c++、pthread 和静态回调。 "this" returns 指向派生的基础 class inctead 的指针（第 2 部分）

Question

此线程已启动 here 但由于缺少一个完全好的示例（并且为了避免删除所有该问题），它在此处重写。

因此，在下面的示例中，void cppthread::ThreadedFunc() 被派生为作为单独的线程执行。相反，我更喜欢 void ThreadedWrite::ThreadedFunc() 被执行。我怎样才能做到这一点？ （代码后面有更多细节）

cppthread.hpp

#ifndef CPPTHREAD_HPP
#define CPPTHREAD_HPP

#include <pthread.h>

using namespace std;

class cppthread
{
    public:
        cppthread();
        virtual ~cppthread();

        virtual void threadedFunc();
        ///parentObj (ie "this" pte from caller") is 
        ///necessary in order to execute the correct
        ///threadedFunc() even when the derived class
        ///wants to spawn a thread.
        int spawn(void *parentObj = NULL);
        void terminate();

    protected:
        pthread_mutex_t mtx;
        bool exitThread;

    private:
        /* add your private declarations */
        int join();

        pthread_t threadId;
};

#endif /* CPPTHREAD_HPP */ 

cppthread.cpp

#include <stdio.h>
#include <errno.h>
#include <stdlib.h>

#include "cppthread.hpp"

void* threadCallback(void* obj);

cppthread::cppthread()
{
    exitThread = false;
    pthread_mutex_init(&mtx, NULL);

}


cppthread::~cppthread()
{
    if (!exitThread)
        terminate();
    pthread_mutex_destroy(&mtx);
}

void cppthread::threadedFunc()
{
    while ( !exitThread )
    {
        printf("Hello from cppthread::threadfunc. This should not be run once derived and redefined.\n");
    }
    if (exitThread)
    {
        printf("graceful exit from cppthread::threadfunc. This should not be run once derived and redefined.\n");
    }
    pthread_exit((void*)0);
}

int cppthread::spawn(void* parentObj)
{
    int ret;
    printf("parentObj = %p\n", parentObj);
    if (parentObj == NULL)
    {
        ret = pthread_create(&threadId, 0, &threadCallback, this);
        printf("cppthread_create with \"this\" \n");
    }
    else
    {
        ret = pthread_create(&threadId, 0, &threadCallback, parentObj);
        printf("cppthread_create with parentObj\n");
    }

    if (ret != 0)
    {
        printf("cppthread_create error\n");
        exit(EXIT_FAILURE);
    }
    else
    {
        //printf("cppthread::threadID= %lu\n",threadId);
    }
    return ret;
}

void cppthread::terminate()
{
    exitThread = true;
    join();
}

int cppthread::join()
{
    int status , ret;
    //printf("cppthread::join_threadID= %lu\n",threadId);
    ret = pthread_join(threadId,(void**)&status);
    if (ret != 0)
    {
        printf("cppthread_join error: ");
        switch (ret)
        {
            case EDEADLK: printf("deadlock\n"); break;
            case EINVAL: printf("thread not joinable\n"); break;
            case ESRCH: printf("threadID not found\n"); break;
            default : printf("unknown error\n"); break;
        }
    }
    return status;
}

//----------------------------------------------------------------------
void* threadCallback(void* obj)
{
    static_cast<cppthread*>(obj)->threadedFunc();
    return(0);
} // callback

threadedwrite.hpp

#ifndef THREADEDWRITE_HPP
#define THREADEDWRITE_HPP

#include "cppthread.hpp"

using namespace std;

class ThreadedWrite : public cppthread
{
    public:
        ThreadedWrite(ThreadedWrite* mySelf);
        virtual ~ThreadedWrite();

        void threadedFunc();
        void rrdupdate_thread();

        ///inherited significant members: from cppthread
        ///   int spawn();
        ///   void terminate();
        ///protected
        ///   pthread_mutex_t mtx;
        ///   bool exitThread;

    private:
        ThreadedWrite* instancePtr;
};

#endif /* THREADEDWRITE_HPP */ 

threadedwrite.cpp

#include <iostream>
#include "threadedwrite.hpp"


ThreadedWrite::ThreadedWrite(ThreadedWrite* mySelf):instancePtr(mySelf)
{
    cout << "instancePtr = " << instancePtr << endl;
}

ThreadedWrite::~ThreadedWrite()
{

}

void ThreadedWrite::threadedFunc()
{
    if ( !exitThread )
    {
        cout << "this is the ThreadedWrite::threadedFunc() running!" << endl;
    }
    else
    {
        cout << "ThreadedWrite::threadedFunc must exist now" << endl;
    }

    pthread_exit((void*)0);
}

void ThreadedWrite::rrdupdate_thread()
{
    cout << "about to spawn" << endl;
    spawn(instancePtr);
}

main.cpp

#include <iostream>

#include "threadedwrite.hpp"

using namespace std;

//-------main body------------------------------------------------------
int main(int argc, char* argv[])
{
    ThreadedWrite  thrrdupd(&thrrdupd);
    cout << "hello from main 1 " << &thrrdupd << endl;
    thrrdupd.rrdupdate_thread();
    cout << "hello from main 2 " << &thrrdupd << endl;
    return 0;
}

以上产生输出（对我来说）：

instancePtr = 0x7fff39d17860
hello from main 1 0x7fff39d17860
about to spawn
parentObj = 0x7fff39d17860
cppthread_create with parentObj
hello from main 2 0x7fff39d17860
graceful exit from cppthread::threadfunc. This should not be run once derived and redefined.

因此，从派生的 class "ThreadedWrite" 中执行上述 cppthread::spawn() 调用，实际上提供了指向 callback() 函数的 "this" 指针，该函数指向 cppthread::ThreadedFunc()，而不是 ThreadedWrite::ThreadedFunc()。

您还可以看到我尝试（通过 "instancePtr" 基础结构）将指向 "ThreadedWrite" 实例的指针传递回回调函数。但这也失败了。

此外，最好我希望 cppthread class 尽可能通用，以便能够在可能的情况下使用它。

请注意，如果我从 threadedwrite.cpp 中删除 "spawn(instancePtr)"，并像这样从 main.cpp 中调用 spawn

int main(int argc, char* argv[])
{
        ThreadedWrite  thrrdupd(&thrrdupd);
        cout << "hello from main 1 " << &thrrdupd << endl;
        thrrdupd.rrdupdate_thread();
        thrrdupd.spawn();
        cout << "hello from main 2 " << &thrrdupd << endl;
        return 0;
}

我得到的输出是预期的（和想要的）输出，它看起来像这样：

instancePtr = 0x7ffd24b04ed0
hello from main 1 0x7ffd24b04ed0
about to spawn
parentObj = (nil)
cppthread_create with "this" 
hello from main 2 0x7ffd24b04ed0
this is the ThreadedWrite::threadedFunc() running!

Answer 1

您需要等待线程终止，然后才能从 main return，因为这会破坏您的对象。

否则你有竞争条件：

1. 线程已启动。
2. thrrdupd 在您离开时开始被破坏 main。
3. ~ThreadedWrite 运行；此时对象不再是 ThreadedWrite 而是 cppthread.
4. ~cppthread 运行并等待线程。
5. 线程调用回调，因为对象现在具有动态类型 cppthread，cppthread::threadedFunc 被调用。

5. 有可能发生在 3. 之前，在这种情况下您将获得预期的输出。

如果您确保等待线程在第 3 步完成，那么它将正常工作。也许你可以在 ~ThreadedWrite?

中调用 terminate