White Paper: Esound

Overview

Esound (ESD) is a stand-alone sound daemon which abstracts the system sound device to multiple clients. Under Linux using the Open Sound System (OSS), as well as other UNIX systems, typically only one process may open the sound device. This is not acceptable in a desktop environment like GNOME, as it is expected that many applications will be making sounds (music decoders, event based sounds, video conferencing, etc). The ESD daemon connects to the sound device and accepts connections from multiple clients, mixing the incoming audio streams and sending the result to the sound device. Connections are only allowed to clients which can authenticate successfully, alleviating the concern that unauthorized users can eavesdrop via the sound device. In addition to accepting client connections from the local machine, ESD can be configured to accept client connections from remote hosts which authenticate successfully.

Applications wanting to contact the ESD daemon do so using the libesd library. Much like with file i/o, a ESD connection is first opened. The ESD daemon will be spawned automatically by libesd if a daemon is not already present. Data is then either read or written to the ESD daemon. For a ESD client local to the machine which the ESD daemon is running on, the data is transferred through a local socket, then written to the sound device by the ESD daemon. For a client on a remote machine, the data is sent by libesd on the remote machine over the network to the ESD daemon. The process is completely transparent to the application using ESD.

Figure 1. The ESD Process