Perl XS unused variable 'Perl___notused' warnings

Question

I am learning how to call C++ code from Perl and to start I am trying to create a basic C++ object from a Perl script.

To do this, I started by executing the h2xs command:

h2xs -A -nMyClass

Then I added the following two arguments to the generated Makefile.PL to use the g++ compiler.

CC => 'g++',
LD => 'g++',

I created my simple class in the .xs file and wrote the XS code to map it with Perl

MyClass.xs

#ifdef __cplusplus
extern "C" {
#endif
#include "EXTERN.h"
#include "perl.h"
#include "XSUB.h"
#include "ppport.h"
#ifdef __cplusplus
}
#endif

class MyClass {
public:
    MyClass(int value) {
        value_ = value;
    }
    ~MyClass() {}

    int value() { return value_; }

    void set_value(int value) {
        value_ = value;
    }
private:
    int value_;
};

MODULE = MyClass                PACKAGE = MyClass

MyClass *
MyClass::new(int value)

void
MyClass::DESTROY()

int
MyClass::value()

void
MyClass::set_value(int value)

Then I created the typemap file to map the new type to Perl.

typemap

TYPEMAP

MyClass *       O_OBJECT

######################################################################
OUTPUT

# The Perl object is blessed into 'CLASS', which should be a
# char* having the name of the package for the blessing.
O_OBJECT
        sv_setref_pv( $arg, CLASS, (void*)$var );

######################################################################
INPUT

O_OBJECT
        if( sv_isobject($arg) && (SvTYPE(SvRV($arg)) == SVt_PVMG) )
                $var = ($type)SvIV((SV*)SvRV( $arg ));
        else{
                warn( \"${Package}::$func_name() -- $var is not a blessed SV reference\" );
                XSRETURN_UNDEF;
        }

Finally I created a simple test.

t/MyClass.t

use Test::More tests => 2;
BEGIN { use_ok('MyClass') };

my $obj = MyClass->new(1);
ok($obj->isa('MyClass'), 'MyClass object constructed');

I then successfully built the code and ran the tests.

perl Makefile.PL
make
make test

---

Although everything works fine, I get some warnings with the build:

MyClass.c: In function 'void XS_MyClass_new(PerlInterpreter*, CV*)':
MyClass.c:95: warning: unused variable 'Perl___notused'
MyClass.c: In function 'void XS_MyClass_DESTROY(PerlInterpreter*, CV*)':
MyClass.c:119: warning: unused variable 'Perl___notused'
MyClass.c: In function 'void XS_MyClass_value(PerlInterpreter*, CV*)':
MyClass.c:145: warning: unused variable 'Perl___notused'
MyClass.c: In function 'void XS_MyClass_set_value(PerlInterpreter*, CV*)':
MyClass.c:174: warning: unused variable 'Perl___notused'
MyClass.c: In function 'void boot_MyClass(PerlInterpreter*, CV*)':
MyClass.c:203: warning: unused variable 'Perl___notused'

I searched all over trying to find the cause for these warnings, and can't figure out what is going on. All of the warnings seem come from the same repeated section in the code that occurs at the beginning of every function definition.

inside MyClass.c

XS(XS_MyClass_new); /* prototype to pass -Wmissing-prototypes */
XS(XS_MyClass_new)
{
#ifdef dVAR
    dVAR; dXSARGS; // <-- warning occurs here
#else
    dXSARGS;
#endif
    // function body continues...

Can someone please tell me the root cause of these warnings?

I am using Perl v5.10.1 and g++ version 4.4.7

Answer 1

The solution is to use a newer version of Perl. v5.10.1 is very outdated and these problems don't occur in later versions.

I am posting an answer to my own question because it was answered in the comments, but an official answer was never posted.