Order of running threads in pthreads

Question

In the following program, what are the possibilities for the ordering of threads? Assuming "function" will print thread id which is unique (since here we have only one process). I always get the order th1,th2!

#include <stdlib.h>
#include <stdio.h>
#include <pthread.h>
int main()
{
            pthread_t th1;
            pthread_t th2;
            pthread_create(&th1, NULL, function, NULL);
            pthread_create(&th2, NULL, function, NULL);
            pthread_join(th1, NULL);
            pthread_join(th2, NULL);
}

   return 0;
}

score 3 · Answer 1 · edited Jun 01 '18 at 01:09

The start tell the OS "start doing this". The joins say "wait until this is done".

Now you are telling the OS "do this" and "do this". The OS can pick any order. But most of the time it will just do it in the order you told it to.

It's like ordering two sammies at subways. 99% of the time you'll get them made in the same order you asked for them. But every blue moon, you won't. But you are still waiting for both of them before you pay :)

score 2 · Answer 2 · answered Jun 18 '13 at 05:24

2

I don't think there will be any specific ordering. Threads on the modern machines are executed in parallel and its not deterministic which thread's print statements will get executed first !

The only ordering constraint that can be assumed is that thread 2 will come before thread 1 because of second pthread_join

answered Jun 18 '13 at 05:24

voidMainReturn

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Thanks Tejas. I was expecting so but I only get one order which is th1,th2 and I'm wondering to know the reason! – Sara Jun 18 '13 at 05:26
1

No order. Most of the time th1 will do it's thing first, but there is no guarantee. – starmole Jun 18 '13 at 05:27
2

The `pthread_join()` calls do not force any ordering between the execution of thread 1 and thread 2. In this case, all they force is that both thread 1 and thread 2 must exit before the main thread exits. – caf Jun 18 '13 at 05:32

caf · Accepted Answer · 2013-06-18T05:33:29.993

2

The only ordering guarantees here are that pthread_join(th1, NULL); will not return until thread 1 has exited and pthread_join(th2, NULL); will not return until thread 2 has exited. Consequently, the main() function will not return (and the process will not exit) until both thread 1 and thread 2 have exited.

There is no ordering imposed between thread 1 and thread 2 - their execution can be arbitrarily interleaved.

edited Jun 18 '13 at 05:33

answered Jun 18 '13 at 05:27

caf

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Thanks @caf, so you mean the "only" possibility is th1,th2?! – Sara Jun 18 '13 at 05:28
@sweet: No. There is no ordering imposed between thread 1 and thread 2. – caf Jun 18 '13 at 05:34

score 1 · Answer 4 · answered Jun 18 '13 at 05:33

It is like a factory. Got two workers. One needs a bit of coffee and therefore starts a little later at the same task as the other one (well takes time for the kettle). Then that worker is firing on all cylinders but the coffee takes effect. Needs to use the loo. So the one that started earlier finished first but spent more time working. If only that worker had a bucket?!

Order of running threads in pthreads

4 Answers4

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