Transcript Systems Programming (Introduction)

Systems Programming
Meeting 1:
Systems Technology: Program,
Process & Thread Overview
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Objectives
•Value / Motivation (Experiment to Start)
•Course Mechanics
•Course Topic Overview
•Defining Systems Programming
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Thought Experiment
•Everyone chose role:
hardware (I/O, CPU, memory, system
bus & clock), OS (processes, signals),
application (pseudo code), operator
•Need: A card from deck
•Walk thru (clock cadence):
- Operator ‘runs’ processes (stored in
memory), supplying parameter(s)
- Operands read from file (input),
processes retrieved from memory files,
ALU perform calculations in registers,
writes results to separate file (output) …
•What if some hurdle was added (i.e.,
process file relocation)?
•Lessons Learned?
•This is Systems Programming!
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Systems Programming
Motivation
•Familiar with the ‘dominant OS’
incompatibilities?
•Exploding growth of web uses/
users (linear?) exacerbates issue
•Systems & Applications:
designed to perform specific tasks
and functions. Do they?
•Goes wrong examples: IEEE,
NASA/ESA, Power Grids & Plant,
costs to $BBs
•Fix it sooner or later?
(Easier, faster vs. ‘hero’)
{Web version: sorry, no movie clip}
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Systems Programming
Motivation (more …)
•At Operating Systems & Application Programming
intersection: Learn to develop precise code where
required or systems library use where available
•Fundamentals (UNIX/LINUX programs, processes, files,
directories, I/O, programming tools, token ring)
•Asynchronous Events (signals, timers)
•Concurrency (threads, synchronization, critical sections,
semaphores, inter-process communication, SMP/Cray)
•Communication (connection-oriented, wireless,
MANET/ProxiNet)
•Project (develop actual release modules for open source
reusable library for scripting programs—expert in our midst!)
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Course Mechanics
Course Web Page:
www.seas.gwu.edu/~csci259/
(materials, links, assignments, exams)
Email to:
[email protected] (questions, anytime)
[email protected] (assignments, exams)
Office Hours: Monday 5 - 6pm, Phillips Hall
720 (or by appt.)
Prereqs: CSci 210; ‘C’ & UNIX experience;
SEAS (or other) UNIX account access
A little background?
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Course Mechanics (more)
Text: UNIX Systems Programming: Communication,
Concurrency, and Threads, by Kay A. & Steven
Robbins (skim before, study after class),
supplementary materials provided and can be
found (web links, bookstore)
Class time is discussion; not simply lecture
Homework from web page, due by email:
collaboration allowed in study; final product,
your own; course project ~ 3 person groups.
Programming assignments require archive file with
‘readme’ (contents, instructions), source code,
executable (SEAS UNIX), test cases & results
Coding Guidelines (3 D’s: Document, document…)
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Relationship to other coursework
• Programming (learn to use those involving
aspects of hardware and system calls)
• Operating Systems (addresses how
distributed systems programs use OS services)
• Networks (natural progression from Internet
Protocols’ specifics to general case)
• Architecture (how components interact)
• Formal Methods (critical systems calls either
robust or complicit in system crashes)
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But What Would I Use It For?
•Single Computer Applications
•Networked, Complex Systems
•Team & Individual Projects
•Safety Critical Applications
•Timing Critical Assignments
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What will I get out of this?
• Understand & apply course concepts & logic, their strengths
& weaknesses
• Choose appropriate methods for particular system classes
• Describe relations between various levels of abstraction
• OS ‘Primeval mud’ of computer system: demystify
• OS instance of Complex System: similar to biological (ants,
DNA, neurons), weather, social (cities, bf/gf), internet
examples; all emergent, huge, parallel, not well understood
• Strategy: Use abstraction, modularity & iteration
{fail early, often, grow from working simple “Hello World”
model!}
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Operating Systems
•Operating systems: the
applications’ interface to
hardware
•Even if hardware doesn’t
change (though it will), every
patch, version update, etc.
may appear as hardware
change to applications
•Challenges/Benefits of
multiple applications/users
•Adds security (or insecurity)
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Acquiring Systems
Programming Tools
Links will be available to
academically available
materials
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Systems Programming
•Key Characteristics? (Brainstorm)
•Reliability, Robustness, Maintainability
•Reliability: Specification provided & validated,
within resources (budget) {Note: the latter is
promoted by early definition of the former –
using methods we will cover – but multiple
iterations expected}
•Robustness: Graceful degradation response to
unanticipated exceptions {Software failure rate
inversely (vs. hardware, directly) related to time}
•Maintainability: Handle circumstances
changing with time (operators, hardware,
operating systems, archive media; ex: storage
access vs. processor speed gap widens)
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Embedded Processor OS
•Critical / Safety
Applications
•Cars: networks
< $10 per node
•Chemical plants
•Avionics
•Remote spacecraft
Robustness and Security issues not confined to
this context, but clear here
Consider remote ‘OnStar’ login & buffer overflow
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Design Process:
Abstractions & Mathematics
• Mathematical models (abstractions) represent
real world systems; OS abstracts hardware &
provides greater platform portability
• Formal methods & reasoning improve system
design & production ability (reveal design flaws,
define requirements precisely, implement
rational production)
• Various logical models have differing strengths
and weaknesses
• Abstraction: focus on important problem aspects
• Mathematics: reason about problem solutions
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Where did these systems begin?
• Evolution from: abacus, slide rule, mechanical calculators
& difference engines
• Turing machine realized with room of vacuum tubes;
programming by physical switches, patch wire and lights;
“time sharing” by assigned time (hour) on machine
• Enhanced input methods: (paper) card & tape readers,
magnetic tape and disk storage, now optical and soon
holographic means; interface evolved first along typewriter
path, then GUI
• Output improvements followed similar path
• Meanwhile time sharing progressed from batch processing
to multitasking mainframes with ‘dumb’ terminals; then
stand-alone single-user PCs, now networked thru Internet
• Resource sharing of certain network elements (e.g.,
printers, gateways)
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What OS Tools handle Concurrency?
• Interrupts (handling, context switches, device drivers for
particular I/O)
• Signals (synchronous or asynchronous) as software event
notification
• I/O with significantly different handling characteristics
(cache access vs. user keyboard entry)
• Processes (how different from programs?) started using
fork; communication through pipes (common ancestor)
or signals, FIFOs/named pipes, semaphores, shared
memory, or messages
• Threads (how different from processes?) provide a further
level of concurrency & further potential resource conflicts
• Multiple processors (SMP or ASMP) with distributed
[network] or shared [Cray/IBM] memory (consider ant
colonies & brain neurons)
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Programs, Processes & Threads
• Compiled program organized into functions, linked with
necessary libraries, with certain defined parameters, stored on
disk
• Process is a program instance copied by OS into particular
memory space, allowing for tracking variable values (process
state), and assigns a unique process ID to distinguish it
• Each flow of control thru the process is a thread; each of which
will have a subset of memory space assigned, unique state &
identification
• Threads each have an execution stack, program counter, set of
registers and variable state; synchronization to share resources
is key
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Program Image
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Executable appears in contiguous memory block
Diagram on board …
Environment & command line parameters
Activation records for each function call on stack
Heap memory allocations
Uninitialized static data
Initialized static data
NOTE: static variables may make program unsafe for
threads (Key: Handling re-entrant functions)
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System Function Calls, Process
Environment & Termination
• System libraries provide wealth of functions to
interface hardware with application; properly used
and handling all errors, powerful; improperly
handled: system crash (Ex: Koopman & MS VP)
• Environmental variables provide an array of pointers
to strings to pass system- and/or user- specific
information upon starting a new process for
initialization
• Proper termination of a process includes notifying
parent process (or init if a zombie, no parent) and
releasing resources, resetting OS status info
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Conclusion
•The cost of poor systems programming is significant
and rising (MS?)
•Systems programming alternatives:
- poor design and significant modification during
testing, validation and operation; or
- good design and minor changes during system
validation
•Course will develop your knowledge of operating
systems fundamentals and concepts of asynchronous
event notification, concurrency & communication;
applicable to most, primarily focus on Unix (POSIX)
•Plenty of interaction in class, demonstration of skills
on assignments, projects & exams
•Questions? Comments?
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