Extern variables and linking

[Dave]

Why does the program below link successfully? (Yep, what's shown is all
there is to it.) Does an extern variable actually have to be referenced to
be bound to an address? Is this Standard-compliant behavior?


int main()
{
extern int a;
}

 

[Rob Williscroft]

Dave wrote in

Why does the program below link successfully? (Yep, what's shown is
all there is to it.) Does an extern variable actually have to be
referenced to be bound to an address?

Sorry that last sentence just doesn't parse.

Is this Standard-compliant behavior?
Yes.



int main()
{
extern int a;

You're telling the compiler such an object 'a' exists, but since you
*never* reference it the compiler never uses the information. If this
wern't the case including headers would link huge amounts of unused
code and data (just so the optimizer can throw it all away again .

Try adding:

a = 1;

}

Rob.

 

[E. Robert Tisdale]

Why does the program below link successfully?
(Yep, what's shown is all there is to it.)
Does an extern variable actually have to be referenced
to be bound to an address?
Is this Standard-compliant behavior?

int main() {
extern int a;
}

cat main.c

int main(int argc, char* argv[]) {
extern int a;
return 0;
}

g++ -Wall -ansi -pedantic -c main.c

main.c: In function `int main(int, char**)':
main.c:2: warning: unused variable `int a'

nm main.o

00000000 T main

cat main.c

int main(int argc, char* argv[]) {
extern int a;
return a;
}

g++ -Wall -ansi -pedantic -c main.c
nm main.o

U a
00000000 T main