weird output when using pthread_create

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?

score 2 · Answer 1 · answered Apr 20 '16 at 08:26

2

Your main() isn't joining the threads before returning and thereby terminating the program, so it's more or less random whether you see any given thread's output.

answered Apr 20 '16 at 08:26

Tony Delroy

102,968
15
177
252

score 2 · Accepted Answer · edited May 23 '17 at 11:45

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 ?

edited May 23 '17 at 11:45

Community

1
1

answered Apr 20 '16 at 08:30

Caduchon

4,574
4
26
67

Thank you! and its because i'm doing a school project in which we're not allowed to use boost. tough life – Alex Goft Apr 20 '16 at 08:32
*"any reason to not use threads from boost ?"* - Standard Library threads should be preferred for >= C++11-capable compilers.... – Tony Delroy Apr 20 '16 at 09:02
@TonyD : I agree with that, I'm restricted to gcc 4.1.2 at work, then I'm still thinking C++98. – Caduchon Apr 20 '16 at 09:14
@Caduchon: with the time I spend getting and rebuilding 3rd party packages someone once built with VS2008 and committed binaries for - so we can finish our move to VS2013 and 64 bit and use C++11 - I'd seriously consider settling for gcc 4.1.2 too. Cheers. – Tony Delroy Apr 20 '16 at 09:42

weird output when using pthread_create

2 Answers2