From: Norton Allen <allen@huarp.harvard.edu>
To: Bruno Haible <bruno@clisp.org>, cygwin@cygwin.com
Subject: Re: rand is not ISO C compliant in Cygwin
Date: Fri, 10 Nov 2023 16:39:42 -0500 [thread overview]
Message-ID: <e099714b-9820-4647-ba40-9db5e471e9aa@huarp.harvard.edu> (raw)
In-Reply-To: <9938355.c9vzh5UkMf@nimes>
On 11/10/2023 3:19 PM, Bruno Haible via Cygwin wrote:
> ISO C 23 § 7.24.2.1 and 7.24.2.2 document how rand() and srand() are
> expected to behave. In particular:
> "The srand function uses the argument as a seed for a new sequence
> of pseudo-random numbers to be returned by subsequent calls to rand.
> If srand is then called with the same seed value, the sequence of
> pseudo-random numbers shall be repeated. ...
> The srand function is not required to avoid data races with other
> calls to pseudo-random sequence generation functions. ..."
>
> The two attached programs call srand() in one thread and then rand()
> in another thread. There is no data race, since the second thread
> is only created after the call to srand() has returned. The behaviour
> in Cygwin is that the values in the second thread ignore the srand()
> call done in the first thread.
Since the standard is trying to be precise, this reads to me as though
Cygwin/(newlib?) has chosen to avoid race conditions by making
pseudo-random sequences in different threads independent. Although the
standard does not require this, it does not prohibit it either.
>
> How to reproduce the bug:
>
> $ x86_64-pc-cygwin-gcc -Wall rand-in-posix-thread.c
> $ ./a
>
> or
>
> $ x86_64-pc-cygwin-gcc -Wall rand-in-isoc-thread.c
> $ ./a
>
> Expected output:
>
> Value from main thread: 1583559764
> Value from separate thread: 1583559764
>
> Actual output:
>
> Value from main thread: 1583559764
> Value from separate thread: 1481765933
>
>
next prev parent reply other threads:[~2023-11-10 21:39 UTC|newest]
Thread overview: 18+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-11-10 20:19 Bruno Haible
2023-11-10 21:39 ` Norton Allen [this message]
2023-11-10 22:27 ` Bruno Haible
2023-11-11 16:50 ` Allen, Norton T.
2023-11-11 18:25 ` René Berber
2023-11-11 20:18 ` Allen, Norton T.
2023-11-13 14:17 ` Corinna Vinschen
2023-11-13 14:25 ` Bruno Haible
2023-11-13 14:38 ` Corinna Vinschen
2023-11-13 16:21 ` Corinna Vinschen
2023-11-13 16:44 ` [EXTERNAL] " Lavrentiev, Anton (NIH/NLM/NCBI) [C]
2023-11-13 21:33 ` Bruno Haible
2023-11-13 22:14 ` Glenn Strauss
2023-11-14 10:20 ` Corinna Vinschen
2023-11-14 10:11 ` Corinna Vinschen
2023-11-14 11:52 ` Bruno Haible
2023-11-14 16:59 ` Corinna Vinschen
2023-11-14 18:06 ` Bruno Haible
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