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Please note that on FreeBSD things work a little bit differently for non-blocking sockets:
http://www.freebsd.org/cgi/man.cgi?query=sendfile&sektion=2
In details I'm talking about:
> When using a socket marked for non-blocking I/O, sendfile() may send
> fewer bytes than requested. In this case, the number of bytes 
> success-fully written is returned in *sbytes (if specified), and the 
> error EAGAIN is returned.
...and the similar note about EBUSY, later in the page.
Something like this should work:
ret = sendfile(in, out, offset, &sbytes, &sf, flags);
...
if (ret == -1) {
 if ((errno == EAGAIN) || (errno == EBUSY)) {
 return Py_BuildValue("ll", sbytes, offset + sbytes);
 } 
 return posix_error();
}