Creating callback from XS

XSTesting.xs

/*
 * Usage:
 
 my $x = XSTesting::makecb("my arg");
 say $x->();

*/

#include "EXTERN.h"
#include "perl.h"
#include "XSUB.h"

#include "ppport.h"

STATIC MGVTBL null_mg_vtbl = {
    NULL, /* get */
    NULL, /* set */
    NULL, /* len */
    NULL, /* clear */
    NULL, /* free */
#if MGf_COPY
    NULL, /* copy */
#endif /* MGf_COPY */
#if MGf_DUP
    NULL, /* dup */
#endif /* MGf_DUP */
#if MGf_LOCAL
    NULL, /* local */
#endif /* MGf_LOCAL */
};


XS(cb_test) {
	dVAR;
	dXSARGS;
	dORIGMARK;

	PERL_CONTEXT *cx = &cxstack[cxstack_ix];
	
	U32 cxtype = CxTYPE (cx) & CXTYPEMASK;
	char *type;
	switch(cxtype)
	{
		case CXt_NULL:   type = "null"; break;
		case CXt_SUB:    type = "sub"; break;
		case CXt_EVAL:   type = "eval"; break;
		case CXt_SUBST:  type = "subst"; break;
		case CXt_BLOCK:  type = "block"; break;
		case CXt_FORMAT: type = "format"; break;
		default:
			warn("Unknown context type 0x%lx\n", cxtype);
			type = "(unknown)";
	}
	
	// CxTYPE (cx) here is CXt_BLOCK, not CXt_SUB
	// I can't yet find a way to access my CV *.
	warn("my type is %d: %s", CxTYPE(cx), type);
	
	SP = ORIGMARK;
	PUSHs(&PL_sv_yes);
}

MODULE = XSTesting		PACKAGE = XSTesting

void makecb(SV *var)
	PPCODE:
		CV *myxs = newXS(0, cb_test, __FILE__);
		void *context = (void *) var;
		
		// Currently I have an ideas:
		// - Assign context ptr to magic of CV; Something like:
		//		sv_magicext((SV *) myxs, NULL, PERL_MAGIC_ext, &null_mg_vtbl, context, 0);
		// - Create PADLIST for CV and store context there
		//		how?
		
		ST(0) = newRV_noinc( (SV *) myxs );
		XSRETURN(1);