extern local variables

[j0mbolar]

void myfunction(void )
{
extern int myvariable;
return ;
}

what is the point in allowing local variables to have external linkage?
its scope is only myfunction so it can't be used anywhere else.

[Dan Pop]

Re: extern local variables

In <2d31a9f9.04030 40513.5f9550b7@ posting.google. com> j0mbolar@engine er.com (j0mbolar) writes:
[color=blue]
>void myfunction(void )
>{
> extern int myvariable;
> return ;
>}
>
>what is the point in allowing local variables to have external linkage?[/color]

You're confused.

extern int myvariable;

does NOT *define* myvariable, it merely *declares* myvariable as an
object defined somewhere else with external linkage.
[color=blue]
>its scope is only myfunction so it can't be used anywhere else.[/color]

The scope of the myvariable *identifier* is only myfunction. The actual
object was *defined* elsewhere and it's still available to any function
*declaring* it.

Example:

fangorn:~/tmp 264> cat test.c
#include <stdio.h>

void foo(void)
{
extern int myvariable;
printf("%d\n", myvariable);
}

void foo2(void)
{
extern int myvariable;
printf("%d\n", myvariable * 2);
}

int main()
{
foo();
foo2();
return 0;
}

int myvariable = 42;

fangorn:~/tmp 265> gcc -ansi -pedantic test.c
fangorn:~/tmp 266> ./a.out
42
84

You can also move the definition of myvariable to another source file.

Dan
--
Dan Pop
DESY Zeuthen, RZ group
Email: Dan.Pop@ifh.de

[Leor Zolman]

Re: extern local variables

On 4 Mar 2004 05:13:31 -0800, j0mbolar@engine er.com (j0mbolar) wrote:
[color=blue]
>void myfunction(void )
>{
> extern int myvariable;
> return ;
>}
>
>what is the point in allowing local variables to have external linkage?
>its scope is only myfunction so it can't be used anywhere else.[/color]

The /name/ may have local scope, but the variable it refers to most
certainly will not. This would be something you might choose to do
(personally, I don't think I've ever written a declaration like this) to
bring a file-scope variable defined in, say, other another translation unit
into scope within the body of this function, in case it isn't in scope
already. (Another possibility is that it is brought into file scope /below/
the point where this function is defined.)

Of course, if myvariable is already declared at file scope at the point
where your function definition resides, this would effectively constitute a
no-op declaration.
-leor



Leor Zolman
BD Software
leor@bdsoft.com
www.bdsoft.com -- On-Site Training in C/C++, Java, Perl & Unix
C++ users: Download BD Software's free STL Error Message
Decryptor at www.bdsoft.com/tools/stlfilt.html

[Derk Gwen]

Re: extern local variables

j0mbolar@engine er.com (j0mbolar) wrote:
# void myfunction(void )
# {
# extern int myvariable;
# return ;
# }
#
# what is the point in allowing local variables to have external linkage?
# its scope is only myfunction so it can't be used anywhere else.

What's the point in disallowing it? It's statically allocated and visible
to all other declarations of the same extern name.

--
Derk Gwen http://derkgwen.250free.com/html/index.html
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