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devel / comp.unix.programmer / Re: POSIX example for pthread_join()

SubjectAuthor
* POSIX example for pthread_join()Spiros Bousbouras
`* POSIX example for pthread_join()Scott Lurndal
 `- POSIX example for pthread_join()Richard Kettlewell

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POSIX example for pthread_join()

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From: spi...@gmail.com (Spiros Bousbouras)
Newsgroups: comp.unix.programmer
Subject: POSIX example for pthread_join()
Date: Fri, 14 Jul 2023 11:05:52 -0000 (UTC)
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 by: Spiros Bousbouras - Fri, 14 Jul 2023 11:05 UTC

https://pubs.opengroup.org/onlinepubs/9699919799/functions/pthread_join.html :

((subarray *)arg)->ar[i]++;

This seems to me to be undefined behaviour since arr[] is not
initialised.

Re: POSIX example for pthread_join()

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 by: Scott Lurndal - Fri, 14 Jul 2023 15:23 UTC

Spiros Bousbouras <spibou@gmail.com> writes:
>https://pubs.opengroup.org/onlinepubs/9699919799/functions/pthread_join.html :
>
> ((subarray *)arg)->ar[i]++;
>
>This seems to me to be undefined behaviour since arr[] is not
>initialised.

it's true that arr[] is not initialized. ar[] is, however.

int ar[1000000];
....
sb1.ar = &ar[0];
sb1.n = 500000;
(void) pthread_create(&th1, NULL, incer, &sb1);

Re: POSIX example for pthread_join()

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From: inva...@invalid.invalid (Richard Kettlewell)
Newsgroups: comp.unix.programmer
Subject: Re: POSIX example for pthread_join()
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 by: Richard Kettlewell - Fri, 14 Jul 2023 16:25 UTC

scott@slp53.sl.home (Scott Lurndal) writes:
> Spiros Bousbouras <spibou@gmail.com> writes:
>>https://pubs.opengroup.org/onlinepubs/9699919799/functions/pthread_join.html :
>>
>> ((subarray *)arg)->ar[i]++;
>>
>>This seems to me to be undefined behaviour since arr[] is not
>>initialised.
>
> it's true that arr[] is not initialized. ar[] is, however.
>
>
> int ar[1000000];
> ...
> sb1.ar = &ar[0];
> sb1.n = 500000;
> (void) pthread_create(&th1, NULL, incer, &sb1);

There is no arr, but that’s just a typo.

I agree with Spiros. sb1.ar and sb2.ar are initialized; they point to
ar[0] and ar[500000] respectively. The contents of the ar array on
main’s stack are not initialized, so both threads will do an
uninitialized read on every iteration.

If the example was simpler it would be more likely to be correct l-)

--
https://www.greenend.org.uk/rjk/

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