How to fix the problem if a mutex is a local…

A mutex declared as a local variable within a function will be destroyed when exiting that function, which can lead to a race condition or undefined behavior when attempting to access the protected resource from other threads.

To fix this issue, it is necessary to ensure the mutex's lifecycle covers the entire period during which all threads access the protected resource. Possible solutions:

Declare the mutex globally or statically:

A global mutex is accessible from any part of the program.
A static mutex inside a function or class retains its state throughout the program's lifetime.

#include <mutex>
#include <thread>
#include <iostream>

// Global mutex
std::mutex global_mutex;
int shared_resource = 0;

void increment_global() {
    global_mutex.lock();
    shared_resource++;
    global_mutex.unlock();
}

class Example {
public:
    void increment_static() {
        // Static mutex inside class
        static std::mutex class_static_mutex;
        class_static_mutex.lock();
        shared_resource++; // Access to shared resource (for example)
        class_static_mutex.unlock();
    }
};

void increment_static_func() {
    // Static mutex inside function
    static std::mutex func_static_mutex;
    func_static_mutex.lock();
    shared_resource++; // Access to shared resource (for example)
    func_static_mutex.unlock();
}

Declare the mutex as a class or struct member:

If the resource is a member of a class/struct, then the mutex protecting it should also be a member of the same class/struct. This ensures encapsulation.

#include <mutex>
#include <thread>

class ProtectedResource {
public:
    void access_resource() {
        lock_.lock();
        // Access to the protected resource
        // ...
        lock_.unlock();
    }

private:
    std::mutex lock_; // Mutex as a class member
    // Resource to be protected
    // int data;
};

Use smart pointers or RAII wrappers to manage the mutex's lifecycle:
- In more complex scenarios where the resource and its mutex are created dynamically, std::shared_ptr or std::unique_ptr can be used to manage their lifetime.
- RAII (Resource Acquisition Is Initialization) approach, such as with std::lock_guard or std::unique_lock, guarantees mutex release upon leaving the scope, but does not solve the problem of a local mutex that disappears itself. However, RAII is crucial for correct mutex handling when declared with the proper lifetime.

When choosing a solution, consider the scope of the resource and the threads that will access it. The most preferred approach in object-oriented design is to declare the mutex as a member of the class protecting specific data of that class.

How to fix the problem if a mutex is a local variable?