Try starting with something that works, and then add to it.
For example, I have no trouble with: 
use warnings;
use strict;

use Inline ('C' => 'DATA');

callback();

sub p_callback {
    print "in perl callback\n";
}

__DATA__
__C__


void callback(){
    dSP;
    PUSHMARK(SP);
    call_pv("p_callback", G_DISCARD|G_NOARGS);
}

[download]
Does that work ok for you ? Make sure there's an empty line after the end of the C code.
I don't have libwiringPi, and can't really help with the rest. (If I don't have code that I can actually test then I'm likely to end up presenting misinformation.)

Are you aware of the Inline::C configuration option CLEAN_AFTER_BUILD=>0
It leaves the build directory intact so you can actually inspect the generated XS file and C file - which is sometimes helpful when things aren't working as expected.

Cheers,
Rob

In reply to Re^5: XS: Passing an external library's function a Perl XS callback by syphilis
in thread SOLVED: XS: Passing an external library's function a Perl XS callback by stevieb

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