Unfortunately not. If I substitute SvOK( sv ) for my more complex test, then I get this output from my testcases:

c:\test>test
billfred
Use of uninitialized value in subroutine entry at c:\test\test.pl line
+ 30.
fred
billfred
fred
fred1
[download]

Compare that with the output from the original

c:\test>test
billfred
fred
billfred
1fred
1fred1
[download]

The problem is that it detects that (N|I|U)OK is true, but it doesn't cause the PV to be set to reflect their contents. So, when you call sv_catsv the value in the NV|IV|UV gets discarded. Or at least that's my best interpretation of what I am seeing.

You're getting the undef warning via the SvPV_nolen macro. If you change that to use sv_setpvn you won't get that anymore. This...

if (!SvOK(sv)) 
    sv_setpvn(sv, "", 0);
[download]

... is the XS equivalent of this...

$sv = "" unless defined $sv;
[download]

--
Marvin Humphrey
Rectangular Research ― http://www.rectangular.com

Once again thankyou. That does greatly simplify the code and produce the desired results:

SV* test( SV *a, SV *b ) {
    if( SvREADONLY( a ) )
        a = newSVpv( SvPVX( a ), 0 );
    else
       SvREFCNT_inc( a );

    if( !SvOK( a ) )
        sv_setpvn( a, "", 0 );

    sv_catsv( a, b );
    return a;
}

...
__END__
c:\test>test
billfred
fred
billfred
1fred
1fred1
[download]

---

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