Sleep in Fortran

Question

Does anyone know of an way to sleep for a given number of milliseconds in Fortran? I do not want to use non-portable system calls so anything intrinsic to Fortran or C libraries would be preferred.

Answer 1

Using the Fortran ISO C Binding to use the C library sleep to sleep in units of seconds:

   module Fortran_Sleep

   use, intrinsic :: iso_c_binding, only: c_int

   implicit none

   interface

      !  should be unsigned int ... not available in Fortran
      !  OK until highest bit gets set.
      function FortSleep (seconds)  bind ( C, name="sleep" )
          import
          integer (c_int) :: FortSleep
          integer (c_int), intent (in), VALUE :: seconds
      end function FortSleep

   end interface

end module Fortran_Sleep


program test_Fortran_Sleep

   use, intrinsic :: iso_c_binding, only: c_int

   use Fortran_Sleep

   implicit none

   integer (c_int) :: wait_sec, how_long

   write (*, '( "Input sleep time: " )', advance='no')
   read (*, *) wait_sec
   how_long = FortSleep ( wait_sec )

   write (*, *) how_long

   stop

end program test_Fortran_Sleep

Answer 2

You can use Fortran standard intrinsic functions to do this without C binding:

program sleep
!===============================================================================
implicit none
character(len=100) :: arg ! input argument character string
integer,dimension(8) :: t ! arguments for date_and_time
integer :: s1,s2,ms1,ms2  ! start and end times [ms]
real :: dt                ! desired sleep interval [ms]
!===============================================================================
! Get start time:
call date_and_time(values=t)
ms1=(t(5)*3600+t(6)*60+t(7))*1000+t(8)

! Get the command argument, e.g. sleep time in milliseconds:
call get_command_argument(number=1,value=arg)
read(unit=arg,fmt=*)dt

do ! check time:
  call date_and_time(values=t)
  ms2=(t(5)*3600+t(6)*60+t(7))*1000+t(8)
  if(ms2-ms1>=dt)exit
enddo
!===============================================================================
endprogram sleep

Assuming the executable is slp:

~$ time slp 1234

real        0m1.237s
user        0m1.233s
sys         0m0.003s 

Add a special case to this program if you are worried it will break around midnight :)