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From: Ross Johnson <>
To: Brian Cole <>
Subject: Re: When is the pthread_key_create destructor called?
Date: Sun, 23 Mar 2008 01:51:00 -0000	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <>


Brian Cole wrote:
> Is it safe to assume that when a thread exits its destructor functions
> for thread specific values have finished executing before the thread
> can be joined?
Yes, pthread_join() waits and continues only after all destructors have 
returned. As described by the manual page and the standard 
the library rechecks all keys and their values in case a specific 
destructor needs to be called again. A key will be checked at most 
PTHREAD_DESTRUCTOR_ITERATIONS  (= 4, IIRC) times and if the key value is 
not null by that time the key is forcibly deleted.

> For example, is the following free of race conditions?
> static pthread_key_t key;
> void dtor(void *val)
> {
>   // safely push the data pointed to by val into some global data structure
> }
> void *thread_func(void *)
> {
>   pthread_setspecific(key, //pointer to some data that will be pushed
> into a global data structure on exit);
>   pthread_exit(NULL); // What happens if I don't call this and let it
> run off the end of the function?
> }
You can let your thread routine run off the end - pthread_exit() is is 
called implicitly in that case.
> int main()
> {
>   pthread_key_create(key, dtor);
>   pthread_t thrd;
>   pthread_create(&thrd, NULL, thread_func, NULL);
>   void *ret;
>   pthread_join(thrd, &ret);
>   // safe to assume data that was in thread local storage is all in my
> global data structure?
> }
It is safe to assume your destructors have been called and returned.
> If this is not the case, would adding a pthread_key_delete achieve the
> desired behavior?
No, pthread_key_delete() removes the key for ALL threads, but does not 
call the destructor for ANY threads. If you think of TSD keys as a two 
dimensional array, with threads across and keys down and each cell as a 
TSD association between a key and a thread. Exiting a thread removes a 
column of TSD associations, deleting a key removes a row of TSD 
>  And further, how cross-platform is this behavior
> seeing as I can't find it explicitly stated in any pthreads
> documentation.
This is Single Unix Specification (nee POSIX) behaviour. The manual 
pages for each of these routines pretty much reflects the standard 
descriptions. See, for example:-
(In the left frame, select the "System Interfaces" volume and then 
section 3 "System Interfaces")


> To further complicate the matter my threads are actually OpenMP
> threads. Is it safe to assume they will properly clean up thread
> specific data in the same semantic fashion?
I'm not familiar with OpenMP.
> Thanks,
> Brian


      reply	other threads:[~2008-03-23  1:51 UTC|newest]

Thread overview: 2+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2008-03-22 15:13 Brian Cole
2008-03-23  1:51 ` Ross Johnson [this message]

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