Reader Writer Problem Solution using semaphore

Duration: 12 min

This video lesson is available to enrolled students.

Return to /learn/RSSB-SENIOR-COMPUTER-INSTRUCTOR/operating-systems/process-synchronization/classical-problems/asset-reader-writer-problem-solution-using-semaphore after enrolling

Inside: a video lesson and guided study material.

Module outline

  1. About the Course: About the Course
  2. Paper-1 & 2 | Basic Numeracy: अनुपात और समानुपात / Ratio and Proportion, Divisibility Rules, मिश्रण और एलिगेशन / Mixture and Alligation, Permutation and Combination, Logarithm, संख्या पद्धति / Number System, महत्तम समापवर्तक और लघुत्तम समापवर्त्य / HCF and LCM, Sequence and Series (Series), समय, चाल और दूरी / Time, Speed and Distance, प्रतिशत / Percentage, औसत / Average, घन क्षेत्रमिति / Solid Mensuration, Age Problems, Arithmetic, Statistics, Probability, लाभ, हानि और बट्टा / Profit, Loss and Discount, साधारण और चक्रवृद्धि ब्याज / Simple and Compound Interest, Partnership, Speed Mathematics and Basic Mathematics, Algebra, सरलीकरण, घातांक और करणी / Simplification, Indices and Surds, समय और कार्य / Time and Work, Pipe and Cistern, RSSB Specific Videos - Basic Numeracy, समुच्चय, संबंध, फलन और विविक्त गणित / Sets, Relations, Functions and Discrete Mathematics, संख्या पद्धति और संख्या सिद्धांत / Number Systems and Number Theory, निर्देशांक ज्यामिति, सदिश और त्रिविमीय ज्यामिति / Coordinate Geometry, Vectors and 3D Geometry, बहुपद, समीकरण और सम्मिश्र संख्याएँ / Polynomials, Equations and Complex Numbers
  3. Paper-1 & 2 | Data Interpretation: आँकड़ा निर्वचन / Data Interpretation
  4. Paper-1 & 2 | General Mental Ability: Mental Ability, RSSB Specific Videos - Mental Ability
  5. Paper-1 & 2 | Logical Reasoning and Analytical Ability: श्रेणी / Series, कूट लेखन एवं कूट वाचन / Coding-Decoding, Data Sufficiency, अशाब्दिक तर्क / Non-Verbal Reasoning, Syllogisms, बैठक व्यवस्था एवं पहेलियाँ / Seating Arrangement and Puzzles, रक्त संबंध / Blood Relations, दिशा एवं दूरी / Direction and Distance, Analytical Reasoning (Counting Figures Reasoning), गणितीय संक्रियाएँ एवं लुप्त संख्या / Mathematical Operations and Missing Numbers, कैलेंडर एवं घड़ी / Calendar and Clock, Clock, घन, पासा एवं आकृतियों की गिनती / Cubes, Dice and Figure Counting, वेन आरेख, न्याय वाक्य एवं असमानता / Venn Diagrams, Syllogism and Inequalities, सादृश्यता एवं वर्गीकरण / Analogy and Classification, क्रम एवं रैंकिंग / Order and Ranking, कथन आधारित तार्किक विवेचन / Statement-Based Reasoning, Data Arrangement, वर्णमाला एवं संख्या परीक्षण / Alphabet and Number Test, Crypt Arithmetic
  6. Paper-1 & 2 | Decision Making and Problem Solving: आँकड़ों की पर्याप्तता एवं निर्णय क्षमता / Data Sufficiency and Decision Making
  7. Paper-1 | General Science: वैज्ञानिक उपलब्धियाँ और अनुसंधान / Scientific Achievements and Research, Biology, Chemistry, RSSB Specific Videos
  8. Paper-1 | General Awareness: राज्य विशेष सामान्य ज्ञान / State-Specific General Knowledge, समसामयिकी / Current Affairs
  9. Paper-2 | Fundamentals of Computer: Basics of COA, Number System, Floating Point Rep, Number Representation
  10. Paper - 2 | Digital Logic Systems: Digital Systems & Boolean Basics, Logic Gates & Hardware, Boolean Expression, Boolean Minimization, Combinational Circuit, Sequential Circuits
  11. Paper-2 | Operating Systems: Introduction to OS, Process Management, CPU Scheduling, Process Synchronization, Deadlock, Threads & Process Creation, Memory Management, Virtual Memory, Disc Scheduling, File Management, Linux OS, Distributed Systems
  12. Paper-2 | Database Management System: Basics of DBMS, ER Diagram, Relational Model & Functional Dependencies, Keys & Integrity Constraints, Normalization (1NF - BCNF), Decomposition Properties & 4NF, Relational Algebra, SQL, Relational Calculus, File Organization & Indexing, Transaction Management, Concurrency Control
  13. Paper-2 | Data Structures: Introduction to DS, Array, Stack, Queue, Linked List, Tree, Graphs, Hashing
  14. Paper-2 | C Programming: C Fundamentals, Control Flow, Functions, Arrays & Pointers, Storage Classes, Structures & Enums, DMA, Macros, Scoping & File Handling
  15. Paper 2 | Oops, C++, Java: Language Design, OOPS with C++, Java, .Net Programming, Java Fundamentals
  16. Paper 2 | Python: Introduction to Python, Tokens, Variables, Data Types, I/O, Operators & Expressions, Control Flow & Looping, Collections & Strings, Functions & Modules, File Handling & Data Analysis
  17. Paper 2 | Emerging Trends and AI: Emerging Tech Trends, कंप्यूटर के मूल सिद्धांत / Computer Fundamentals, व्यापार, विपणन एवं उपभोक्ता संरक्षण / Trade, Marketing and Consumer Protection, कंप्यूटर नेटवर्क / Computer Networks
  18. Paper-2 | Algorithms: Algorithm Analysis, Time Complexity Analysis, Sorting Algorithms, Greedy Algorithms, Advanced Algorithms
  19. Paper 2 | Data Communication and Network Security: Introduction to CN, Data Communication, DLL: Access Control, DLL: Flow Control, DLL: Error Control, Data Link Layer - Ethernet, Net Layer: IPv4 & Proto, Net Layer: IP Addressing, Net Layer:Routing Protocol, Transport Layer Services, TL: Congestion & UDP, Application Layer, Hardware basics, Network Security, Ethical Hacking
  20. Paper-2 I Software Engineering: Fundamentals of Software Engineering, Software Requirements and Quality Assurance, Software Design, Estimation and Metrics, Software Testing, Software Maintenance and Configuration Management
  21. Paper-2 | Computer Organization and Architecture: Cache Memory Organization, Input Output Organisation, Instr Formats & Modes, Control Unit Design, Register Transfer and Microoperations
  22. Paper-2 | Web Development: JavaScript-Basics, HTML Basics, CSS, XML
  23. Pedagogy: वृद्धि और विकास / Growth and Development, Concept of Pedagogy, संज्ञानात्मक प्रक्रियाएँ / Cognitive Processes, विकास के सिद्धांत / Theories of Development, समावेशी शिक्षा / Inclusive Education, Experiential Pedagogy, समाजीकरण और जेंडर / Socialisation and Gender, National Education Policy (NEP) 2020
  24. Live Sessions: RSSB Live Sessions
  25. RSSB Paper 1 - Full Mock Tests:
  26. RSSB Paper 2 - Full Mock Tests:
  27. SCI Previous Year Papers:
AI summary & chapters

AI Summary

An AI-generated summary of this video lecture.

The video provides a detailed lecture on solving the Readers-Writers synchronization problem using semaphores. The instructor defines shared variables (mutex, wrt, readcount) and constructs the logic for Writer and Reader processes. He explains the necessity of protecting the readcount variable with a mutex and using the wrt semaphore to block writers when readers are active. The lecture culminates in a complete code structure that implements a readers-preference solution, ensuring writers wait if readers are present.

Chapters

  1. 0:00 – 2:00 00:00-02:00

    The video begins with the instructor introducing the 'Solution using Semaphores' for the Readers-Writers problem. On the slide, he lists the shared data structures: semaphore mutex = 1, semaphore wrt = 1, and int readcount = 0. He explains that these variables are shared among all reader processes. He clarifies the role of mutex as a binary semaphore used to protect the readcount variable from race conditions, ensuring that only one process updates the count at a time. He also defines wrt as a semaphore used to manage access to the shared resource, specifically to block writers when readers are active. He points to the text on the screen to emphasize the initialization values.

  2. 2:00 – 5:00 02:00-05:00

    The instructor moves to the whiteboard to outline the logic for the Writer process. He writes Wait(wrt) before the Critical Section (CS) for writing and Signal(wrt) after. He explains that this simple structure ensures that only one writer can access the resource at a time and that writers are blocked if readers are currently reading. He then shifts focus to the Reader process, writing Readcount++ to increment the count when a reader enters and Readcount-- when it leaves. He emphasizes that these operations must be atomic to prevent data corruption and that a separate mechanism is needed to protect the counter.

  3. 5:00 – 10:00 05:00-10:00

    The instructor refines the Reader process logic by adding synchronization primitives. He writes Wait(mutex) before Readcount++ and Signal(mutex) after, ensuring the counter update is safe. He then introduces the critical logic for the first and last readers. He writes If(readcount == 1) followed by wait(wrt), explaining that the first reader must block writers. Conversely, he writes If(readcount == 0) followed by signal(wrt), explaining that the last reader must release the resource for waiting writers. He draws arrows to show the flow of execution and the conditions under which writers are blocked or unblocked. He also writes R1 and R2 to denote different readers.

  4. 10:00 – 12:11 10:00-12:11

    The instructor completes the solution by filling in the full table for both Writer and Reader processes. He writes the complete code sequence: Writer waits on wrt, enters CS, signals wrt. Reader waits on mutex, increments readcount, checks if it is the first reader (waits on wrt), signals mutex, enters CS, waits on mutex, decrements readcount, checks if it is the last reader (signals wrt), and signals mutex. He reviews the entire flow, pointing out how the wrt semaphore acts as a gatekeeper for writers, ensuring they wait if any readers are present, thus implementing a readers-preference solution. He highlights the specific lines of code on the board to reinforce the logic.

The lecture systematically builds a solution to the Readers-Writers problem. It starts with defining shared resources, then constructs the Writer process to ensure exclusive access. It then develops the Reader process, introducing a counter (readcount) protected by a mutex. The core logic involves the first reader blocking writers and the last reader releasing them. The final output is a complete, synchronized code structure that prioritizes readers.

Loading lesson…