private synchronized int expelBuffer(long blockFor) throws IOException {
    if (_buffer == null) return 0;
    int result = flushBuffer();
    _buffer.compact();
    if (!_endp.isBlocking()) {
      while (_buffer.space() == 0) {
        boolean ready = _endp.blockWritable(blockFor);
        if (!ready) throw new IOException("Write timeout");

        result += flushBuffer();
        _buffer.compact();
      }
    }
    return result;
  }
  public synchronized void addFrame(byte flags, byte opcode, byte[] content, int offset, int length)
      throws IOException {
    // System.err.printf("<< %s %s
    // %s\n",TypeUtil.toHexString(flags),TypeUtil.toHexString(opcode),length);

    long blockFor = _endp.getMaxIdleTime();

    if (_buffer == null)
      _buffer = (_maskGen != null) ? _buffers.getBuffer() : _buffers.getDirectBuffer();

    boolean last = WebSocketConnectionD06.isLastFrame(flags);
    opcode = (byte) (((0xf & flags) << 4) + 0xf & opcode);

    int space = (_maskGen != null) ? 14 : 10;

    do {
      opcode = _opsent ? WebSocketConnectionD06.OP_CONTINUATION : opcode;
      _opsent = true;

      int payload = length;
      if (payload + space > _buffer.capacity()) {
        // We must fragement, so clear FIN bit
        opcode &= (byte) 0x7F; // Clear the FIN bit
        payload = _buffer.capacity() - space;
      } else if (last) opcode |= (byte) 0x80; // Set the FIN bit

      // ensure there is space for header
      if (_buffer.space() <= space) expelBuffer(blockFor);

      // write mask
      if ((_maskGen != null)) {
        _maskGen.genMask(_mask);
        _m = 0;
        _buffer.put(_mask);
      }

      // write the opcode and length
      if (payload > 0xffff) {
        bufferPut(
            new byte[] {
              opcode,
              (byte) 0x7f,
              (byte) 0,
              (byte) 0,
              (byte) 0,
              (byte) 0,
              (byte) ((payload >> 24) & 0xff),
              (byte) ((payload >> 16) & 0xff),
              (byte) ((payload >> 8) & 0xff),
              (byte) (payload & 0xff)
            });
      } else if (payload >= 0x7e) {
        bufferPut(new byte[] {opcode, (byte) 0x7e, (byte) (payload >> 8), (byte) (payload & 0xff)});
      } else {
        bufferPut(opcode);
        bufferPut((byte) payload);
      }

      // write payload
      int remaining = payload;
      while (remaining > 0) {
        _buffer.compact();
        int chunk = remaining < _buffer.space() ? remaining : _buffer.space();

        if ((_maskGen != null)) {
          for (int i = 0; i < chunk; i++) bufferPut(content[offset + (payload - remaining) + i]);
        } else _buffer.put(content, offset + (payload - remaining), chunk);

        remaining -= chunk;
        if (_buffer.space() > 0) {
          // Gently flush the data, issuing a non-blocking write
          flushBuffer();
        } else {
          // Forcibly flush the data, issuing a blocking write
          expelBuffer(blockFor);
          if (remaining == 0) {
            // Gently flush the data, issuing a non-blocking write
            flushBuffer();
          }
        }
      }
      offset += payload;
      length -= payload;
    } while (length > 0);
    _opsent = !last;
  }