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! Copyright (C) 2018-2024 Free Software Foundation, Inc.
! Contributed by Steven G. Kargl <>
! This file is part of the GNU Fortran runtime library (libgfortran).
! Libgfortran is free software; you can redistribute it and/or
! modify it under the terms of the GNU General Public
! License as published by the Free Software Foundation; either
! version 3 of the License, or (at your option) any later version.
! Libgfortran is distributed in the hope that it will be useful,
! but WITHOUT ANY WARRANTY; without even the implied warranty of
! GNU General Public License for more details.
! Under Section 7 of GPL version 3, you are granted additional
! permissions described in the GCC Runtime Library Exception, version
! 3.1, as published by the Free Software Foundation.
! You should have received a copy of the GNU General Public License and
! a copy of the GCC Runtime Library Exception along with this program;
! see the files COPYING3 and COPYING.RUNTIME respectively. If not, see
! <>.
! WARNING: This file should never be compiled with an option that changes
! default logical kind from 4 to some other value or changes default integer
! kind from 4 to some other value.
! There are four combinations of repeatable and image_distinct. The
! language below is from the F2018 standard (actually, J3/18-007r1).
! This routine is only used for non-coarray programs or with programs
! compiled with -fcoarray=single. Use of -fcoarray=lib or -fcoarray=shared
! requires different routines due to the need for communication between
! images under case(iv).
! Technically, neither image_distinct nor image_num are now needed. The
! interface to _gfortran_random_init() is maintained for libgfortran ABI.
! Note, the Fortran standard requires the image_distinct argument, so
! it will always have a valid value, and the frontend generates an value
! of 0 for image_num.
impure subroutine _gfortran_random_init(repeatable, image_distinct, image_num)
implicit none
logical, value, intent(in) :: repeatable
logical, value, intent(in) :: image_distinct
integer, value, intent(in) :: image_num
logical, save :: once = .true.
integer :: nseed, lcg_seed
integer, save, allocatable :: seed(:)
if (repeatable) then
if (once) then
once = .false.
call random_seed(size=nseed)
lcg_seed = 57911963
call _gfortran_lcg(seed)
end if
call random_seed(put=seed)
call random_seed()
! This cannot happen; but, prevent gfortran complaining about
! unused variables.
if (image_num > 2) then
use iso_fortran_env, only : error_unit
write(error_unit, '(A)') 'whoops: random_init(.false., .false.)'
if (image_distinct) error stop image_num + 1
error stop image_num
end block
end if
end if
! SK Park and KW Miller, ``Random number generators: good ones are hard
! to find,'' Comm. ACM, 31(10), 1192--1201, (1988).
! Implementation of a prime modulus multiplicative linear congruential
! generator, which avoids overflow and provides the full period.
impure elemental subroutine _gfortran_lcg(i)
implicit none
integer, intent(out) :: i
integer, parameter :: a = 16807 ! Multiplier
integer, parameter :: m = huge(a) ! Modulus
integer, parameter :: q = 127773 ! Quotient to avoid overflow
integer, parameter :: r = 2836 ! Remainder to avoid overflow
lcg_seed = a * mod(lcg_seed, q) - r * (lcg_seed / q)
if (lcg_seed <= 0) lcg_seed = lcg_seed + m
i = lcg_seed
end subroutine _gfortran_lcg
end subroutine _gfortran_random_init