Hi,
the example we provide for the usage of the random_seed intrinsic
could be better. At least one user has already been tripped over by
the fact that on some targets the first call to system_clock returns
0, resulting in a poor seed. Below is an improved(?!) example program.
Ok for trunk/4.8/4.7? (Changelog + patch is of course trivial once
there is agreement on the code itself)
! Seed the random generator with some random input
module rseed_rand
contains
subroutine rseed()
implicit none
integer, allocatable :: seed(:)
integer :: i, n, un, istat, dt(8), pid, t(2), s
integer(8) :: count, tms
call random_seed(size = n)
allocate(seed(n))
! First try if the OS provides a random number generator
open(newunit=un, file="/dev/urandom", access="stream", &
form="unformatted", iostat=istat)
if (istat == 0) then
read(un) seed
close(un)
else
! Fallback to XOR:ing the current time and pid. The PID is
! useful in case one launches multiple instances of the same
! program in parallel.
call system_clock(count)
if (count /= 0) then
t = transfer(count, t)
else
call date_and_time(values=dt)
tms = (dt(1) - 1970) * 365_8 * 24 * 60 * 60 * 1000 &
+ dt(2) * 31_8 * 24 * 60 * 60 * 1000 &
+ dt(3) * 24 * 60 * 60 * 60 * 1000 &
+ dt(5) * 60 * 60 * 1000 &
+ dt(6) * 60 * 1000 + dt(7) * 1000 &
+ dt(8)
t = transfer(tms, t)
end if
s = ieor(t(1), t(2))
pid = getpid()
s = ieor(s, pid)
if (n >= 3) then
seed(1) = t(1)
seed(2) = t(2)
seed(3) = pid
if (n > 3) then
seed(4:) = s + 37 * (/ (i, i = 0, n - 4) /)
end if
else
seed = s + 37 * (/ (i, i = 0, n - 1 ) /)
end if
end if
call random_seed(put=seed)
end subroutine rseed
end module rseed_rand
--
Janne Blomqvist
