How Can Thread Synchronization Be Achieved in C 0x Without Using Semaphores?

C 0x Without Semaphores: Achieving Thread Synchronization

C 0x introduces various enhancements, but one notable omission is the inclusion of semaphores. Semaphores are essential for thread synchronization, enabling threads to signal events and wait for their completion.

Problem Statement

In the absence of semaphores, how can we implement a mechanism for thread synchronization as exemplified in the followingPOSIX semaphore usage:

void thread0(...)
{
  doSomething0();

  event1.wait();

  ...
}

void thread1(...)
{
  doSomething1();

  event1.post();

  ...
}

Solution Using Mutex and Condition Variable

One effective method is to create a semaphore-like functionality using a mutex and a condition variable. Here's the code:

#include <mutex>
#include <condition_variable>

class semaphore {
    std::mutex mutex_;
    std::condition_variable condition_;
    unsigned long count_ = 0; // Initialized as locked.

public:
    void release() {
        std::lock_guard<decltype> lock(mutex_);
        ++count_;
        condition_.notify_one();
    }

    void acquire() {
        std::unique_lock<decltype> lock(mutex_);
        while(!count_) // Handle spurious wake-ups.
            condition_.wait(lock);
        --count_;
    }

    bool try_acquire() {
        std::lock_guard<decltype> lock(mutex_);
        if(count_) {
            --count_;
            return true;
        }
        return false;
    }
};</decltype></decltype></decltype></condition_variable></mutex>

Usage

This custom semaphore can be employed in the following manner:

semaphore event1;

void thread0(...)
{
  doSomething0();

  event1.acquire();

  ...
}

void thread1(...)
{
  doSomething1();

  event1.release();

  ...
}

Compared to Mutex Only

This approach offers several advantages over using a mutex alone:

• It eliminates the ambiguity of acquiring and releasing the same mutex in different threads, which can lead to deadlock.
• It ensures that thread1 does not acquire the mutex before thread0.
• It provides an additional try_acquire() method for non-blocking acquisition attempts.

By utilizing this custom semaphore implementation, you can effectively achieve thread synchronization in C 0x without explicit semaphore support.

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