The alsa-gobject project

2022/06/30 Takashi Sakamoto

Introduction

This project is a sub project in Advanced Linux Sound Architecture a.k.a ALSA, to produce libraries compatible with GObject introspection (g-i) for language bindings.

The latest release is 0.3.0.

Libraries produced by the project

libalsactl0

For APIs to operate ALSA control character device

libalsatimer0

For APIs to operate ALSA timer character device

libalsaseq0

For APIs to operate ALSA seq character device, depending on libalsatimer0

libalsahwdep0

For APIs to operate ALSA hwdep character device

libalsarawmidi0

For APIs to operate ALSA rawmidi character device

Namespaces of g-i produced by the project

ALSACtl-0.0

For bindings to use the libalsactl0

ALSATimer-0.0

For bindings to use the libalsatimer0

ALSASeq-0.0

For bindings to use the libalsaseq0

ALSAHwdep-0.0

For bindings to use the libalsahwdep0

ALSARawmidi-0.0

For bindings to use the libalsarawmidi0

Documentation

https://alsa-project.github.io/alsa-gobject-docs/

Python 3 Samples

See samples directory.

License

This software is licensed under GNU Lesser General Public License version 3 or later

Dependencies

• GLib https://gitlab.gnome.org/GNOME/glib
• GObject introspection https://gi.readthedocs.io/
• libudev1 https://www.freedesktop.org/wiki/Software/systemd/
• Linux kernel version 4.5 or later

Requirements to build

• UAPI header of Linux kernel
• Meson build system https://mesonbuild.com/
• Ninja build system https://ninja-build.org/
• PyGObject https://pygobject.readthedocs.io/ (optional to run unit tests)
• gi-docgen https://gnome.pages.gitlab.gnome.org/gi-docgen/ (optional to generate API documentation)

How to build

Preparation

$ meson (--prefix=install-directory) build-directory

Build

$ meson compile -C build-directory

Test

$ meson test -C build-directory
(PyGObject <https://pygobject.readthedocs.io/> is required)

Install

$ meson install -C build-directory

After installed, C headers for APIs are available and pkg-config returns arguments for them. For example

$ pkg-config --cflags --libs alsactl
-I/usr/include/alsa-gobject/ctl -I/usr/include/glib-2.0 -I/usr/lib/x86_64-linux-gnu/glib-2.0/include -lalsactl

Generate documentation

$ meson --prefix=install-directory -D doc=true build
$ meson install -C build
$ xdg-open (install-directory)/share/doc/alsa-gobject/index.html

Design note

• These libraries are independent of alsa-lib, to focus on interaction to kernel land directly by system calls without alsa-lib's configuration space and plugin framework.
• The way to enumerate any device is based on sysfs, programmed with libudev1.
• GObject-drived object is used for structures in UAPI of Linux sound subsystem with reserved space.
• Boxed object is used for structures in UAPI of Linux sound subsystem without reserved space.
• The type which has variable size of storage in 32/64 bit ABI (e.g. long) is not used for properties of GObject-derived objects.

Supplemental information for language bindings

• PyGObject https://pygobject.readthedocs.io/ is a dynamic loader for libraries compatible with g-i.
• alsa-gobject-rs https://github.com/alsa-project/alsa-gobject-rs/ includes creates to use these libraries.

Valgrind suppression file for leak detected in glib

The execution of valgrind for executable binaries which link the libraries detects some leaks inner glib, mainly due to allocation of type information. This is expected and unavoidable. The valgrind-glib.suppressions file is included in this repository and available to suppress the report.

For example

$ valgrind --suppressions=valgrind-glib.suppressions --leak-check=full your-executable

Loss of backward compatibility between v0.2/0.3 releases

Following to GNOME convention for throw function

In GNOME convention, the throw function to report error at GError argument should return gboolean value to report the overall operation finished successfully or not. At v0.3 release, the most of public API are rewritten according to it.

Using GObject Interface

GObject Interface is utilized for some cases to express structure with union.

• ALSACtl.ElemInfoCommon and ALSACtl.ElemInfoSingleArray for struct snd_ctl_elem_info
• ALSASeq.QueueTimerCommon for struct snd_seq_queue_timer

Therefore some GObject-derived objects implements the interfaces.

• ALSACtl.ElemInfoIec60958
• ALSACtl.ElemInfoBoolean
• ALSACtl.ElemInfoBytes
• ALSACtl.ElemInfoInteger
• ALSACtl.ElemInfoInteger64
• ALSACtl.ElemInfoEnumerated
• ALSASeq.QueueTimerAlsa

Some boxed structures are obsoleted and removed.

• ALSACtl.ElemInfo
• ALSACtl.QueueTimer
• ALSACtl.QueueTimerDataAlsa

GObject Interface is utlized to define common feature of hwdep device as well. ALSAHwdep.DeviceCommon interface is added for the purpose.

Event expression for ALSA Sequencer

ALSASeq.Event boxed structure is newly added to each event, and ALSASeq.EventCntr is simplified to include deserializer only. ALSASeq.EventError domain is newly added to express event handling problem.

Name consistency in time stamp expression

The word tstamp is renamed to real time in the most of functions, enumerations, object names, and methods for name consistency.

• ALSATimer.TstampEvent boxed structure is renamed to ALSATimer.RealTimeEvent
• ALSATimer.EventType enumeration is renamed to ALSATimer.RealTimeEventType
• ALSATimer.EventDataType enumeration is renamed to ALSATimer.EventType
• ALSATimer.InstanceStatus.get_tstamp() method is renamed to ALSATimer.InstanceStatus.get_time()
• ALSATimer.get_tstamp_source() function is renamed to ALSATimer.get_real_time_clock_id()

In time stamp, the counterpart of real time is tick. ALSATimer.TickEvent boxed structure is renamed to ALSATimer.TickTimeEvent. ALSATimer.Event, ALSATimer.EventDataTick and ALSATimer.EventDataTstamp are obsoleted and removed. ALSATimer.UserInstance::handle-event is obsoleted as well. The alternatives are available to retrieve corresponding event.

• ALSATimer.UserInstance::handle-tick-time-event
• ALSATimer.UserInstance::handle-real-time-event

ALSASeq.Tstamp is obsoleted and removed as well. The accessor methods to it are rewritten to retrieve either tick time or real time.

• ALSASeq.Event.get_tick_time()
• ALSASeq.Event.get_real_time()
• ALSASeq.Event.get_tick_time_data()
• ALSASeq.Event.get_real_time_data()
• ALSASeq.EventDataQueue.get_tick_time_param()
• ALSASeq.EventDataQueue.get_real_time_param()
• ALSASeq.RemoveFilter.new_with_tick_time()
• ALSASeq.RemoveFilter.new_with_real_time()

The word tstamp still remains in the enumerations and properties to affects both tick and real time.

• ALSASeq.EventTstampMode
• ALSASeq.Event.get_tstamp_mode()
• ALSASeq.PortInfo:tstamp-mode
• ALSASeq.PortInfo:tstamp-overwrite
• ALSASeq.SusbscribeData:has-tstamp
• ALSASeq.SusbscribeData:tstamp-mode

Some properties are added to ALSASeq.SubscribeData to obsolete ALSASeq.PortSubscribeFlag.

• ALSASeq.SubscribeData:is-exclusive
• ALSASeq.SubscribeData:has-tstamp
• ALSASeq.SubscribeData:tstamp-mode
• ALSASeq.SubscribeData:queue-id

Rewrite setter method in ALSACtl.ElemValue

Below methods are rewritten to retrieve the pointer in internal storage instead of copying to given buffer as fixed-sized array:

• ALSACtl.ElemValue.get_bool
• ALSACtl.ElemValue.get_int
• ALSACtl.ElemValue.get_enum
• ALSACtl.ElemValue.get_bytes
• ALSACtl.ElemValue.get_iec60958_user_data
• ALSACtl.ElemValue.get_iec60958_channel_status
• ALSACtl.ElemValue.get_int64

Rewrite ALSASeq.RemoveFilter

ALSASeq.RemoveFilter is largely written so that it's GObject-derived object instead of boxed structure for flexible configuration of filter condition.