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.
$ mkdir build && cd build
$ cmake -DCMAKE_INSTALL_PREFIX=/usr -DCMAKE_BUILD_TYPE=Release ..
$ make
# make install
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
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.
$ 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.
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.
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.
Source code is distributed under the terms of the MIT License. See LICENSE.MIT for full text.
/3rdparty/pulseaudio-headers contains part of PulseAudio project and is distributed
under LGPLv2.1+ terms. See content of the files for details.