external variables in a multifile program

[murschech]

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____________________________

#include <iostream>
using namespace std;

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

________________file 2____________________________

int f(int x)
{   return (a*x);
}

[Dave Sinkula]

Does anyone have any insights to offer?

I'll try.

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.

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.

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]