c++ - pthread_mutex_t 作为类成员导致死锁

Question

我是并行计算的新手，所以我正在研究 pthread 和互斥量。运行以下代码会导致死锁。谁能解释我为什么，以及如何以正确的方式做到这一点？我的目的是保护成员变量不被交叉写入。非常感谢!

#include <pthread.h>
#include <iostream>

class mutex_class
{

    public:
        mutex_class();
        void print_stars(int count);
        void print_dots(int count);

    private:
        pthread_mutex_t mutex;

};

mutex_class::mutex_class()
{
    pthread_mutex_t mutex;
    pthread_mutex_init (&mutex, NULL);
}

void mutex_class::print_stars(int count)
{
    pthread_mutex_lock(&this->mutex);
    for(int i=0; i<count; i++)
    {
        std::cout << "*" << std::flush;
    }
    pthread_mutex_unlock(&this->mutex);
}

void mutex_class::print_dots(int count)
{
    pthread_mutex_lock(&this->mutex);
    for(int i=0; i<count; i++)
    {
        std::cout << "." << std::flush;
    }
    pthread_mutex_unlock(&this->mutex);
}

void* call_dots(void* m)
{
    (*(mutex_class*)m).print_dots(20);
}

void* call_stars(void* m)
{
    (*(mutex_class*)m).print_stars(20);
}

int main()
{
    mutex_class m1 = mutex_class();
    pthread_t t1, t2;
    pthread_create(&t1, NULL, call_dots, (void*)&m1);
    pthread_create(&t2, NULL, call_stars, (void*)&m1);
    pthread_join(t1, NULL);
    pthread_join(t2, NULL);
    return(0);
}

Answer 1

问题出在这里:

mutex_class::mutex_class()
{
    pthread_mutex_t mutex;             // <<< remove this line
    pthread_mutex_init (&mutex, NULL);
}

您正在初始化一个本地互斥量，并使成员变量未初始化。

除非您出于某种原因被困在过去，否则请考虑使用标准 C++ 线程库；与使用 C 风格的 API 相比，它更便携，更不容易出错。这是您使用更 C++ 习惯用法的程序:

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

class mutex_class
{
    public:
        // No need for a special constructor

        void print_stars(int count);
        void print_dots(int count);

    private:
        std::mutex mutex;
};

void mutex_class::print_stars(int count)
{
    std::lock_guard<std::mutex> lock(mutex);  // unlocks automatically
    for(int i=0; i<count; i++)
    {
        std::cout << "*" << std::flush;
    }
}

void mutex_class::print_dots(int count)
{
    std::lock_guard<std::mutex> lock(mutex);  // unlocks automatically
    for(int i=0; i<count; i++)
    {
        std::cout << "." << std::flush;
    }
}

// No need to muck around with C-style 'void*' functions

int main()
{
    mutex_class m1;
    std::thread t1([&]{m1.print_dots(20);});
    std::thread t2([&]{m1.print_stars(20);});
    t1.join();
    t2.join();
}