Can C 0x Semaphores Be Emulated Using Mutexes and Condition Variables?

Can You Use Semaphores in C 0x?

Semaphore-based synchronization is essential for thread communication. In the absence of semaphores in C 0x, it seems like a deadlock.

Semaphore Emulation with Mutex and Condition Variables

Fortunately, a semaphore can be recreated using a mutex and a condition variable. Here's a straightforward implementation:

#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(mutex_)> lock(mutex_);
        ++count_;
        condition_.notify_one();
    }

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

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

This emulation provides the fundamental behaviors of semaphores:

• release() signals an event that may be waiting on the semaphore.
• acquire() waits for a signal before continuing execution.
• try_acquire() checks for and grabs a permit if available, without blocking.

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