Specialized Topics Continued

Specialized Topics Continued

1. Networking programming

   • Essential Concepts
     • Each system has a virtual "loopback" network adapter
     • IP range 127.x.x.x is reserved for loopback. Default 127.0.0.1 (localhost)
     • TCP and UDP use 216 ≈ 65000 ports
     • Ports 1 – 1024 are reserved and on many systems need admin privileges
     • A unique port is needed at each part of the communication
     • Address 0.0.0.0 is used as any/all address
   • Client Networking (TCP)
     • The client initiates the connection by opening a connection to server port
     • The client "knows" which ip address and port to use
     • The TCP stack takes care of providing two channels (sending and receiving)
   • Server Networking
     • The server binds for requests on a predefined port on a predefined address (or 0.0.0.0 for all)
     • The server then waits for a connection by calling accept to receive a new connection
     • Accepted connections get their own local port so that multiple connections are possible
     • Server now needs to wait for new connections as well as handle the current one
   • Sockets
     • Abstraction for network access
     • Origin in Unix
     • Abstracts away from implementation and networking protocol
     • Provides file handles that can be read or written to
     • Sockets don't have a "file" length
     • End of file when connection is closed
   • Client Sockets
     • Specify address family (AF_INET, AF_INET6, AF_UNIX), and address (including port) of destination
     • Local port can be specified, but generally isn't (OS manages this)
     • Connect will initiate the connection
     • Connection can then be written to or read from.
   • Server Sockets
     • Specify address family (AF_INET, AF_INET6, AF_UNIX), and port (and address) to listen to
     • Accept will wait for a connection and return a new open socket for the connection
     • Connection can then be written to or read from.
   • Synchronous Networking
     • Accept, recv, send (or read, write) are blocking (they return only when finished)
     • Network operations can be really slow
     • You might want to do something while waiting (like update the GUI and respond to user events) – so you need threads
   • Asynchronous Networking
     • Give commands, the OS handles them
     • Callbacks on completion/error
     • Polling to see if new data is available
     • The only way to do I/O in Javascript
     • No synchronization problems (race conditions etc.), but harder programming model.
   • Http Basics
     • <COMMAND> <PATH> <PROTOCOL> eg: "GET / HTTP/1.1"
     • <HEADERS> eg: "Accept-Encoding: gzip"
   • Implementation of a server
     • Serve "basic" HTTP/1.0
     • Ignore most headers
     • Return the request in text/plain
   • Server Workflow
     • Create a new socket. Bound to 0.0.0.0:80
     • While not shutting down
       • accept a new connection
       • Read the first line in the connection
       • Read a line. If it's empty stop processing, If not, add it to the known headers
       • If there is a content-length header. Read the content
       • Process the request based on the path

2. Advanced I/O

   • What is I/O
     • Computers only make sense if they can interact
     • That means they need to be able to receive inputs
     • And produce outputs
   • I/O Devices
     • Keyboard
     • Mouse
     • Screen
     • Scanner
     • Printer
     • Hard Disk
   • Hardware perspective
     • CPU
       • Devices live on buses (PCI, USB)
       • Buses are middle-men (often layered)
       • Devices are exposed in two ways (not exclusive)
         • I/O ports (mostly used in Memory Mapped mode)
         • Memory access (put video memory into cpu address space)
     • I/O ports (on x86 architecture)
       • Maximum 32 bits
       • Addressed one by one
       • In total 65,536 available (each device only gets a limited amount)
       • Ports are unique across a computer (PnP needed to dynamically assign ports)
       • Can be exposed as "plain" memory (memory mapped I/O)
     • Memory based I/O
       • Ports are made available as memory addresses (no longer a big issue with large address spaces)
       • Device based memory is made available in main address space
       • Creates (imperfect) illusion that everything is memory
     • DMA (Direct Memory Access)
       • I/O in many cases is really slow
       • Memory mapped I/O doesn't stop slowness
       • Copying I/O from hardware to memory is a "simple" operation
       • Special purpose helper system (DMA engine)
       • Hardware needs to "cooperate" / be designed for it.
     • OS role
       • All these technical details are primarily relevant for driver / OS writers
       • But there are performance and design consequences that leak into programs
   • Programming I/O
     • Kinds of I/O
       • Disk – Arbitrated through File System layer
       • Video – Arbitrated through windowing system
       • Audio – Arbitrated through audio layer
       • Network – Arbitrated through networking layer
       • Mouse, Keyboard etc. Arbitrated through windowing system
       • General I/O – Arbitrated through device drivers exposing specific or generic interface
     • Common thread
       • There can only be one program controlling a device
       • Many programs want access
       • The OS must arbitrate and often control
       • Programs receive OS regulated access to smaller abstractions of the device
     • I/O Models
       • Synchronous – Stop the thread while performing an operation
       • Asynchronous – Don't wait for results (but optionally be notified on completion)
     • Access Modes
       • Command based – Use special commands to access the I/O device
       • Memory mapped – Get access as if it is a range in memory (where this makes sense) – A bit more complex, but faster, and reuses virtual memory techniques
     • Java
       • Did not use to support many advanced modes
       • With NIO (New IO) supports many more modes
     • I/O Buffering
       • Unbuffered – When using small reads this can be very slow
       • Buffered – Read in large blocks, let users use it as small
       • Unless requesting "direct I/O" always has OS cache between program and file
     • Access modes
       • Read – Only read the file
       • Write – Remove existing content and write
       • Append – Add to the end of the file
       • Random Access – Be able to move through the file (and reading/writing)
       • Read+ – Read access that does not disallow writing
     • Files kinds
       • Binary files. Store sequences of bytes. When storing/reading values larger than bytes, byte order matters (big/small endian)
       • Text files. Store sequences of characters. Depends on character encoding (mapping bytes in the file to unicode), and line ending

3. Callbacks and Lambdas

   • Functors
     • Literally function objects
     • Allow functionality to be provided outside object control
     • In some cases could be replaced with inheritance
     • uses
       • Comparator<T>
       • OnClickListener
       • Runnable
     • Difference from function
       • Function only has address / name
       • Functor / function object may have state
   • Function ref
     • Address to a function
     • Type that allows for correct calling
     • Since 1.7 there is a MethodHandle in java that “does this”
     • Traditional Method objects are also references
     • In C/C++ it is a pointer (32 or 64 bits) to the address where the executable code starts
   • Poor man's objects (with virtual methods)
     • Struct(ured) type with method pointers
     • The pointed to functions have the object as first parameter, and operate on it
     • The function pointers don't change (or are stored in a shared “class” object)
     • Example
   • 2 kinds of data - java
     • Objects – Always references, inherit from Object, all methods are virtual
     • Primitives – Limited, fixed set of stack based numbers that are the building blocks of everything else
   • Other entities
     • Classes/types – Have a runtime “existence” through Class objects
     • Methods – Available through reflection
     • You can see them, but they are not straightforward (or cheap) to use.
     • They are second class citizens.
   • First class functions
     • When you don't have data
     • “like static methods”
     • Parameters can be functions
     • Functions can return functions
     • Other first class "things"
       • Primitives
         • can be calculated
       • Objects
         • Can be instantiated (new Object)
     • Creating first class functions
       • Lambda (λ) (Yes the Greek letter)
       • Lambda calculus is a formalisation of programming
       • Lambda functions are functions that create functions.
   • Why lambda
     • Lambda calculus teaches us the awesome power of “dynamic” function creation
     • Very compact way to specify functions in-line
     • Allows for easy use of very powerful algorithms
     • Use new Thread(() -> { System.out.println(); });
   • Lambda usage
     • map(iterable, function) → new iterable storing the results
     • filter(collection, function) → shortened collection containing only some functions
     • Event handlers
     • Create a function that incorporates a link to an object and calls a method on it as a replacement of a functor.

4. Generics/Templates

   • Core programming concepts
     • Abstraction
     • Handling complexity
     • Code reuse
       • Using libraries
       • Designing your own code
     • Correctness
   • Design for reuse
     • Support various uses
     • Accept any type
     • Keep code correct
     • Make types and other things a part of the "parameters" to types and functions
   • Introspection
     • Type info available at run-time
     • Instantiate objects at run-time
     • Create method calls at run-time
     • Java Class objects
   • Code generation
     • Special code
       • Android's R.java – Link resources to java
       • Qt's MOC (Meta Object Compiler) to support a library feature
     • Generate code from UML (even round-tripping
     • Compiler (generates assembler code)
   • Runtime code generation
     • Better(reified) generics (C# style)
     • Just in time compilation
     • Various Java libraries with Classloader
   • Compile time code generation
     • Special purpose tools
     • One-off programs
     • Framework tools
       • Android R.java
       • JSP processing
   • Generics
     • Allow compiler to enforce rules on types
     • Language specifies type dependencies in classes / functions
     • Only work on types
     • Generic types are highly related (but not actually the same type)
     • One body of code
   • Generic implementation
     • Type erasure (java) – At run-time generics have disappeared.
     • Reification (.net) – Remember the used type, allowing for things like arrays of generics (up to having separate implementation for primitives)
   • What is an object in Java/C#
   • In place use of objects
   • Memory usage
     • In place - Memory is part of owner (object / stack)
     • Pointer/reference - Separately allocated memory, Owner has a pointer (size does not depend on type)
   • Templates
     • C++ solution to code reuse (especially for containers)
     • Unlike generics, is not a type of itself
     • Arbitrary specialisations are allowed
     • Tells compiler how to create function or class based on parameter type or number
   • Example Template

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