pthread_join() logical error using pthreads in c++

Question

I'm learning pthreads in c++. I've stucked at some point and searched on the web, however the same sentence exists in all pages, however not any explanation about this sentence. I couldn't understand the below sentence.

A joining thread can match one pthread_join() call. It is a logical error to attempt multiple joins on the same thread.

It is from this link : http://www.bogotobogo.com/cplusplus/multithreading_pthread.php

Could you explain what this sentence mean? Could you explain what the logical error is by an example?

What is "joining thread"? Say that, Main thread creates a child thread, main thread waits for child thread to complete its job. OK. in this case, which one is the joining thread , main thread or child thread?

Once a thread has been joined, it doesn't exist any more, so it would clearly be an error of trying to join the (now non-existing) thread again. — Some programmer dude
– Some programmer dude, Commented Mar 1, 2015 at 16:40

P.P · Accepted Answer · 2015-03-01 16:39:17Z

1

For example, if main() thread joins on a thread twice.

 pthread_t t1;

 pthread_create(&t1, ....);
 pthread_join(t1, NULL); 
 pthread_join(t1, NULL); /* The quoted sentence refers to cases like this */

The same is applicable for any other thread.

answered Mar 1, 2015 at 16:39

P.P

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8 Comments

oiyio Over a year ago

Ok. If more than one thread tries to join the same thread, what happens, is it ok?

P.P Over a year ago

It's undefined behaviour to attempt to join more than once with the same thread. If you do so, the program is invalid henceforth.

Lightness Races in Orbit Over a year ago

@oiyio: You're the one who quoted, in the question, that it's a logical error. That's what your whole question is about. So, no, it's not ok!!

P.P Over a year ago

Yes, but it's not "more than one thread" but "more than once" by difference threads also. E.g. main() joins with thread1 and thread2 joins with thread1. This is invalid too.

P.P Over a year ago

It's also thread1. Remember it's not necessary to call pthread_join() on every thread. It's just an example.

|

Lightness Races in Orbit · Accepted Answer · 2015-03-01 16:40:31Z

1

It's simple: you may only join a thread once.

                   main        worker 1
                     |
                     |
  pthread_create     +----------->+
                     |            |
                     |            |
  pthread_join       ⁞            |
        |            ⁞            | (done now)
        ┴            +<-----------+
                     |
                     |
                    (etc).

answered Mar 1, 2015 at 16:40

Lightness Races in Orbit

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2 Comments

oiyio Over a year ago

Say that, Main thread creates thread1 and thread2. 1 - Does it possible "main thread and thread2 simultaneously join thread2"? 2 - Does it possible "thread1 join thread2", in other words can a thread be joined by a thread which is not its parent?

Lightness Races in Orbit Over a year ago

@oiyio: The answer to both is in the documentation.

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