* @param data A pointer to a buffer containing data to already queue for sending, or NULL if there is no data to send.
* After meshlink_send() returns, the application is free to overwrite or free this buffer.
* @param len The length of the data, or 0 if there is no data to send.
* @param data A pointer to a buffer containing data to already queue for sending, or NULL if there is no data to send.
* After meshlink_send() returns, the application is free to overwrite or free this buffer.
* @param len The length of the data, or 0 if there is no data to send.