Talking to PipeWire

March 2026

Why PipeWire?

Some challenges from last year's topic:

• Can I detect whether / when a microphone is plugged in?
• Can I record / play sound without using an external program?
• Can I prevent PulseAudio and Jack from stepping on each other's toes?

PipeWire seemed to be a solution. Unfortunately, there's nothing on CPAN.

About PipeWire

Wikipedia says:

PipeWire is a low-level server and multimedia framework for handling audio and video streams on Linux. Created by Wim Taymans at Red Hat, it aims to unify audio and video processing by providing low-latency capture and playback functionality. PipeWire facilitates advanced multimedia routing and pipeline processing, and is designed to replace and be compatible with existing sound systems.

PipeWire is available on current Linux versions

PipeWire comes with...

• Online documentation, containing
  • A long list of CLI programs
    ...which could already replace SoX or aplay
  • An API tutorial with C programs
    ...which turned out to be very helpful
  • A list of Examples
    Not well documented, but hey, you get what you pay for

The Procedure

1. Run the examples from the API tutorial
   Easy.
2. Run the examples from Perl, using Inline::C
   Also easy, but not too informative.
3. Try h2xs on the PipeWire header
   Well, not really. I shall revisit that at some time.
4. Go for FFI::Platypus

The h2xs Experience

I had no idea what the next step would be to "translate" the C programs from the tutorial to Perl.

I could have read a XS tutorial, but...

Dave Mitchell had announced that he would rewrite the XS documentation, so I decided to postpone XS. He also recently said he intends to rewrite the XS tutorial.

Go for FFI::Platypus

FFI::Platypus suggests that you can build interfaces to C without writing XS.

FFI = Foreign Function Interface: libffi

From the DESCRIPTION: Platypus is a library for creating interfaces to machine code libraries written in languages like C, C++, Go, Fortran, Rust, Pascal.

WTF?

WTF?

Ah. I missed something.

Some C Code Required

(no XS, though, as promised)

PipeWire uses static inline functions or functions implemented as preprocessor macros a lot.

Luckily, FFI::Platypus makes adding own C code quite easy.

Add C Code

• Drop your C code to a sub-directory ./ffi
• FFI::Platypus takes care for compiling, linking and explains what to add to Makefile.PL

Uses for Bundled C Code

• Make static functions or macros available from Perl
• Debugging / Diagnostics / Troubleshooting
• Inline::C on steroids thanks to the Platypus environment

Tutorial 2: Enumerating Objects V

Define the callback routine

Complex Data Types

Issues and Limitations

• A large number of structs means: Much manual "translating" of C declarations
• Several constructs used by PipeWire can not be mapped to FFI::Platypus declarations

Enter Convert::Binary::C(CBC)

• CBC operates on header files (and sources), not on binaries
• Its methods pack and unpack use C struct names as templates
• False start: Failed to process stdargs.h
  • The maintainer provided a fix within hours (not yet on CPAN)
• Does not understand GCC extensions
  • ... which PipeWire uses frequently
  • CBC is "bugfix only", no enhancements planned

CBC for Code Generation

• CBC offers insight into the parsed C constructs
• The #defined constants can be used to create use constant equivalents for ~350 values
• The struct method helps to create Perl classes equivalent to C structs
  • Could also help with XS typemaps

An Example Class

Tutorial 4: Playing Sound

Using what I have learned plus code generation for about a dozen classes should be easy, shouldn't it?

Tutorial 4: Playing Sound

Using what I have learned plus code generation for about a dozen classes should be easy, shouldn't it?

This time, it is not my fault.

It works with Perls built without threads.

Portability Summary

• PipeWire works on current "stable" Linux versions
  • So no demo with my presentation PC (Win11)
• Needs a fairly recent GCC to compile
  • ...with a somewhat fragile CBC configuration (356 lines)
• Works only with non-threaded Perl
  • No system Perl: Distributions ship threaded Perl
• This is very unlikely to go anywhere

FFI::Platypus Summary

• 😀 Easy to learn (docs, lots of examples)
• 😀 Easy to add own C code
• 😐 Works with libraries in various languages
• 🙄 Needs FFI declarations for every function call and every data type
• 🙁 The libraries I tried so far use many data types not supported by FFI::Platypus
  • Workarounds are possible, but tedious

Convert::Binary::C Summary

• 😀 Is a companion to FFI::Platypus (or XS), not a replacement
• 😀 Works on C sources and header files
• 😀 Allows introspection / code generation for FFI or XS wrappers
  • 🙄 Does not parse function signatures and function pointers
• 🙄 Does not support GCC extensions
• 🙄 Configuring can be tedious and brittle

Miscellaneous Topics

• Defining callback routines needs a workaround for FFI::Platypus' assumption that a code reference must be passed as a formal parameter
• Integrating PipeWire into another event loop (in my case: Prima) is possible by adapting PipeWire's GTK example to Prima::File

Obscure Objects of Desire

• The Perl source code - because the class feature lacks a debugger and has not yet a MOP
  • Should be easier for CBC because it is more portable
    • Spoiler: It is not ☹️.
  • Should be more difficult because of inconsistent style
• Vulkan - because OpenGL is retired and OpenGL::Modern is disappointing (tests? examples?)
  • Is currently my XS playground

See Also...

• Alternatives to FFI::Platypus:
  • PDL::PP for number crunching
  • Affix - by the author of SDL3
  • perlxs - much closer to Perl's internals
• Alternatives to Convert::Binary::C:
  • C::Scan "for easily recognized constructs"
  • MarpaX::Languages::C::Scan stopped at ISO-ANSI-C-2011