Re: rand is not ISO C compliant in Cygwin

[Corinna Vinschen <vinschen@redhat.com>]

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