A Guide Through The Linux Sound API Jungle

Post Syndicated from Lennart Poettering original https://0pointer.net/blog/projects/guide-to-sound-apis.html

At the Audio MC at the Linux Plumbers Conference one
thing became very clear: it is very difficult for programmers to
figure out which audio API to use for which purpose and which API not
to use when doing audio programming on Linux. So here’s my try to
guide you through this jungle:

What do you want to do?

I want to write a media-player-like application!: Use GStreamer! (Unless your focus is only KDE in which cases Phonon might be an alternative.)
I want to add event sounds to my application!: Use libcanberra, install your sound files according to the XDG Sound Theming/Naming Specifications! (Unless your focus is only KDE in which case KNotify might be an alternative although it has a different focus.)
I want to do professional audio programming, hard-disk recording, music synthesizing, MIDI interfacing!: Use JACK and/or the full ALSA interface.
I want to do basic PCM audio playback/capturing!: Use the safe ALSA subset.
I want to add sound to my game!: Use the audio API of SDL for full-screen games, libcanberra for simple games with standard UIs such as Gtk+.
I want to write a mixer application!: Use the layer you want to support directly: if you want to support enhanced desktop software mixers, use the PulseAudio volume control APIs. If you want to support hardware mixers, use the ALSA mixer APIs.
I want to write audio software for the plumbing layer!: Use the full ALSA stack.
I want to write audio software for embedded applications!: For technical appliances usually the safe ALSA subset is a good choice, this however depends highly on your use-case.

You want to know more about the different sound APIs?

GStreamer: GStreamer is the de-facto
standard media streaming system for Linux desktops. It supports decoding and
encoding of audio and video streams. You can use it for a wide range of
purposes from simple audio file playback to elaborate network
streaming setups. GStreamer supports a wide range of CODECs and audio
backends. GStreamer is not particularly suited for basic PCM playback
or low-latency/realtime applications. GStreamer is portable and not
limited in its use to Linux. Among the supported backends are ALSA, OSS, PulseAudio. [Programming Manuals and References]
libcanberra: libcanberra
is an abstract event sound API. It implements the XDG
Sound Theme and Naming Specifications. libcanberra is a blessed
GNOME dependency, but itself has no dependency on GNOME/Gtk/GLib and can be
used with other desktop environments as well. In addition to an easy
interface for playing sound files, libcanberra provides caching
(which is very useful for networked thin clients) and allows passing
of various meta data to the underlying audio system which then can be
used to enhance user experience (such as positional event sounds) and
for improving accessibility. libcanberra supports multiple backends
and is portable beyond Linux. Among the supported backends are ALSA, OSS, PulseAudio, GStreamer. [API Reference]
JACK: JACK is a sound system for
connecting professional audio production applications and hardware
output. It’s focus is low-latency and application interconnection. It
is not useful for normal desktop or embedded use. It is not an API
that is particularly useful if all you want to do is simple PCM
playback. JACK supports multiple backends, although ALSA is best
supported. JACK is portable beyond Linux. Among the supported backends are ALSA, OSS. [API Reference]
Full ALSA: ALSA is the Linux API
for doing PCM playback and recording. ALSA is very focused on
hardware devices, although other backends are supported as well (to a
limit degree, see below). ALSA as a name is used both for the Linux
audio kernel drivers and a user-space library that wraps these. ALSA — the library — is
comprehensive, and portable (to a limited degree). The full ALSA API
can appear very complex and is large. However it supports almost
everything modern sound hardware can provide. Some of the
functionality of the ALSA API is limited in its use to actual hardware
devices supported by the Linux kernel (in contrast to software sound
servers and sound drivers implemented in user-space such as those for
Bluetooth and FireWire audio — among others) and Linux specific
drivers. [API
Reference]
Safe ALSA: Only a subset of the full ALSA API works on all backends ALSA
supports. It is highly recommended to stick to this safe subset
if you do ALSA programming to keep programs portable, future-proof and
compatible with sound servers, Bluetooth audio and FireWire audio. See
below for more details about which functions of ALSA are considered
safe. The safe ALSA API is a suitable abstraction for basic,
portable PCM playback and recording — not just for ALSA kernel driver
supported devices. Among the supported backends are ALSA kernel driver
devices, OSS, PulseAudio, JACK.
Phonon and KNotify: Phonon is high-level
abstraction for media streaming systems such as GStreamer, but goes a
bit further than that. It supports multiple backends. KNotify is a
system for “notifications”, which goes beyond mere event
sounds. However it does not support the XDG Sound Theming/Naming
Specifications at this point, and also doesn’t support caching or
passing of event meta-data to an underlying sound system. KNotify
supports multiple backends for audio playback via Phonon. Both APIs
are KDE/Qt specific and should not be used outside of KDE/Qt
applications. [Phonon API Reference] [KNotify API Reference]
SDL: SDL is a portable API
primarily used for full-screen game development. Among other stuff it
includes a portable audio interface. Among others SDL support OSS,
PulseAudio, ALSA as backends. [API Reference]
PulseAudio: PulseAudio is a sound system
for Linux desktops and embedded environments that runs in user-space
and (usually) on top of ALSA. PulseAudio supports network
transparency, per-application volumes, spatial events sounds, allows
switching of sound streams between devices on-the-fly, policy
decisions, and many other high-level operations. PulseAudio adds a glitch-free
audio playback model to the Linux audio stack. PulseAudio is not
useful in professional audio production environments. PulseAudio is
portable beyond Linux. PulseAudio has a native API and also supports
the safe subset of ALSA, in addition to limited,
LD_PRELOAD-based OSS compatibility. Among others PulseAudio supports
OSS and ALSA as backends and provides connectivity to JACK. [API
Reference]
OSS: The Open Sound System is a
low-level PCM API supported by a variety of Unixes including Linux. It
started out as the standard Linux audio system and is supported on
current Linux kernels in the API version 3 as OSS3. OSS3 is considered
obsolete and has been fully replaced by ALSA. A successor to OSS3
called OSS4 is available but plays virtually no role on Linux and is
not supported in standard kernels or by any of the relevant
distributions. The OSS API is very low-level, based around direct
kernel interfacing using ioctl()s. It it is hence awkward to use and
can practically not be virtualized for usage on non-kernel audio
systems like sound servers (such as PulseAudio) or user-space sound
drivers (such as Bluetooth or FireWire audio). OSS3’s timing model
cannot properly be mapped to software sound servers at all, and is
also problematic on non-PCI hardware such as USB audio. Also, OSS does
not do sample type conversion, remapping or resampling if
necessary. This means that clients that properly want to support OSS
need to include a complete set of converters/remappers/resamplers for
the case when the hardware does not natively support the requested
sampling parameters. With modern sound cards it is very common to
support only S32LE samples at 48KHz and nothing else. If an OSS client
assumes it can always play back S16LE samples at 44.1KHz it will thus
fail. OSS3 is portable to other Unix-like systems, various differences
however apply. OSS also doesn’t support surround sound and other
functionality of modern sounds systems properly. OSS should be
considered obsolete and not be used in new applications. ALSA and
PulseAudio have limited LD_PRELOAD-based compatibility with OSS. [Programming Guide]

All sound systems and APIs listed above are supported in all
relevant current distributions. For libcanberra support the newest
development release of your distribution might be necessary.

All sound systems and APIs listed above are suitable for
development for commercial (read: closed source) applications, since
they are licensed under LGPL or more liberal licenses or no client
library is involved.

You want to know why and when you should use a specific sound API?

GStreamer: GStreamer is best used for very high-level needs: i.e. you want to
play an audio file or video stream and do not care about all the tiny
details down to the PCM or codec level.
libcanberra: libcanberra is best used when adding sound feedback to user input
in UIs. It can also be used to play simple sound files for
notification purposes.
JACK: JACK is best used in professional audio production and where interconnecting applications is required.
Full ALSA: The full ALSA interface is best used for software on “plumbing layer” or when you want to make use of very specific hardware features, which might be need for audio production purposes.
Safe ALSA: The safe ALSA interface is best used for software that wants to output/record basic PCM data from hardware devices or software sound systems.
Phonon and KNotify: Phonon and KNotify should only be used in KDE/Qt applications and only for high-level media playback, resp. simple audio notifications.
SDL: SDL is best used in full-screen games.
PulseAudio: For now, the PulseAudio API should be used only for applications
that want to expose sound-server-specific functionality (such as
mixers) or when a PCM output abstraction layer is already available in
your application and it thus makes sense to add an additional backend
to it for PulseAudio to keep the stack of audio layers minimal.
OSS: OSS should not be used for new programs.

You want to know more about the safe ALSA subset?

Here’s a list of DOS and DONTS in the ALSA API if you care about
that you application stays future-proof and works fine with
non-hardware backends or backends for user-space sound drivers such as
Bluetooth and FireWire audio. Some of these recommendations apply for
people using the full ALSA API as well, since some functionality
should be considered obsolete for all cases.

If your application’s code does not follow these rules, you must have
a very good reason for that. Otherwise your code should simply be considered
broken!

DONTS:

Do not use “async handlers”, e.g. via
snd_async_add_pcm_handler() and friends. Asynchronous
handlers are implemented using POSIX signals, which is a very
questionable use of them, especially from libraries and plugins. Even
when you don’t want to limit yourself to the safe ALSA subset
it is highly recommended not to use this functionality. Read
this for a longer explanation why signals for audio IO are
evil.
Do not parse the ALSA configuration file yourself or with
any of the ALSA functions such as snd_config_xxx(). If you
need to enumerate audio devices use snd_device_name_hint()
(and related functions). That
is the only API that also supports enumerating non-hardware audio
devices and audio devices with drivers implemented in userspace.
Do not parse any of the files from
/proc/asound/. Those files only include information about
kernel sound drivers — user-space plugins are not listed there. Also,
the set of kernel devices might differ from the way they are presented
in user-space. (i.e. sub-devices are mapped in different ways to
actual user-space devices such as surround51 an suchlike.
Do not rely on stable device indexes from ALSA. Nowadays
they depend on the initialization order of the drivers during boot-up
time and are thus not stable.
Do not use the snd_card_xxx() APIs. For
enumerating use snd_device_name_hint() (and related
functions). snd_card_xxx() is obsolete. It will only list
kernel hardware devices. User-space devices such as sound servers,
Bluetooth audio are not included. snd_card_load() is
completely obsolete in these days.
Do not hard-code device strings, especially not
hw:0 or plughw:0 or even dmix — these devices define no channel
mapping and are mapped to raw kernel devices. It is highly recommended
to use exclusively default as device string. If specific
channel mappings are required the correct device strings should be
front for stereo, surround40 for Surround 4.0,
surround41, surround51, and so on. Unfortunately at
this point ALSA does not define standard device names with channel
mappings for non-kernel devices. This means default may only
be used safely for mono and stereo streams. You should probably prefix
your device string with plug: to make sure ALSA transparently
reformats/remaps/resamples your PCM stream for you if the
hardware/backend does not support your sampling parameters
natively.
Do not assume that any particular sample type is supported
except the following ones: U8, S16_LE, S16_BE, S32_LE, S32_BE,
FLOAT_LE, FLOAT_BE, MU_LAW, A_LAW.
Do not use snd_pcm_avail_update() for
synchronization purposes. It should be used exclusively to query the
amount of bytes that may be written/read right now. Do not use
snd_pcm_delay() to query the fill level of your playback
buffer. It should be used exclusively for synchronisation
purposes. Make sure you fully understand the difference, and note that
the two functions return values that are not necessarily directly
connected!
Do not assume that the mixer controls always know dB information.
Do not assume that all devices support MMAP style buffer access.
Do not assume that the hardware pointer inside the (possibly mmaped) playback buffer is the actual position of the sample in the DAC. There might be an extra latency involved.
Do not try to recover with your own code from ALSA error conditions such as buffer under-runs. Use snd_pcm_recover() instead.
Do not touch buffering/period metrics unless you have
specific latency needs. Develop defensively, handling correctly the
case when the backend cannot fulfill your buffering metrics
requests. Be aware that the buffering metrics of the playback buffer
only indirectly influence the overall latency in many
cases. i.e. setting the buffer size to a fixed value might actually result in
practical latencies that are much higher.
Do not assume that snd_pcm_rewind() is available and works and to which degree.
Do not assume that the time when a PCM stream can receive
new data is strictly dependant on the sampling and buffering
parameters and the resulting average throughput. Always make sure to
supply new audio data to the device when it asks for it by signalling
“writability” on the fd. (And similarly for capturing)
Do not use the “simple” interface snd_spcm_xxx().
Do not use any of the functions marked as “obsolete”.
Do not use the timer, midi, rawmidi, hwdep subsystems.

DOS:

Use snd_device_name_hint() for enumerating audio devices.
Use snd_smixer_xx() instead of raw snd_ctl_xxx()
For synchronization purposes use snd_pcm_delay().
For checking buffer playback/capture fill level use snd_pcm_update_avail().
Use snd_pcm_recover() to recover from errors returned by any of the ALSA functions.
If possible use the largest buffer sizes the device supports to maximize power saving and drop-out safety. Use snd_pcm_rewind() if you need to react to user input quickly.

FAQ

What about ESD and NAS?: ESD and NAS are obsolete, both as API and as sound daemon. Do not develop for it any further.
ALSA isn’t portable!: That’s not true! Actually the user-space library is relatively portable, it even includes a backend for OSS sound devices. There is no real reason that would disallow using the ALSA libraries on other Unixes as well.
Portability is key to me! What can I do?: Unfortunately no truly portable (i.e. to Win32) PCM API is
available right now that I could truly recommend. The systems shown
above are more or less portable at least to Unix-like operating
systems. That does not mean however that there are suitable backends
for all of them available. If you care about portability to Win32 and
MacOS you probably have to find a solution outside of the
recommendations above, or contribute the necessary
backends/portability fixes. None of the systems (with the exception of
OSS) is truly bound to Linux or Unix-like kernels.
What about PortAudio?: I don’t think that PortAudio is very good API for Unix-like operating systems. I cannot recommend it, but it’s your choice.
Oh, why do you hate OSS4 so much?: I don’t hate anything or anyone. I just don’t think OSS4 is a
serious option, especially not on Linux. On Linux, it is also
completely redundant due to ALSA.
You idiot, you have no clue!: You are right, I totally don’t. But that doesn’t hinder me from recommending things. Ha!
Hey I wrote/know this tiny new project which is an awesome abstraction layer for audio/media!: Sorry, that’s not sufficient. I only list software here that is known to be sufficiently relevant and sufficiently well maintained.

Final Words

Of course these recommendations are very basic and are only intended to
lead into the right direction. For each use-case different necessities
apply and hence options that I did not consider here might become
viable. It’s up to you to decide how much of what I wrote here
actually applies to your application.

This summary only includes software systems that are considered
stable and universally available at the time of writing. In the
future I hope to introduce a more suitable and portable replacement
for the safe ALSA subset of functions. I plan to update this text
from time to time to keep things up-to-date.

If you feel that I forgot a use case or an important API, then
please contact me or leave a comment. However, I think the summary
above is sufficiently comprehensive and if an entry is missing I most
likely deliberately left it out.

(Also note that I am upstream for both PulseAudio and libcanberra and did some minor contributions to ALSA, GStreamer and some other of the systems listed above. Yes, I am biased.)

Oh, and please syndicate this, digg it. I’d like to see this guide to be well-known all around the Linux community. Thank you!

Noise