Sharing double with C and Perl?

[Tim Pushor]

First of all, I am completely overwhelmed with XS, so this may be a dumb 
question ;-)

Good or bad, I want to be able to share a double with C and Perl. The 
complicated part is that I need to be able to take the address of the 
double in C (as a global) and use it through various functions. i.e.

double *globalvar;
void setvar (double *var) {
        globalvar=var;
}
void process1 (double value) {
        *globalvar=value;
}
void show (void) {
        printf ("XS: Var is now %f\n",*globalvar);
}
And in perl:
$a=0;
setvar($a);
printf ("Perl: Var is initially $var\n";
process1(5);
printf ("Perl: Var is now $var\n";
$a=10;
show();
----
The result should be
Perl: Var is initially 0
Perl: Var is now 5
XS: Var is now 10.000000
---
Is this possible somehow? Can I get the address of the perl variable (I 
am thinking I need to use a reference somehow)

Thanks for any and all advice!
Tim