Question about "extern"-clarification

**Drogin** · 08-30-2009

Another related question:
Say I have the following 3 files:
-main.c
-writeFunction.c
-header.h

And I want main.c and writeFunction.c to have a common global variable.
So that I can modify the variable in main, and then the variable in the function in "writeFunction.c" would be updated as well.

One possibilty is to define the variable in for example main, and in "header.h", include

Code:

extern int globalCommonVariable;

But, would it be correct to instead drop defining the globalCommonVariable, and instead just do like this in the header:

Code:

int globalCommonVariable;

I have compiled and tested it, and if I do it like this, it works, and the variable is shared, with only 1 definition, and no use what so ever of extern.
However...is it safe, and is it correct according to standards and all that stuff?

**Elysia** · 08-30-2009

No, it's not correct because you'd be defining the variable twice in the two source files, and you cannot have variables with the same name in multiple source files on a global level.
Therefore you have to define it in our source file and use a declaration everywhere you use so the compiler knows it exists.

**Drogin** · 08-30-2009

I'll give you my 3 files...I've compiled and linked, and it works :S

main.c

Code:

#include "headerFil.h" // get the global variable "test"

int main(int argc, char *argv[]) {
  test = 10; // Modify the global variable 
  moo(); // Write the global variable, using a function from fil1.c
  system("PAUSE");	
  return 0;
}

fil1.c

Code:

#include "headerFil.h" // Get the global variable "test"

void moo(void) {
  printf("variabelen er:%d\n", test); // Just print the global variable
}

headerFil.h

Code:

#ifndef MOFF
#define MOFF
void moo(void);
int test; // Global variable to be used in 2 files
#endif

So..is it just because my compiler/linker are very friendly, and smart, that this works?

**tabstop** · 08-30-2009

Originally Posted by Drogin

Another related question:
Say I have the following 3 files:
-main.c
-writeFunction.c
-header.h

And I want main.c and writeFunction.c to have a common global variable.
So that I can modify the variable in main, and then the variable in the function in "writeFunction.c" would be updated as well.

One possibilty is to define the variable in for example main, and in "header.h", include

Code:

extern int globalCommonVariable;

But, would it be correct to instead drop defining the globalCommonVariable, and instead just do like this in the header:

Code:

int globalCommonVariable;

I have compiled and tested it, and if I do it like this, it works, and the variable is shared, with only 1 definition, and no use what so ever of extern.
However...is it safe, and is it correct according to standards and all that stuff?

Some compilers will save you from yourself. For instance, I just tried with gcc; if I only typed "int globalCommonVariable" I got away with it -- if I did "int globalCommonVariable = 3" I got a duplicate symbol error.

**abachler** · 08-30-2009

Originally Posted by tabstop

Some compilers will save you from yourself. For instance, I just tried with gcc; if I only typed "int globalCommonVariable" I got away with it -- if I did "int globalCommonVariable = 3" I got a duplicate symbol error.

That's not saving you from yourself, thats standard behavior. declaration can take place in multiple places, initialization can occur only once.

**tabstop** · 08-30-2009

Originally Posted by abachler

That's not saving you from yourself, thats standard behavior. declaration can take place in multiple places, initialization can occur only once.

And since global variables are automatically initialized to zero .... what's your point?

**Drogin** · 08-30-2009

So, that the code from this headerfile works, is because the compiler is saving me?

Code:

#ifndef MOFF
#define MOFF
void moo(void);
int test; // Global variable to be used in 2 files
#endif

**tabstop** · 08-30-2009

Originally Posted by Drogin

So, that the code from this headerfile works, is because the compiler is saving me?

Code:

#ifndef MOFF
#define MOFF
void moo(void);
int test; // Global variable to be used in 2 files
#endif

Well, you know, maybe I'm wrong here. I've been reading through trying to decide whether int test here is a definition or just a declaration. For some reason I had thought that all file scope declarations were definitions, but I can't find that now, or anything close to that. This might be a difference between C++ and C, since in C++ this is illegal. More updates as events warrant.

UPDATE: Oh hey, this is in the C++ forum! So, yes, this is illegal in C++.

**Elysia** · 08-30-2009

Originally Posted by tabstop

UPDATE: Oh hey, this is in the C++ forum! So, yes, this is illegal in C++.

Ah, but the OP's example is C, though.

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