Gravitational Waves with GStreamer Workshop

Canadian Institute for Theoretical Astrophysics

November 8–11, 2010

Held in association with the 20th Annual Midwest Relativity Meeting

GSTLAL Anatomy

• Gstlal is a software package written in C and Python.
• The C code in gstlal sits on top of the GStreamer and LAL C libraries.
• GStreamer is built on top of the GLib and GObject C libraries.

The following is a simplified depiction of the package hierarchy.

GLib

• GLib is a compatibility library, used to ease the porting of software across platforms.
  • memory allocation
  • I/O
  • threading
  • byte order macros
  • things that are in the C library but behave differently on different systems, like some string manipulation functions
• GLib provides things you wish were in the C standard library.
  • dynamically-sized strings
  • doubly- and singly-linked lists
  • queues and stacks
  • trees
  • hash tables
  • caches
• GLib contains GObject.
  • they are provided as a single source tar ball
  • logically, GObject is built on top of GLib
  • the two are nearly universally distributed by OSes as distinct packages
  • confusion:  gobject's documentation refers to itself as the "glib type system",  there is not a gobject web page, and so on.

GObject

• GObject is a run-time type system written in portable C.
• What is that?
  • Consider the following Python function:

    def f(x):
    	return x * 2

    You can pass any object to f() for which "multiply by 2" is defined.
  • For example

    >>> f(3)
    6
    >>> f(3.0)
    6.0
    >>> f("hello")
    'hellohello'

  • How would you do this in C?
  • Define a structure to hold a piece of data and a description of the type of data held therein.
  • All functions are written to accept and return those structures, instead of native C types like ints and floats.
  • How do you "multiply by 2"?
  • Add a function pointer to the structure holding the address of a function that implements multiplication.
  • How do you initialize one, how do you print one?
  • How do you allow programmers to define new ones?
  • ahhh ... the sweater unravels, and you eventually come to have written a C library that in some sense defines a new language.
• That's what GObject is.
• GObject promotes "event-based programming".
  • procedural programming:
    • write your program's flow chart explicitly, step-by-step, with loops and conditionals
  • event-based programming:
    • write a collection of functions to be invoked when various conditions occur,
    • hand it all over to an "event loop",
    • then let whatever happens happen.
  • event-based programming typical of user interface code: when button X is clicked invoke button_X_pressed(), and so on.
  • GObject evolved out of the GTK+ user interface library.
  • more about this later.

GStreamer

• GStreamer is a C library built on top of GObject for implementing multimedia data processing applications.
• Gstreamer provides several object definitions to be used as the building blocks.
  • GstBuffer:  an object to store a block of data
  • GstElement:  an object representing a piece of a pipeline
  • GstPad:  an object representing the interface between an elements
  • etc...
• GStreamer is a suite of pre-written pipeline elements
  • filesrc:  read bytes from a file and push downstream
  • fdsrc:  read bytes from a file descriptor (e.g., stdin) and push downstream
  • audioresample:  adjust the sample rate of time series data
  • autoaudiosink:  send time series data to your computer's speakers
  • etc...
• Gstreamer provides a library of utilities to assist with writing elements
  • GstAdapter:  an object to concatenate buffers in memory
  • etc...
• A GStreamer-based program:
  • links together a graph of elements,
    • including a source element
    • and a sink element
  • hands pipeline graph to GStreamer library
  • "presses Play"
  • waits for and responds to events
    • errors
    • EOS — end of stream
    • other events your pipeline generates