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Date: Tue, 14 Nov 2023 11:20:24 +0100
To: Glenn Strauss <gs-cygwin.com@gluelogic.com>
Subject: Re: rand is not ISO C compliant in Cygwin
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From: Corinna Vinschen via Cygwin <cygwin@cygwin.com>
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Cc: Corinna Vinschen <vinschen@redhat.com>, Bruno Haible <bruno@clisp.org>,
        newlib@sourceware.org, cygwin@cygwin.com
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On Nov 13 17:14, Glenn Strauss wrote:
> On Mon, Nov 13, 2023 at 10:33:48PM +0100, Bruno Haible via Cygwin wrote:
> > POSIX does not have these two sentences, but instead has:
> > 
> >   "The rand() function need not be thread-safe."
> 
> I read the above as requiring *reentrancy*, but not *thread-safety*.
> 
> If multiple threads are accessing rand() and rand() accesses global
> state, the global state should not get corrupted; the sequence
> generated by rand() should remain intact.  Since thread-safety is not
> guaranteed, is it theoretically possible that multiple threads enter
> rand() at nearly the same time and both get the *same* random value in
> the sequence.  (Whether or not that is undesirable behavior is
> application-specific.)  A mutex can avoid this theoretical duplication,
> as can using thread-local state (with difference seeds) instead of
> global state.  If the seed value is the same in multiple threads using
> thread-local state, the sequence of random values generated in each
> thread will be repeated in each thread.  This may be surprising behavior
> to some when srand() is called, then multiple threads spawned, and each
> thread subsequently gets the same sequence of values from rand().

That's a good common sense argument for changing rand() to use a global
state.


Corinna


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