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Date: Mon, 13 Nov 2023 17:21:02 +0100
To: Bruno Haible <bruno AT clisp DOT org>
Subject: Re: rand is not ISO C compliant in Cygwin
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From: Corinna Vinschen via Cygwin <cygwin AT cygwin DOT com>
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On Nov 13 15:38, Corinna Vinschen wrote:
> On Nov 13 15:25, Bruno Haible wrote:
> > Corinna Vinschen wrote:
> > > The rand() function would still not use locking but AFAICS that's
> > > not actually required by POSIX or ISO C.
> > 
> > Correct. Those who want an MT-safe rand-like function need to use random(),
> > not rand().

I took a look into POSIX and I'm a bit puzzled now.  From
https://pubs.opengroup.org/onlinepubs/9699919799/functions/rand.html

RATIONAL

  The ISO C standard rand() and srand() functions allow per-process
                                                  ^^^^^ (not requires)

  pseudo-random streams shared by all threads. Those two functions need
  not change, but there has to be mutual-exclusion that prevents
  interference between two threads concurrently accessing the random
  number generator.

Ok, so, *iff* rand/srand share per-process state, then they have to
use locking to prevent MT interference.  POSIX continues:

  With regard to rand(), there are two different behaviors that may be
  wanted in a multi-threaded program:

  1. A single per-process sequence of pseudo-random numbers that is
     shared by all threads that call rand()

  2. A different sequence of pseudo-random numbers for each thread that
     calls rand()
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

I read this as the newlib technique being one way of correctly
implementing rand/srand, no?


Corinna


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