s, using unix write.
Although it can be used with blocking file descriptors, it is mainly
intended for use with asynchronous procedures which write blocks of
data, to enable them to do so efficiently.
</p>
<p>This procedure provides a &rsquo;begin&rsquo; parameter indicating the start of
the sequence of bytes to be written, as an index.  &rsquo;fd&rsquo; is the file
descriptor of the device to be written to, and it should normally be
non-blocking.  &rsquo;bv&rsquo; is a bytevector containing the bytes to be