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external variables in a multifile program

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Join Date: Dec 2004

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murschech

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external variables in a multifile program

Jul 28th, 2005

I have a question on external variables in a multifile program.
Quoting from K&R (first addition, page 72)

"By default, external variables are also "global", so that all references to such a variable by the same name (even from functions compiled separately) are references to the same thing."

Here's an example that doesn't work. The variable 'a' in file 1 is external (since it's defined outside of any function) so, according to the above paragraph, it should have the same value in file 2. The
program, however, doesn't compile. If the line "int a" is added to
file 2 it compiles and runs, giving the wrong answer 0. If instead the
line "extern int a" is added then it runs and gives the correct
answer 12. I understand why the last version works, but shouldn't
the first one also (according to K&R)?

Does anyone have any insights to offer?

_____________________file 1____________________________

 Help with Code Tags 
 C++ Syntax (Toggle Plain Text) 
#include <iostream>
using namespace std;
 
int a=6;
int f(int);
main()
{   cout << f(2) << endl;
}
#include <iostream>
using namespace std;

int a=6;
int f(int);
main()
{   cout << f(2) << endl;
}

________________file 2____________________________

 Help with Code Tags 
 C++ Syntax (Toggle Plain Text) 
int f(int x)
{   return (a*x);
}
int f(int x)
{   return (a*x);
}

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Dave Sinkula Dave Sinkula is offline

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long time no c

Re: external variables in a multifile program

Jul 28th, 2005

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Originally Posted by murschech

Does anyone have any insights to offer?

I'll try.

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Originally Posted by murschech

Here's an example that doesn't work. The variable 'a' in file 1 is external (since it's defined outside of any function) so, according to the above paragraph, it should have the same value in file 2. The program, however, doesn't compile.

As it shouldn't. Identifiers need to be declared. The linkage of an identifier is a bit separate.

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Originally Posted by murschech

If the line "int a" is added to file 2 it compiles and runs, giving the wrong answer 0.

I get a linker error because adding this definition conflicts with the definition in the other file.

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Originally Posted by murschech

If instead the line "extern int a" is added then it runs and gives the correct answer 12. I understand why the last version works, but shouldn't the first one also (according to K&R)?

I'll borrow Chris Torek's words of wisdom: "At file scope, 'extern' means, approximately, 'read this as a declaration, not a definition'".

[aside]Using K&R1 as a reference for C++98 seems to leave room for a number of gaps.[/aside]

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