libao documentation

libao version 1.2.0 - 20130122

libao Drivers

Libao supports both live output drivers and file output drivers. Live output drivers send audio data to sound cards and sound daemons. File output drivers write audio to disk using a particular file format (such as WAV, AU, etc.). You must invoke the ao_open_????() function that corresponds to the type of driver you are using, but otherwise live and file drivers are treated identically in libao.

Driver options may be passed to the drivers via the *options argument to ao_open_live() and ao_open_file(), or they may be set in the configuration file as name=value pairs.

Options Understood by All Drivers

Standard Driver Options

These are options that have the same use in each driver, but may not be supported by all drivers (due to the option being meaningless or unimplemented for a given audio backend).

Live Output Drivers


IBM AIX sound driver. According to the author, "tested on AIX 5.1 with the Crystal chipsets only (found as internal audio in the 7043-140 and on the MCA adapter DFE5 (7-6)), but it should work with other AIX releases and the ACPA also."

Option keys:


Advanced Linux Sound Architecture (API versions 0.9.x/1.x.x; earlier API versions are now deprecated).

Option keys:


aRts Sound Daemon live output driver.

Option keys:


ESounD audio driver. Although declining in poularity, this sound daemon is still used on some Linux systems. It permits multiple programs to play sound simultaneously and sound to be sent to networked computers.

Option keys:


IRIX audio driver. This was inherited from the original libao, but has not been tested. Use at your own risk. (Better yet, fix it! I don't have access to an IRIX system.)


MacOS X AUHAL live output driver. This driver supports MacOS X 10.5 and later (10.4 and earlier uses an earlier, incompatable interface).

Option keys:


Network Audio System live output driver.

Option keys:


Null driver. This is just a test device which does not write the audio data anywhere.


Open Sound System driver. This is the audio system for older Linux and FreeBSD as well as some other UNIX-like systems.

Option keys:


PulseAudio live audio sound driver. Pulse is a sound server daemon used by the modern Gnome desktop on UNIX-like systems.

Option keys:


RoarAudio live audio sound driver.

Option keys:


SNDIO is the modern audio interface used by OpenBSD.

Option keys:


Sun audio driver. This is the audio system for NetBSD, OpenBSD, and Solaris.

Option keys:


Windows MMSound output driver for Win98 and later.

Option keys:

File Output Drivers


Sun audio file driver. Writes a .au file from audio output. This driver can write usable data to unseekable files (like standard out), which the wav driver cannot do.


Raw sample driver. Writes the sound to disk in uncompressed, headerless form using the byte order specified.

Option keys:


Windows 'WAV' sound file output. Because of the way WAV files are structured, this driver cannot correct files unless the target file is seekable. Writing WAVs to stdout will result in broken files. Use either the raw or the au driver instead.

Default Driver Detection

In the absence of
configuration files to explicit identify a default driver, the library will try to detect a suitable default driver. It does this by testing every available live output driver (using ao_plugin_test()) and finding the driver with the highest priority (see the ao_info struct) that works. Drivers with priority 0, such as the null and file output drivers, are never selected as the default.

The ranking system currently used is:

50 pulse, roar
45 arts*
40 esd
35 alsa
30 macosx, sndio
20 aixs, oss, irix, sun, wmm
15 arts*
10 nas
0 null, all file output

*arts priority depends on whether or not the arts install was built with HAVE_ARTS_SUSPENDED; when present, the default aRts priority is 45, else it is 15.

Clearly, any ranking scheme will fail to make everybody happy. For such cases, the configuration files can be easily used to define an appropriate default output device.

Adding debug to the libao configuration file on a line by itself will cause libao to print what static and dynamic drivers are available for use, as well as print the testing order.

copyright © 2001-2003 Stan Seibert, 2010-2013 Monty

libao documentation

libao version 1.2.0 - 20130122