fortran 95 Initialize a pseudo-random number sequence

! from https://gcc.gnu.org/onlinedocs/gfortran/RANDOM_005fSEED.html#RANDOM_005fSEED subroutine init_random_seed() use iso_fortran_env, only: int64 implicit none integer, allocatable :: seed(:) integer :: i, n, un, istat, dt(8), pid integer(int64) :: t 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", action="read", status="old", 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(t) if (t == 0) then call date_and_time(values=dt) t = (dt(1) - 1970) * 365_int64 * 24 * 60 * 60 * 1000 & + dt(2) * 31_int64 * 24 * 60 * 60 * 1000 & + dt(3) * 24_int64 * 60 * 60 * 1000 & + dt(5) * 60 * 60 * 1000 & + dt(6) * 60 * 1000 + dt(7) * 1000 & + dt(8) end if pid = getpid() t = ieor(t, int(pid, kind(t))) do i = 1, n seed(i) = lcg(t) end do end if call random_seed(put=seed) contains ! This simple PRNG might not be good enough for real work, but is ! sufficient for seeding a better PRNG. function lcg(s) integer :: lcg integer(int64) :: s if (s == 0) then s = 104729 else s = mod(s, 4294967296_int64) end if s = mod(s * 279470273_int64, 4294967291_int64) lcg = int(mod(s, int(huge(0), int64)), kind(0)) end function lcg end subroutine init_random_seed