First, keep in mind I've never used XS. I can't guarantee the following works or even its soundness.

If you check the system typemap (lib/ExtUtils/typemap), you'll see the definition of T_PTROBJ. (What is T_OBJECT?) We can create a new rule based on T_PTROBJ to suit our needs. 

# typedef struct bar*      Bar;
# typedef struct type_foo* Foo;

TYPEMAP
#######

Bar    T_NULLABLE_PTROBJ
Foo    T_NULLABLE_PTROBJ


INPUT
#####

T_NULLABLE_PTROBJ
    if ( !SvOK( $arg ) ) {  /* if not defined */
        $var = NULL;
    }
    else if ( sv_derived_from( $arg, \"${ntype}\" ) ) {
        IV tmp = SvIV( (SV*)SvRV( $arg ) );
        $var = INT2PTR( $type, tmp );
    }
    else {
        Perl_croak( aTHX_ \"$var is not of type ${ntype}\" )
    }


OUTPUT
######

T_NULLABLE_PTROBJ
    if ( $var )  /* if defined */
        sv_setref_pv( $arg, \"${ntype}\", (void*)$var );
    else
        $arg = &PL_sv_undef;

[download]

The usage would be 

my $foo = undef;
my $bar = Foo::create_bar( $foo );

[download]

Update: I don't think addressing the problem in the typemap is the best method. This makes pointers of that type always nullable, whereas that should be decided on a per-function (and even per-argument) basis. That means we should addresses the issue in the XS stub. 

# typedef struct bar*      Bar;
# typedef struct type_foo* Foo;

# In typemap:
# Bar    T_PTROBJ
# Foo    T_PTROBJ

struct bar*
create_bar(foo_sv = 0)
    SV* foo_sv

CODE:
    struct type_foo* foo = NULL;

    if ( !foo_sv ) {
        /* Use default of NULL. */
    }
    else if ( !SvOK( foo_sv ) ) {
        /* Use default of NULL. */
    }
    else if ( sv_derived_from( foo_sv, 'type_foo' ) ) {
        IV tmp = SvIV( (SV*)SvRV( foo_sv ) );
        foo = INT2PTR( struct type_foo*, tmp );
    }
    else {
        Perl_croak( aTHX_ "\$foo is not of type foo_type" )
    }

    RETVAL = my_c_create_bar(foo);

OUTPUT:
    RETVAL

[download]

Bonus: The = 0 and the if ( !foo_sv ) allows the parameter to be omitted. Feel free to remove this.

Update: Now uses tabs in typemap, as required.

Update: struct bar* and struct type_foo* are no good for type T_PTROBJ — search for T_PTROBJ_SPECIAL in perlxs — so I added typedefs.

In reply to Re: Passing NULL pointer through XS without "Use of uninitialized value in subroutine entry" by ikegami
in thread Passing NULL pointer through XS without "Use of uninitialized value in subroutine entry" by kscaldef

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