Print list of available audio drivers

[gavv]

Intro

We have four concepts in roc_sndio module:

• audio backend

  Audio backend provides one or multiple audio drivers, and each driver can be used to work with audio sources and audio sinks of some specific type.

  In code, a backend is an implementation of sndio::IBackend interface.

  Examples of backends are: SoxBackend (opens sources and sinks using libsox; it supports numerous drivers, e.g. "wav" or "alsa"), PulseaudioBackend (opens sources and sinks using libpulse; it supports a single driver "pulseaudio").

• audio driver

  Audio driver works with sources and sinks of some specific type.

  In code, a driver is just a string passed to backend implementation when opening a source or a sink. It depends on the specific backend how to interpret this string.

  Examples of drivers are: "wav" (WAV file), "alsa" (ALSA device), "pulseaudio" (PulseAudio device).

• audio source

  Audio source represents a recording audio device or input audio file. We can read sound stream from a source.

  In code, a source is an implementation of sndio::ISource. Sources are opened using IBackend::open_source() method.

• audio sink

  Audio sink represents a playback audio device or output audio file. We can write sound stream to a sink.

  In code, a sink is an implementation of sndio::ISink. Sinks are opened using IBackend::open_sink() method.

Backends are not visible to the user. When the user starts roc-recv or roc-send, they specify the driver and the input name (in roc-send) or an output name (in roc-recv). The name can be a file name or a device name, depending on the driver.

The driver and the name are passed to BackendDispatcher. It looks at the driver name and selects a backend that can handle this driver. Then it passes the driver name and the input or output name to the backend, and the backend creates a source (in roc-send) or a sink (in roc-recv).

Task

This task is rather simple: we need to be able to print the list of supported drivers.

Something like this:

$ roc-recv -L
pulseaudio
alsa
wav
...

Implementation

The following implementation is suggested:

• Add a new structure sndio::DriverInfo. It should contain at list one field const char* name. We can also add other fields, like backend name, driver type (a file or a device), but this can be done later.

• Add a new method IBackend::get_drivers(core::Array<DriverInfo>&) that will add info to the list about every driver supported by the backend. If a driver is already present in the list, it should not be added. It can happen because the same driver can be supported by different backends (e.g. "pulseaudio" driver is supported by PulseaudioBackend and SoxBackend).

• Implement this method for all IBackend implementations.

  • For PulseaudioBackend, the only supported driver is "pulseaudio".

  • For SoxBackend, the list of supported drivers can be retrieved using sox_get_format_fns() function.

• Implement BackendDispatcher::get_drivers(core::Array<DriverInfo>&) that will collect drivers from all registered backends.

• Add --list-drivers (-L) option to roc-recv and roc-send. It should invoke BackendDispatcher::get_drivers() and then print all collected drivers to stdout.

[gavv]

Assigned (#262).

[gavv]

PR is merged.