func (b *Reader) Read(p []byte) (n int, err error) {
	n = len(p)
	if n == 0 {
		if b.Buffered() > 0 {
			return 0, nil
		}
		return 0, b.readErr()
	}
	if b.r == b.w {
		if b.err != nil {
			return 0, b.readErr()
		}
		if len(p) >= len(b.buf) {
			// Large read, empty buffer.
			// Read directly into p to avoid copy.
			n, b.err = b.rd.Read(p)
			if n < 0 {

Assembly can refer to the size of this struct
as <code>reader__size</code> and the offsets of the two fields
as <code>reader_buf</code> and <code>reader_r</code>.
Hence, if register <code>R1</code> contains a pointer to
a <code>reader</code>, assembly can reference the <code>r</code> field
as <code>reader_r(R1)</code>.
</p>

<p>
If any of these <code>#define</code> names are ambiguous (for example,