Improve RANDOM_SEED example.

2013-05-22  Janne Blomqvist  <jb@gcc.gnu.org>

	* intrinsic.texi (RANDOM_SEED): Improve example.

From-SVN: r199182

gcc/fortran/ChangeLog

+2013-05-22  Janne Blomqvist  <jb@gcc.gnu.org>
+
+	* intrinsic.texi (RANDOM_SEED): Improve example.
+
 2013-05-21  Tobias Burnus  <burnus@net-b.de>
 
 	PR fortran/57035

gcc/fortran/intrinsic.texi

@@ -10173,9 +10173,12 @@ end program
 Restarts or queries the state of the pseudorandom number generator used by 
 @code{RANDOM_NUMBER}.
 
-If @code{RANDOM_SEED} is called without arguments, it is initialized to
-a default state. The example below shows how to initialize the random 
-seed based on the system's time.
+If @code{RANDOM_SEED} is called without arguments, it is initialized
+to a default state. The example below shows how to initialize the
+random seed with a varying seed in order to ensure a different random
+number sequence for each invocation of the program. Note that setting
+any of the seed values to zero should be avoided as it can result in
+poor quality random numbers being generated.
 
 @item @emph{Standard}:
 Fortran 95 and later
@@ -10203,20 +10206,53 @@ the @var{SIZE} argument.
 
 @item @emph{Example}:
 @smallexample
-SUBROUTINE init_random_seed()
-  INTEGER :: i, n, clock
-  INTEGER, DIMENSION(:), ALLOCATABLE :: seed
-
-  CALL RANDOM_SEED(size = n)
-  ALLOCATE(seed(n))
-
-  CALL SYSTEM_CLOCK(COUNT=clock)
-
-  seed = clock + 37 * (/ (i - 1, i = 1, n) /)
-  CALL RANDOM_SEED(PUT = seed)
-
-  DEALLOCATE(seed)
-END SUBROUTINE
+subroutine init_random_seed()
+  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", 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(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() + 1099279 ! Add a prime
+     s = ieor(s, pid)
+     if (n >= 3) then
+        seed(1) = t(1) + 36269
+        seed(2) = t(2) + 72551
+        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 init_random_seed
 @end smallexample
 
 @item @emph{See also}: