使用pthread_create时的奇怪输出

Question

Consider the next piece of code :

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

using namespace std;

pthread_t tid[3];

void* printMe(void* arg)
{
    pthread_t id = pthread_self();
    if(pthread_equal(id,tid[0]))
    {
        cout << "first thread's in function" << endl;
    }
    if(pthread_equal(id,tid[1]))
    {
        cout << "second thread's in function" << endl;
    }
    if(pthread_equal(id,tid[2]))
    {
        cout << "third thread's in function" << endl;
    }

}

int main() {

    int i = 0;
    int err;

    while (i < 3)
    {
        err = pthread_create(&(tid[i]), NULL, printMe, NULL);

        if (err != 0)
        {
            cout << "failed to create thread number " << i << strerror(i) << endl;
        }
        else
        {
            cout << "main() : creating thread number " << i << endl;
        }

        i++;
    }

    return 0;
}

I don't understand when does a thread invoke his function? Is that right as it's created? (while, simultaneously, the main thread keeps on creating the other threads?)

Moreover, i don't understand the output -

main() : creating thread number 0
main() : creating thread number 1
main() : creating thread number 2
first thread's in function
first thread's in function

First thread invoked his function twice while none of the other threads invoked theirs.

Then, i compiled again and got -

main() : creating thread number 0
first thread's in function
main() : creating thread number 1
second thread's in function
main() : creating thread number 2

Once again, what about the third thread?

Why " sometimes " threads don't get to invoke their functions?

Answer 1

您的main()在返回并终止程序之前没有加入线程，因此您是否看到任何给定线程的输出或多或少是随机的。

Answer 2

Your main function terminates before the run of the thread. You have to wait the end of all threads before returning from the main. See this question : How can I wait for any/all pthreads to complete?

Note : You flag it as C++, any reason to not use threads from C++11 or boost ?