Here is some typemap code I copied from my own project. I mostly figured this out (aka copy/pasted) from the perlxstut manpage I think.

TYPEMAP

# You need a typedef for BarPointOrNull in your .xs file or a header..
+.
# Like this: typedef bar_t * BarPointerOrNull;
# Then you can use it in the XS parameter list:
# routine( a, b, c )
#     char           * a
#     int              b
#     BarPointerOrNull c

BarPointerOrNull T_BAR_OR_NULL

INPUT

#---------------------------------------------------------------------
+--------
# Map a perl reference to a C pointer...
#---------------------------------------------------------------------
+--------

T_BAR_OR_NULL
    if ( ! SvOK( $arg )) {
        $var = NULL;
    }
    /* Make sure we are given an object of class Bar::Class... */
    else if ( sv_derived_from( $arg, \"Bar::Class\" )) {
        IV tmp = SvIV( (SV *) SvRV( $arg ));
        $var   = INT2PTR( $type, tmp );
    }
    /* Holy crap! I must have copied this from the manpage... nice. */
    else {
        Perl_croak(aTHX_ \"%s: %s is not of type %s\",
            ${$ALIAS?\q[GvNAME(CvGV(cv))]:\qq[\"$pname\"]},
            \"$var\", \"Bar::Class\")
    }


OUTPUT

#---------------------------------------------------------------------
+--------
# Map a C pointer to a perl reference...
#---------------------------------------------------------------------
+--------

T_BAR_OR_NULL
    if ( $var == NULL ) {
        $arg = &PL_sv_unef;
    }
    else {
        /* We bless the pointer as a Bar::Class object... can be handy
         * for wrapping structures in objects.
         */
        sv_setref_pv( $arg, \"Bar::Class\", (void *)$var );
    }

# $var is the C variable, $type is the C type, $arg is the perl variab
+le
# Um that's all I know. OH and always escape your double-quotes!
[download]

EDIT On second thought... this won't work. The XS function will complain about not enough arguments if you don't give it a third argument. You could always wrap it in a pure-perl function, though...

# Silly really... but so easy!
sub myroutine { my ($a, $b, $c) = @_; routine( $a, $b, $c ); }
[download]

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