do I have wrong here?
>

The combination of a given source file and all of the other files
incorporated into it by #include is called a translation unit. In your
case, there's two translation units, one associated with a.c which
#includes the contents of c.h, and a second associated with b.c which
also #includes the contents of c.h

Because of the initializer, "int my_var=0;" is a definition of my_var.
Even without the initializer, it would qualify as a "tentative
definition" - if no other external definition occurs in the same
translation unit, it will be treated as an actual definition with an
implicit value of 0. Because you have such a definition in c.h, every
translation unit that #includes c.h will have such a definition.

If your program contains two or more external definitions of an
identifier, it has undefined behavior. Your program therefore runs afoul
of this rule.

"extern int my_var;" is only a declaration of my_var, not a definition.
You can (and should) have such a declaration in every file where my_var
is either defined or used. That is what you should have in c.h.

However, if your program makes any actual use of a object declared with
external linkage, it must have exactly one external definition of that
object. You could provide that definition by adding "int my_var=0;" to
either a.c or b.c, but not both.

However, as a matter of convention, rather than the rules of the
language, it would be more conventional for functions and objects with
external linkage that are declared in "c.h" to be defined in "c.c",
rather than either a.c or b.c.

> #ifndef __C_H__
> #define __C_H__

Use #pragma once - any relevant compiler supports it.

On 29/05/2021 05:09, stdcerr wrote:
> On Friday, May 28, 2021 at 9:00:59 PM UTC-7, stdcerr wrote:
>> Hi,
>>
>> I have two source files call them a.c & b.c and a header file c.h.
>> In c.h I declare functions and I have a variable defined like "int my_var". (also tried "int my_var = 0") that should be accessible by both c files. In a.c & b.c I "import" the variable with "extern in my_var".
>> It compiles fine but upon linking I get errors like:
>> src.tcpip.proprietary.ftpclient.a.o:(.bss+0x1010): first defined here
>> src.tcpip.proprietary.ftpclient.b.o:(.bss+0x28): multiple definition of `my_var'
>> What do I have wrong here?
> I followed up and built the following test application:
> A.c
> #include "C.h"
>
> int main(void){
> int a = 0;
> printf("A.c a %d\n",a);
> Myexp();
> printf("A.c a %d\n",a);
> return 0;
> B.c
>
> #include "C.h"
>
> int Myexp(void){
> extern int a;
> a=100;
> printf("B.c a %d\n",a);
> }
> C.h
> #ifndef __C_H__
> #define __C_H__
> #include <stdio.h>
> int a;
> int Myexp(void);
> #endif
>
> And compilation would give me:
> gcc -o Myexp A.c B.c
> /usr/bin/ld: /tmp/cczogQht.o:(.bss+0x0): multiple definition of `a'; /tmp/ccVRKYfb.o:(.bss+0x0): first defined here
>

If you want 'a' to be shared between A and B, then declare the files
like this:

----------------------
A.c
----------------------

#include <stdio.h>
#include "C.h"

int a; // can optionally be initialised here

int main(void){
printf("A.c a %d\n",a);
Myexp();
printf("A.c a %d\n",a);
return 0;
}

----------------------
B.c
----------------------
#include <stdio.h>
#include "C.h"

void Myexp(void){ // ret value not used, not given anyway, use 'void'

a=100;
printf("B.c a %d\n",a);
}

----------------------
C.h
----------------------
extern int a;
extern void Myexp(void);