Open Sound System

The Open Sound System (OSS) is an interface for making and capturing sound in Unix and Unix-like operating systems. It is based on standard Unix devices system calls (i.e. POSIX read, write, ioctl, etc.). The term also sometimes refers to the software in a Unix kernel that provides the OSS interface; it can be thought of as a device driver (or a collection of device drivers) for sound controller hardware. The goal of OSS is to allow the writing of sound-based applications that are agnostic of the underlying sound hardware.
OSS was created by Hannu Savolainen and is distributed under four license options, three of which are free software licences, thus making OSS free software.
API.
The API is designed to use the traditional Unix framework of open(), read(), write(), and ioctl(), via special devices. For instance, the default device for sound input and output is /dev/dsp. Examples using the shell:
 cat /dev/random > /dev/dsp # plays white noise through the speaker
 cat /dev/dsp > a.a # reads data from the microphone and copies it to file a.a
Free, proprietary, free.
The project was initially free software, but following the project's success, Savolainen was contracted by the company 4Front Technologies and made his support for newer sound devices and improvements proprietary. In response, eventually the Linux community abandoned the OSS/free implementation included in the kernel and development effort switched to the replacement Advanced Linux Sound Architecture (ALSA).
Some Linux distributions, such as Ubuntu, have chosen to disable OSS support in their kernels and ignore any bugs filed against OSS4 packages (although OSS support may be re-enabled on Ubuntu).
In spite of this, several operating systems, such as FreeBSD, continued to distribute previous versions of OSS, and continue to maintain and improve these versions.
In July 2007, 4Front Technologies released sources for OSS under CDDL for OpenSolaris and GPL for Linux.
In January 2008, 4Front Technologies released OSS for FreeBSD (and other BSD systems) under the BSD License.
Other implementations.
FreeBSD contains an independently developed implementation of the OSS API, which includes, among other things, in-kernel resampling, mixing (vchans), equalizer, surround sound, and independent volume control for each application. It also supports bit-perfect mode.
NetBSD and OpenBSD support a compatibility mode for the OSS API, by providing the codice_1 header file and the codice_2 library, which internally operate using the native Sun-like audio interface.
Solaris and OpenSolaris uses a fork of OSS4 called Boomer. It combines the OSS4 framework (audio and mixer) together with Sun's earlier SADA(/dev/audio) API.
OSS/3D.
OSS/3D is a plugin for music players, which acts as an audio postprocessing engine. Supported players include Winamp, Windows Media Player (9 or later), musicmatch, Sonique, Foobar2000, JetAudio, XMMS. It is ported to Windows and Linux platforms. Unlike the OSS, it is shareware.
4Front OSS3D was later renamed to Joesoft Hear.
OSS in relation to ALSA.
In the Linux kernel, there have historically been two uniform sound APIs used. One is OSS;
the other is ALSA ("Advanced Linux Sound Architecture"). ALSA is available for Linux only, and as there is only one implementation of the ALSA interface, ALSA refers equally to that implementation and to the interface itself.
OSS was the only audio API in Linux up through the 2.4 series of official (kernel.org) Linux kernels. ALSA was added starting with 2.5, and in those versions, Linux kernel authors marked OSS as deprecated. 4Front continued to develop OSS outside the Linux kernel.
ALSA provides an optional OSS emulation mode that appears to programs as if it were OSS. Similarly, there was an ALSA emulation mode in the Linux implementation of OSS.
OSS both as API and as software has been criticized by some developers, such as Paul Davis (of JACK Audio Connection Kit) and Lennart Poettering (of competing PulseAudio). Most of these criticisms were either specific to the Linux implementation and not to the OSS API itself, or have been addressed in OSS version 4.