Question

Hey – I’m having an odd problem with a little toy program I’ve written, to try out threads.
This is my code:

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

using std::cout;
using std::endl;

void *threadFunc(void *arg) {
    cout << "I am a thread. Hear me roar." << endl;

    pthread_exit(NULL);
}

int main() {
    cout << "Hello there." << endl;
    int returnValue;
    pthread_t myThread;

    returnValue = pthread_create(&myThread, NULL, threadFunc, NULL);

    if (returnValue != 0) {
        cout << "Couldn't create thread! Whoops." << endl;
        return -1;
    }

    return 0;
}

With the first cout in main not commented out, the thread prints fine.
However, without it, the thread doesn’t print anything at all.

Any help?

Answer 1

Try this:

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

using std::cout;
using std::endl;

void *threadFunc(void *arg) {
    cout << "I am a thread. Hear me roar." << endl;

    pthread_exit(NULL);
}

int main() {
    //cout << "Hello there." << endl;
    int returnValue;
    pthread_t myThread;

    returnValue = pthread_create(&myThread, NULL, threadFunc, NULL);

    if (returnValue != 0) {
        cout << "Couldn't create thread! Whoops." << endl;
        return -1;
    }

    pthread_join( myThread, NULL);

    return 0;
}

The difference between my code and yours is one line – pthread join. This suspends the main thread until the sub-thread has had chance to complete its actions.

In your code, execution reaches the first cout and it’s processed. Then, you split off another thread and the main thread carries on until the end, which may or may not be reached before the secondary thread is tidied up. That’s where the odd behaviour comes in – what you are experiencing is the case where the main program finishes before the sub-thread has had a chance to, so the program has “returned” and the whole lot is cleaned up by the kernel.