I need to open a filehandle in Perl, store the handle in a hash-based object, then call an object method written in XS which will write something to the filehandle. It seems like pulling a filehandle out a hash and dereferencing it ought to be just as easy as doing the same thing with an arrayref or hashref, but the following code doesn't work (PerlIO_write fails and returns -1)...

    /* Extract the object hash and the output filehandle. */
    SV* obj_sv     = ST(0);
    HV* obj_hash   = (HV*)SvRV(obj_sv);
    SV* out_fh_ref = *(hv_fetch(obj_hash, "fh", 2, 0));
    PerlIO* out_fh = (PerlIO*)SvRV(out_fh_ref);

    err = PerlIO_write( out_fh, buf_start, buf_len );
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I'm all but certain the problem is in casting the dereferenced scalar to a PerlIO*, but I've been unable to figure out an alternative. Snooping the code on some CPAN modules, I've found that a few use GvIOn, but that's not documented in the current perlapi, and I'm not sure what it does.

Any advice?

Update: It appears that this is one solution:

    PerlIO* fh = IoOFP( sv_2io(out_fh_ref) );
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However, the jury is out on whether it is OK to use IoOFP -- it's not documented as a part of the official perlapi.