17. Thread API

Info

This is a summary of the 27th chapter of the book “Operating System: Three Easy Pieces” by Remzi H. Arpaci-Dusseau and Andrea C. Arpaci-Dusseau. The chapter introduces the basics of the thread API, covering thread creation, synchronization mechanisms, and related guidelines for multi-threaded programming.

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Overview

Chapter 27 focuses on the Thread API, providing a foundational understanding of how threads are created, controlled, and synchronized. It explores essential programming constructs such as locks and condition variables and discusses best practices for writing robust multi-threaded programs.

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Key Concepts

• Thread API Basics:

  • Threads are lightweight units of execution, created and managed using APIs such as POSIX pthread.
  • Threads share the same address space but have their own stack for local execution.

• Thread Creation:

  • Use pthread_create() to spawn new threads.
  • Function signature:

    int pthread_create(pthread_t *thread, const pthread_attr_t *attr, void *(*start_routine)(void*), void *arg);

    • Arguments:
      • thread: A pointer to a thread identifier.
      • attr: Thread attributes (e.g., stack size). Pass NULL for defaults.
      • start_routine: Function pointer specifying the thread’s starting point.
      • arg: Argument to be passed to the thread.
  • Threads run independently within the same program’s address space.

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Thread Completion

• Use pthread_join() to wait for a thread to finish execution:

  int pthread_join(pthread_t thread, void **value_ptr);

  • Allows retrieving the thread’s return value via value_ptr.
  • Important: Never return a pointer to a stack-allocated variable from a thread; use heap-allocated memory instead.

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Synchronization Mechanisms

1. Locks:

   • Use locks to ensure mutual exclusion in critical sections:

     pthread_mutex_t lock;
     pthread_mutex_lock(&lock);
     // Critical section
     pthread_mutex_unlock(&lock);

   • Initialize locks using:

     • PTHREAD_MUTEX_INITIALIZER for static initialization.
     • pthread_mutex_init() for dynamic initialization.

   • Advanced lock operations:

     • pthread_mutex_trylock(): Attempts to acquire the lock without blocking.
     • pthread_mutex_timedlock(): Attempts to acquire the lock with a timeout.

2. Condition Variables:

   • Facilitate signaling between threads:

     pthread_cond_wait(pthread_cond_t *cond, pthread_mutex_t *mutex);
     pthread_cond_signal(pthread_cond_t *cond);

   • Example usage:

     pthread_mutex_lock(&lock);
     while (!condition) {
         pthread_cond_wait(&cond, &lock);
     }
     pthread_mutex_unlock(&lock);

   • Signals to wake threads:

     pthread_cond_signal(&cond);

   • Always pair condition variables with locks to avoid race conditions.

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Compilation

• Include the header pthread.h.
• Link against the pthread library using -pthread:

  gcc -o main main.c -Wall -pthread

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Best Practices

• Keep it simple:
  • Write clean, straightforward synchronization code.
• Minimize interactions:
  • Reduce the number of thread interdependencies.
• Initialize properly:
  • Always initialize locks and condition variables.
• Check return codes:
  • Handle errors from thread API calls to prevent unexpected behavior.
• Avoid stack references:
  • Never return or share pointers to stack variables between threads.
• Use condition variables for signaling:
  • Avoid using flags for thread synchronization.

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Conclusion

This chapter lays the groundwork for using the pthread library to build robust multi-threaded applications. While the APIs themselves are straightforward, achieving correct and performant concurrency requires thoughtful design and careful handling of synchronization primitives. Mastering these basics is critical for developing reliable concurrent programs.

Next Chapter: Locks