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From: "sgk at troutmask dot apl.washington.edu" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug libfortran/99740] floating point exception in rand() in gfortran
Date: Thu, 25 Mar 2021 17:17:46 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D99740

--- Comment #4 from Steve Kargl <sgk at troutmask dot apl.washington.edu> -=
--
On Thu, Mar 25, 2021 at 12:52:53PM +0000, pvoytas at gmail dot com wrote:
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D99740
>=20
> --- Comment #3 from Paul A. Voytas <pvoytas at gmail dot com> ---
> I see what you mean--if i test for rand(0)=3D0.d0 I do get hist with gfor=
tran on
> the EL7 machine.=20
>=20
> But it seems like there must still be something different from older vers=
ions.
> The info pages for rand() say "between 0 and 1" which I always took to be
> exclusive of the endpoints (of course there's machine precision). On Cent=
OS6
> with g77 when I run the above code I get no errors--even with 10x more
> attempts--and the test for rand(0)=3D0.d0 never is true. On that same Cen=
tOS6
> machine with gfortran, code does show rand(0)=3D0.d0 cases (and the -log(=
rand(0))
> returns +Infinity.
>=20

g77 has not been supported for nearly 1.5 decades (gcc-3.4.6
released on 6 Mar 2006).  gfortran tries to provide some level
of API compatibility with g77.  I believe there has never been
a commitment to provide bit-for-bit compatibility (especially
for something like rand()).

Having now looked at g77's rand() implementation, I see that it
is a wrapper around the libc routine by the same name rand(3).
The quality of implementation of libc's rand(3) certainly=20
varies across operating systems.  gfortran's rand() implements
the classic Park and Miller LCG RNG with an internal range of
[0, 2**31-1), which yields a random number in [0., 1.).

You really want to switch to using random_number().  The internal
generator used in it has an enormous period and very good
statistical quality, and random_number() is specified by the
actual Fortran standard.=