#! perl -slw
use strict; use Config;
use Inline C => Config => BUILD_NOISY => 1;
use Inline C => <<'END_C',  NAME => 'ICexample', CLEAN_AFTER_BUILD =>0;

static const unsigned __int64 i = 0x7ff0000000000001;

SV*snan2() {
    // Allocate a new SvPV with some junk pv data
    SV *ret = newSVpvn( "JustJunk", 8 );

    sv_setnv( ret, *(double*)&i );

    // Now point the PV pointer directly at the NV memory so we can access it
    // (the NV) from perl without going through pack 'd'/'F'
    SvPV_set( ret, (char*)&( ( (XPVNV*)ret->sv_any )->xnv_u.xnv_nv ) );

    // make it persist
    SvREFCNT_inc_void_NN( ret );

    // Make sure noone can mess with it.
    SvREADONLY( ret );
    return ret;
}

END_C

## Get an SNAN.
my $snan2 = snan2();

##Print its numeric value AND its bit pattern bypassing SVNV?()
printf "%f\n%x\n", $snan2, unpack 'Q', $snan2;

## Same thing as the printf '%x', unpack 'Q' above
## that *might* work on 32-bit that doesn't have Q
print scalar reverse unpack 'h*', $snan2;

__END__
C:\test>snan-ic.pl
Use of uninitialized value in printf at C:\test\snan-ic.pl line 32.
1.#SNAN0
0
4e414e53232e31

##</code><code>##

Notionally, a fully loaded SV looks vaguely like this:

SV*->[ SVANY ]->[   IV   ]
     [ FLAGS ]  [   NV   ]
                [   PV   ]->"some text"
    
What my code above does is this:    

SV*->[ SVANY ]->[   IV   ]
     [ FLAGS ]  [   NV   ]<-+
                [   PV   ]--+    Point the PV at the memory holding the NV.