Reader Writer Problem Solution using semaphore

Duration: 12 min

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Module outline

  1. About the Course: About the Course
  2. Section 1 | Language (Hindi): भाषा, लिपि, ध्वनि, वर्ण, अक्षर, स्वर, व्यंजन, मात्रा और संयुक्ताक्षर, वर्तनी, वर्ण-विन्यास, अनुस्वार, अनुनासिक, चंद्रबिंदु और ‘र’ के रूप, शब्द, पद और शब्द-भेद, संज्ञा और सर्वनाम, विशेषण और क्रिया, लिंग, वचन, पुरुष और कारक, काल, वाच्य, वाक्य और वाक्य-भेद, संधि, समास, विलोम शब्द, पर्यायवाची शब्द, सामान्य अशुद्धियाँ, अनेक शब्दों के लिए एक शब्द, मुहावरे और लोकोक्तियाँ, अपठित गद्यांश और अपठित पद्यांश, Multi-Topic Content (Cross-Concept Material), वर्णमाला
  3. Section 1 | Language (उर्दू भाषा — 22 प्रश्न (निश्चित)): Spelling & Diacritics, Grammar (اسم, فعل, صفت, ضمیر), Vocabulary, Idioms & Proverbs, Comprehension
  4. Section 1 | Language (বাংলা / बांग्ला भाषा — 22 प्रश्न (निश्चित)): Bangla Script and Sounds, Bangla Grammar and Usage, Bangla Word Formation and Vocabulary, Reading and Text Interpretation
  5. Section 1 | Language (English): Vocabulary, Noun, Subject Verb Agreement (Verb Noun Agreement), Adjectives, Tenses, Pronoun, Verb, Adverb, Preposition, Articles, Determiner, Conjunction, Modals, Active Passive Voice (Voice), Direct and Indirect Speech, Sentence Correction (Error Correction), Sentence Completion (Fill in the blanks), Sentence Construction, Sentence Re-arrangements (Para Jumbles) (Narrative Sequencing), Comprehension / Reading Comprehension / Unseen Passages (Critical Reasoning) (Paragraph Questions), Cloze Test (Cloze Passage), Verbal Analogy (Word Based Analogy), Facts, Inference and Judgements
  6. Section 2 | Primary Maths & Reasoning: Ratio and Proportion (Ratios), Divisibility Rules, Mixture and Alligation, Permutation and Combination, Number System, HCF LCM, Sequence and Series (Series), Speed Time and Distance, Series (Number and Letter Series) (Numerical Relations and Reasoning), Coding Decoding, Non Verbal Reasoning (Spatial Aptitude) (Spatial Reasoning) (Visual Reasoning), Percentage, Average, Profit and Loss, Simple Interest and Compound Interest, Time and Work, Mensuration and Geometry, Age Problems, Arithmetic, Powers and Exponents (Surds and Indices), Algebra, Probability, Trigonometry, Syllogisms, Seating Arrangements, Blood Relations, Directions (Direction Test), Analytical Reasoning (Counting Figures Reasoning), Puzzle Solving (Puzzles), Calendar, Clock, Cubes & Dices, Venn Diagram, Data Arrangement, Inequality, Analogy, Ranking, Order and Sequence
  7. Section 2 | General Awareness (GK + Current Affairs): Polity, Physics, Biology, State Wise GK, Chemistry, History, Geography, Indian Economy, Environment, GK- Current Affairs, Static General Knowledge
  8. Section 3 | Computer System and Org. (CSO): Basics of COA, Floating Point Rep, Cache Memory Organization, Instr Formats & Modes, Programming the Basic Computer, Input Output Organisation, Register Transfer and Microoperations, Control Unit Design
  9. Section 3 | Database Management System: Basics of DBMS, ER Diagram, Relational Model & Functional Dependencies, Keys & Integrity Constraints, Normalization (1NF - BCNF), Decomposition Properties & 4NF, SQL, File Organization & Indexing, Relational Algebra, Relational Calculus, Transaction Management, Concurrency Control
  10. Section 3 | Operating System: Introduction to OS, Process Management, CPU Scheduling, Threads & Process Creation, Memory Management, Virtual Memory, Disc Scheduling, File Management, Process Synchronization
  11. Section 3 | Computer Networks: 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, Transport Layer Services, TL: Congestion & UDP, Application Layer, Hardware basics
  12. Section 3 | Data Structures: Introduction to DS, Array, Stack, Queue, Linked List, Tree, Graphs, Hashing, Algorithm Analysis, Time Complexity Analysis, Sorting Algorithms, Greedy Algorithms, Minimum Spanning Trees, Shortest Path Algos
  13. Section 3 | Theory Of Computation & Compiler Design: Introduction to TOC, Deterministic FA (DFA), Non-Deterministic FA, Regular Expressions, Grammar, Regular Language Properties, Moore & Mealy Machines, Pushdown Automata & CFG, Turing Machines, Intro to Compilers, Lexical Analysis, Grammar & CFG, Syntax Analysis: Top-Down, Syntax Analysis: Bottom-Up, Semantic Analysis & SDT, Intermediate Code Gen, Code Optimization
  14. Section 3 | Emerging Trends (AI, Societal Ethics & MS Office): Artificial Intelligence, Emerging Tech Trends, Society, Law & Ethics, MS Office Applications
  15. Section 3 | Software Engineering: Fundamentals of Software Engineering, Software Requirements and Quality Assurance, Software Design, Estimation and Metrics, Software Maintenance and Configuration Management, Software Testing
  16. Section 3 | C Programming: C Fundamentals, Control Flow, Functions, Arrays & Pointers, Storage Classes, Structures & Enums, DMA, Macros, Scoping & File Handling
  17. Section 3 | OOPS, C++, JAVA, HTML: OOPS with C++, Java, HTML Basics, CSS
  18. Section 3 | Java: Java Fundamentals, Array, String, IO & References
  19. Section 3 | Python: Introduction to Python, Tokens, Variables, Data Types, I/O, Operators & Expressions, Control Flow & Looping, Collections & Strings, Functions & Modules, File Handling & Data Analysis
  20. Section 3 | Digital Electronics: Digital Systems & Boolean Basics, Logic Gates & Hardware, Boolean Expression, Boolean Minimization, Combinational Circuit, Sequential Circuits, Number System, Number Representation
  21. Section 3 | Multimedia: Multimedia
  22. BPSC Previous Year Papers:
  23. BPSC Full Mock Tests:
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.

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