You shouldn't be using C IO lib, you should be using Perl's. (Well, I think so at least. I have no experience in this area.)

Update: Yup, that does the trick: 

void c_output() {
    PerlIO_printf(PerlIO_stdout(), "###c_output");
}
void c_output_newline() {
    PerlIO_printf(PerlIO_stdout(), "###c_output_newline\n");
}

[download]
>set PERLIO=stdio

>perl a.pl
===============dry run============
        printing [perl_output_newline]
***perl_output_newline
        calling  [c_output_newline]
###c_output_newline
        printing [perl_output]
***perl_output  calling  [c_output]
###c_output
=============test_global==========
        printing [perl_output_newline]
        calling  [c_output_newline]
        printing [perl_output]
        calling  [c_output]
test_global buffer:
(***perl_output_newline\n###c_output_newline\n***perl_output###c_outpu
+t)


>set PERLIO=perlio

>perl a.pl
===============dry run============
        printing [perl_output_newline]
***perl_output_newline
        calling  [c_output_newline]
###c_output_newline
        printing [perl_output]
        calling  [c_output]
***perl_output###c_output
=============test_global==========
        printing [perl_output_newline]
        calling  [c_output_newline]
        printing [perl_output]
        calling  [c_output]
test_global buffer:
(***perl_output_newline\n###c_output_newline\n***perl_output###c_outpu
+t)

[download]

In reply to Re: Redirecting XS stdout by ikegami
in thread Redirecting XS stdout by innominate

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.