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Multithreading and Condition Variables in C

Table of Contents

  1. Introduction to Condition Variables
  2. Multiple Condition Variables
  3. Pthread Condition Broadcast API
  4. Interview Questions

Introduction to Condition Variables 📝

  • What is a Condition Variable?: A condition variable is often associated with a mutex and a predicate. 🎯
  • Role of Mutex: Ensures mutual exclusivity when a thread checks the predicate. No other thread should interfere with the predicate during the check. 🔒
  • Rules:
    • Many condition variables can be associated with the same mutex.
    • One condition variable cannot be associated with more than one mutex. 🛑

Multiple Condition Variables 🌐

  • Scenario:

    • Multiple Threads: T1, T2, T3
    • Each has its own condition variable: cv1, cv2, cv3
    • One resource R with one mutex M

  • Waiting and Signaling:

    • Threads block themselves if the resource is busy.
    • Threads invoke pthread_condition_wait API.
    • The signaling thread (TS) can signal specific threads to unblock. 🔔

  • Developer Power: The developer can specifically signal which thread to unblock and execute. 👨‍💻

Pthread Condition Broadcast API 📡

  • Purpose: Unblocks all threads that are blocked on the same condition variable.

  • Example: Threads T1, T2, T3 blocked on the same condition variable.

  • How it works:

    • Signaling thread (TS) invokes pthread_condition_broadcast.
    • All threads move from blocked to ready state.
    • OS scheduler determines the sequence they get executed in. 🔄

  • Alternative: Without broadcast, you need to call pthread_condition_signal individually for each thread.

  • Lack of Control: The order in which the threads are unblocked is not in the programmer's control. 🎭

Interview Questions ❓

  1. What is the role of a Mutex in working with condition variables?

    • Answer: The mutex ensures that no other thread interferes with the predicate check, providing mutual exclusivity. 🔐

  2. How can you associate multiple condition variables with the same mutex?

    • Answer: You can associate multiple condition variables with the same mutex by invoking pthread_condition_wait for each thread, passing in the mutex and the thread's respective condition variable as arguments. 🔄

  3. What is the pthread_condition_broadcast API and when would you use it?

    • Answer: This API is used to signal all threads blocked on the same condition variable to resume execution. It's particularly useful when you want to unblock all waiting threads at once. 🌐

  4. What are the limitations of the pthread_condition_broadcast API?

    • Answer: The API doesn't allow you to control the order in which the threads are unblocked; that decision is left to the operating system's scheduling policy. 🛑

  5. How does having individual condition variables for each thread give the programmer more control?

    • Answer: Individual condition variables allow the programmer to selectively unblock threads. This fine-grained control is useful for implementing complex logic. 🔍

Feel free to ask for more information or additional questions you might have! 📚