C ++对线程感到困惑

Question

Basicly this is what I have:

Server::
Server (int port) {
    cout << "Initializing server.\n";

    (...)       

    pthread_t newthread;
    pthread_create(&newthread, NULL, &Server::_startListening, NULL);

    cout << "Exit\n";
    pthread_exit(NULL); // <-- Question
}

void* Server::_startListening (void* param) {
cout << "Start listening for clients ...\n";
return 0;
}

Question: If I don't put pthread_exit(NULL); in the code, it will work when I compile it on Linux (Ubuntu) but it won't work on Mac OSX 10.6.2. When I compile and run it on linux it will say Initializing server, Start listening for clients, Exit while on Mac OSX it will say Initializing for server, Exit, Start listening for clients.

The problem seems to occur around the pthread_exit, if I place it above the cout << Exit. That message will never be displayed (how weird is that).

Am I doing something wrong?

Answer 1

您可能打算使用pthread_join而不是退出。

Answer 2

The problem seems to occur around the pthread_exit, if I place it above the cout << Exit. That message will never be displayed (how weird is that).

Not weird at all. When you call pthread_exit, the current thread stop executing. Since it's only in that thread that you are going to display "exit", once that thread is gone there is nothing left to print it.

In regards to the different order of the print of "exit" and "start listening". Each of those are being printed by a separate thread. Because you have no synchronization, either can be printed first.

Answer 3

The ordering of your output is indeterminate. That's what concurrency means! The other two answers are right: pthread_exit before your output kills the main thread and you probably intend to join before the main exit occurs.

Answer 4

Threading (scheduling, etc) is OS dependent. It looks like on Linux, your OS switches context to your new thread before continuing in the main thread, while on Mac OS, it will continue executing the current thread before switching context. Because this is OS dependent, unless you do some kind of synchronization, there's no reliable way of telling which line of two threads will execute first, and even your tested 'linux will switch contexts' results are unreliable.

pthread_exit, as mentioned above, exits the active current thread (ie: main()), not any other threads (_startListning). You're probably looking for joining (pthread_join) the other thread, not exiting the current.

Answer 5

pthread_exit is supposed to be called from the thread you want to stop. Calling it from the main() thread will terminate the main thread, and your program will run until the listener threads have exited.

Recommend you read the manpages on pthread_exit and pthread_join .

Answer 6

我认为您的问题已经在上面回答了，但是您可能还希望研究Boost线程库，如果您使用Windows，它会为您提供更轻松的跨平台支持，以及一个不错的面向对象的界面。