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PulseAudio emulation for ALSA.

The program provides an alternative partial implementation of the PulseAudio API. It consists of a loader script and a number of shared libraries with the same names as from original PulseAudio, so applications could dynamically load them and think they are talking to PulseAudio. Internally, no separate sound mixing daemon is used. Instead, apulse relies on ALSA's dmix, dsnoop, and plug plugins to handle multiple sound sources and capture streams running at the same time. dmix plugin muxes multiple playback streams; dsnoop plugin allow multiple applications to capture from a single microphone; and plug plugin transparently converts audio between various sample formats, sample rates and channel numbers. For more than a decade now, ALSA comes with these plugins enabled and configured by default.

apulse wasn't designed to be a drop-in replacement of PulseAudio. It's pointless, since that will be just reimplementation of original PulseAudio, with the same client-daemon architecture, required by the complete feature set. Instead, only parts of the API that are crucial to specific applications are implemented. That's why there is a loader script, named apulse. It updates value of LD_LIBRARY_PATH environment variable to point also to the directory where apulse's libraries are installed, making them available to the application.

Name comes from names of both ALSA and PulseAudio. As aoss was a compatibility layer between OSS programs and ALSA, apulse was designed to be compatibility layer between PulseAudio applications and ALSA.

Install

You need ALSA libraries and GLib installed. On Debian-based distributions, they are in packages libasound2-dev and libglib2.0-dev.

To build and install, run in source directory:

$ mkdir build && cd build
$ cmake -DCMAKE_INSTALL_PREFIX=/usr -DCMAKE_BUILD_TYPE=Release ..
$ make
# make install

That will create directory named build, and build there. It's possible to install just by running make install as root, as shown above. But you won't be able to uninstall installed files. That's why it's recommended to wrap files into a package. Use checkinstall, or some alternative.

If you want 32-bit binaries on 64-bit machine (for example, for Skype), use:

$ mkdir build && cd build
$ CFLAGS=-m32 cmake -DCMAKE_INSTALL_PREFIX=/usr -DCMAKE_BUILD_TYPE=Release ..
$ make
# make install

Recent GLib versions use different .pc files for i386 and amd64. To help pkg-config find 32-bit versions, use PKG_CONFIG_PATH variable. So, on Debian it will be something like:

$ PKG_CONFIG_PATH=/usr/lib/i386-linux-gnu/pkgconfig CFLAGS=-m32 cmake -DCMAKE_INSTALL_PREFIX=/usr -DCMAKE_BUILD_TYPE=Release ..

There is a way to configure where apulse libraries will be installed, via APULSEPATH cmake variable. For example, if you want to install libraries into default path, /usr/lib, use

cmake -DAPULSEPATH=/usr/lib -DCMAKE_INSTALL_PREFIX=/usr -DCMAKE_BUILD_TYPE=Release ..

If libraries are installed to a regular library path, you don't need run applications through apulse wrapper.

Usage

$ apulse <program-name> [program-parameters]

Environment variables APULSE_CAPTURE_DEVICE and APULSE_PLAYBACK_DEVICE can be used to configure capture and playback devices. Try hw:0,0, plughw:0,0 and the like. Refer to the ALSA user guide for a full list of device names.

System directory versus separate directory installations

By default, libraries from apulse are installed into a separate directory, in order to hide them from all applications.

Most applications in the wild, that support both PulseAudio and ALSA, try to autodetect which sound system is used. First, applications try to start with PulseAudio. Original client libraries fail early if no PulseAudio daemon is running or can be started. Then they switch to ALSA. Decision is made once, at the beginning. It works fine with PulseAudio, but doesn't work with apulse. Latter has no daemons, it happily says that everything is fine, and it's capable of playing audio. Applications then try to call more functions, and eventually touch unimplemented parts, often with crashes. So, libraries are hidden, and become visible only when a program is called through apulse wrapper script.

It's possible to install apulse libraries to /usr/lib. Wrapper script won't be required, but then all applications will try to use PulseAudio API, despite they can use ALSA.

Per-app RPATH trick

There is the RPATH property of ELF executable format, which is used to specify paths to search for dynamic libraries. It's like LD_LIBRARY_PATH variable, but per-executable. Since all that apulse launcher script does is setting LD_LIBRARY_PATH value before launching an application, it's possible to bake paths to apulse libraries into target executable itself. And so to launch it as usual, without helper script.

For example, for Firefox it would be:

# patchelf --set-rpath /usr/lib/apulse /usr/lib/firefox/libxul.so

For some reason, it doesn't work if RPATH is set for /usr/lib/firefox/firefox itself, so some experiments are required to make it work.

Known issues

Not implemented functions, application crashes

Large portion of PulseAudio API is not implemented. There are functions that do nothing and return some arbitraty values. Often, if application tries to call something not implemented, it crashes while trying to dereference a NULL pointer. By default, tracing level is set to 0, which means no messages are printed to standard output. It's possible to increase that value to 1, which shows unimplemented function calls, or to 2, which shows all function calls.

To change level, use WITH_TRACE parameter when calling cmake. Something like cmake -DWITH_TRACE=1 ..

Building apulse with trace level 1 won't fix issues, but will at least help to identify if crashes are caused by not implemented functions.

Generic errors in do_connect_pcm

Apulse acts as a generic ALSA client. It tries to open audio device, and sometimes fails. At its core, apulse does neither audio mixing nor resampling. Instead, it relies on plug, dmix, and dsnoop ALSA plugins, which are usually enabled by default. These plugins handle multiple audio sources, performing resampling and mixing transparently. For years now ALSA comes with those plugins enabled. Audio just works without configuring anything. But not everybody use default settings.

On custom configurations apulse may fail to output and/or capture audio. There could be no sound at all, or just a single audio stream playing at a time. It's also possible that adapters with hardware mixers, which capable of playing multiple streams, may still be unable to handle multiple capture streams. Depending on hardware, you may still need either dmix or dsnoop plugins. Or both.

In other words, for apulse to work, your setup should be capable of playing and capturing multiple streams simultaneously.

Access errors in do_connect_pcm

If other applications output sound just fine, it's possible that application you are using restricts itself.

For example, Firefox now have a sandbox, that blocks file access. It has predefined list of allowed paths, but ALSA devices are not included by default. Fortunately, it's possible to add those path by hand. Add "/dev/snd/" to "security.sandbox.content.write_path_whitelist" parameter in about:config. Note that trailing slash in "/dev/snd/" is required.

Firefox 58 tabs crashing when trying to play audio

Firefox 58 (Nightly) tightened its sandbox a bit more. Now ioctl() calls are forbidden too, but are used by ALSA libraries. That causes sandbox violation with subsequent process termination. Exception can be added by setting parameter security.sandbox.content.syscall_whitelist in about:config. That field accepts a comma separated list of system call numbers. Add there 16 for x86-64, or 54 for x86 or ARM.

Firefox 60 tighened its content sandbox more, but at the same time moved audio accesses from content processes to the main process. From Firefox 60 onwards no changes to the sandbox settings are necessary.

License

Source code is distributed under the terms of the MIT License. See LICENSE.MIT for full text.

Third party code

/3rdparty/pulseaudio-headers contains part of PulseAudio project and is distributed under LGPLv2.1+ terms. See content of the files for details.