The Perl API provides PerlIO_exportFILE() which can convert a PerlIO handle with a file descriptor to a stdio `FILE` pointer

Yes, I mentioned in my first post that I had also taken a look at that option.

Since PerlIO::Scalar is an "in-memory" file handle without a file descriptor the conversion cannot succeed

Thank you - that explanation makes sense to me.
One other thing that I should probably try out is the earlier (anonymous) suggestion of staying within the perlio framework: 
use warnings;
use strict;

use Inline C  => Config =>
  BUILD_NOISY => 1;

use Inline C => <<'EOC';

void to_FH(PerlIO * stream) {
     PerlIO_printf(stream, "hello");
     PerlIO_flush(stream);
}

EOC

my ($got1, $got2);

$got1 = get_string_1();
chomp($got1); # just in case ...
print "OK 1\n" if $got1 eq "hello";


$got2 = get_string_2();
chomp($got2); # just in case ...
print "OK 2\n" if $got2 eq "hello";


sub get_string_1 {
   # Write to a temporary file
   open TEMPFILE, '+>', undef or die $!;
   to_FH(*TEMPFILE);
   seek TEMPFILE, 0, 0;
   my $ret = <TEMPFILE>;
   close TEMPFILE;
   return $ret;
}


sub get_string_2 {
   # Write to a memory file
   my $out;
   open MEM, '>', \$out or die $!;
   to_FH(*MEM);
   close MEM;
   return $out;
}

__END__

After compilation, outputs:
OK 1
OK 2

[download]
Works fine.
Thanks guys.

Cheers,
Rob

In reply to Re^2: XSub won't write to memory file by syphilis
in thread XSub won't write to memory file by syphilis

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.