多线程(C++)临界区Critical Sections

本文详细介绍了CriticalSection的概念及用法,CriticalSection是一种确保同一时刻只有一个线程能够访问共享资源的数据结构。通过使用CCriticalSection类,开发者可以轻松地实现线程同步,防止多个线程同时访问共享资源导致的问题。

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Critical Sections(功能与Mutex相同,保证某一时刻只有一个线程能够访问共享资源,但是不是内核对象,所以访问速度比Mutex快,但是没有等待超时的功能,所以有可能导致死锁,使用要小心)

 

    当多个线程访问一个独占性共享资源时,可以使用“临界区”对象。任一时刻只有一个线程可以拥有临界区对象,拥有临界区的线程可以访问被保护起来的资源或代码段,其他希望进入临界区的线程将被挂起等待,直到拥有临界区的线程放弃临界区时为止,这样就保证了不会在同一时刻出现多个线程访问共享资源。

CCriticalSection类的用法非常简单,步骤如下:

  1. 定义CCriticalSection类的一个全局对象(以使各个线程均能访问),如CCriticalSection critical_section;
  2. 在访问需要保护的资源或代码之前,调用CCriticalSection类的成员Lock()获得临界区对象:
    critical_section.Lock();
    在线程中调用该函数来使线程获得它所请求的临界区。如果此时没有其它线程占有临界区对象,则调用Lock()的线程获得临界区;否则,线程将被挂起,并放入到一个系统队列中等待,直到当前拥有临界区的线程释放了临界区时为止。
  3. 访问临界区完毕后,使用CCriticalSection的成员函数Unlock()来释放临界区:
    critical_section.Unlock();
  4. 再通俗一点讲,就是线程A执行到critical_section.Lock();语句时,如果其它线程(B)正在执行 critical_section.Lock();语句后且critical_section. Unlock();语句前的语句时,线程A就会等待,直到线程B执行完critical_section. Unlock();语句,线程A才会继续执行。
  5. 注意事项

     1) 因为Critical Sections不是内核对象,所以只能用来统一进程内线程间的同步,不能用来多个不同进程间的线程的同步。

     2) 如果在Critical Sections中间突然程序crash或是exit而没有调用LeaveCriticalSection,则结果是改线程所对应的内核不能被释放,该线程成为死线程。

     3) 要比其他的内核对象的速度要快。

### Critical Section in Concurrent Programming In concurrent programming, the term **critical section** refers to a part of the program where shared resources are accessed or modified. To prevent race conditions and ensure thread safety, it is essential to manage access to these critical sections carefully[^1]. A critical section typically involves operations such as reading from or writing to shared memory locations, updating counters, or performing other synchronized actions. To handle critical sections effectively, synchronization mechanisms like locks (mutexes), semaphores, monitors, or atomic operations can be employed. These tools allow only one thread at a time to execute within the critical section, thereby avoiding conflicts between threads accessing shared data simultaneously. Here’s an example demonstrating how a lock might protect a critical section using Java: ```java import java.util.concurrent.locks.Lock; import java.util.concurrent.locks.ReentrantLock; public class Counter { private int count = 0; private final Lock lock = new ReentrantLock(); public void increment() { lock.lock(); // Acquire the lock before entering the critical section. try { count++; } finally { lock.unlock(); // Ensure the lock is released after exiting the critical section. } } public int getCount() { return count; // This method does not modify state so no locking required here. } } ``` The use of `lock` ensures that multiple threads cannot concurrently enter the critical section defined by the body of the `increment()` function, thus maintaining consistency across all accesses to the shared variable `count`. Additionally, when discussing systems design more broadly, particularly those involving distributed architectures or high-performance computing environments, managing critical paths becomes crucial for optimizing overall throughput while minimizing bottlenecks caused by resource contention[^2]. Finally, understanding what constitutes a “critical operation” also extends beyond just coding practices into areas including messaging reliability through brokers which guarantee delivery semantics under various fault scenarios[^4]; this highlights another dimension wherein ensuring correctness around specific interactions remains paramount regardless whether they occur synchronously versus asynchronously over networks etc..
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