spacepille has asked for the wisdom of the Perl Monks concerning the following question:

The following does not work with a tied handle. 
perlio = IoIFP( sv_2io( sv ) );
PerlIO_read( perlio, ... );

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Does anybody know how to access tied handles from XS? Thank you.

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Re: tied handle in xs
by plobsing (Friar) on Oct 03, 2008 at 04:19 UTC
    I have no experience working with tied objects in XS, but I imagine it would be similar to whatever the perl core does.

    First lookup what op does the same thing you want to do. In this case, read (called pp_read in C and located in mathoms.c). It delegates to pp_sysread in pp_sys.c.

    The definition of this op is in the section following PP(pp_sysread)

    The section that handles tied handles is: 
        if ((PL_op->op_type == OP_READ || PL_op->op_type == OP_SYSREAD)
        && gv && (io = GvIO(gv)) )
    {
        const MAGIC * mg = SvTIED_mg((SV*)io, PERL_MAGIC_tiedscalar);
        if (mg) {
            SV *sv;
            PUSHMARK(MARK-1);
            *MARK = SvTIED_obj((SV*)io, mg);
            ENTER;
            call_method("READ", G_SCALAR);
            LEAVE;
            SPAGAIN;
            sv = POPs;
            SP = ORIGMARK;
            PUSHs(sv);
            RETURN;
        }
    }

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      I have no experience working with tied objects in XS, but I imagine it would be similar to whatever the perl core does.
      There's better ways, perlXStut: EXAMPLE 9 Passing open files to XSes.
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        I've read that. It suggests using typemaps for PerlIO objects (my first resort).

        However, looking at the T_IN and T_INOUT typemaps, they both do $var = IoIFP(sv_2io($arg)).

        That apparently isn't working for spacepille. So I looked for other options.
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      big thank you, it works... 
      IO *io = sv_2io( sv );
PerlIO *perlio = IoIFP( io );
if( perlio != NULL ) {
    PerlIO_read( perlio, ... );
}
else {
    const MAGIC *mg = SvTIED_mg( (SV *) io, 'q' );
    if( mg != NULL ) {
        sv = SvTIED_obj( (SV *) io, mg );
        // call the READ method with sv as object...
    }
    else {
        Perl_croak( aTHX_ "invalid IO handle" );
    }
}

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