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 -- Problem reports: https://cygwin.com/problems.html FAQ: https://cygwin.com/faq/ Documentation: https://cygwin.com/docs.html Unsubscribe info: https://cygwin.com/ml/#unsubscribe-simple
