Multi-thread locks in C++ and how to use them

Multi-thread locks in C and how to use them

When writing multi-threaded programs, in order to ensure data security and avoid race conditions, we need to use locks. A lock is a synchronization mechanism that allows a certain code block to be executed by only one thread at the same time. In C, there are many kinds of locks to choose from, the more commonly used ones are mutex locks, read-write locks and condition variables. This article will introduce the basic concepts, usage and precautions of these locks.

Mutex (Mutex)

Mutex locks are often used to protect shared resources and only allow one thread to access them at the same time. When one thread holds a lock, other threads that want to hold the lock must wait. When the thread releases the lock, other threads can begin to access the shared resource.

In C, the definition of mutex lock is as follows:

#include <mutex>

std::mutex mutex;

The method of using mutex lock is usually as follows:

// 线程1
mutex.lock();
// 访问共享资源
mutex.unlock();

// 线程2
mutex.lock();
// 访问共享资源
mutex.unlock();

It should be noted that when using mutex lock You should always use lock and unlock, otherwise a deadlock may occur. In addition, you should avoid occupying the lock for a long time to avoid affecting the execution of other threads.

Reader-Writer Lock

Read-Writer Lock is a special lock that allows multiple threads to read shared resources at the same time, but only allows one thread to write to the share resource. When a thread writes to a shared resource, other threads cannot read or write until the write operation is completed, and the reading thread cannot continue reading. The definition of read-write lock is as follows:

#include <shared_mutex>

std::shared_mutex rw_mutex;

The method of using read-write lock is as follows:

// 写入线程
rw_mutex.lock();
// 写入共享资源
rw_mutex.unlock();

// 读取线程
rw_mutex.lock_shared();
// 读取共享资源
rw_mutex.unlock_shared();

It should be noted that when using read-write lock, priority should be given to threads reading shared resources to avoid Write operations occupy the lock for a long time.

Condition Variable

Condition variables are usually used for synchronization and communication between threads. It is a synchronization mechanism based on mutex locks that can achieve coordination between threads through waiting and notification mechanisms.

The definition of condition variables is as follows:

#include <condition_variable>

std::condition_variable cond_var;

Condition variables are usually used together with mutex locks. The waiting and waking up process are as follows:

// 线程1
std::unique_lock<std::mutex> lock(mutex);
cond_var.wait(lock);
// 唤醒后执行的代码

// 线程2
std::unique_lock<std::mutex> lock(mutex);
// 执行唤醒操作
cond_var.notify_one();

It should be noted that when using conditions When using variables, they must be locked first, otherwise a deadlock may occur.

Summary

In multi-threaded programming, locks are an essential synchronization mechanism. When selecting a lock, you should consider the characteristics and usage scenarios of different locks to better ensure data security and thread coordination. Remember the basic principles of using locks: granular control and avoiding deadlocks.

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