Fortran call to sleep does not block program - fortran

Question

I have the most basic Fortran program:
program sleep
print*, "Sleeping"
call sleep(30)
print*, "Done"
end program sleep
which I compile with gfortran sleep.f90 (version 9.3.0). From what I understood from the sleep documentation, this program is supposed to sleep for 30 seconds, i.e. I should expect to see "Done" being printed 30 seconds after "Sleeping". This doesn't happen: I see both print statements appearing instantaneously, suggesting that call sleep(30) does not block my program in any way. Doing call sleep(10000) didn't make any difference. I am compiling and running this program on a Windows Subsystem for Linux (WSL Ubuntu 20.04).

Answer 1

So this problem was fixed through a combination of solutions suggested by #roygvib in the comments. The main issue is that sleep in the WSL (Ubuntu 20.04) environment is broken. The first step is to replace the broken /usr/bin/sleep with this Python script:
#!/usr/bin/env python3
import sys
import time
time.sleep(int(sys.argv[1]))
Then, the Fortran program is modified to make a system call to this new sleep executable:
program sleep
print*, "Sleeping"
call system("sleep 30")
print*, "Done"
end program sleep
Until the next update of WSL, this hack will have to do.

Answer 2

The sleep procedure is not part of the Fortran standard and non-portable. Here is a solution that would potentially work on all systems with any standard-compliant Fortran compiler:
module sleep_mod
use, intrinsic :: iso_fortran_env, only: IK => int64, RK => real64, output_unit
implicit none
contains
subroutine sleep(seconds)
implicit none
real(RK), intent(in) :: seconds ! sleep time
integer(IK) :: countOld, countNew, countMax
real(RK) :: countRate
call system_clock( count=countOld, count_rate=countRate, count_max=countMax )
if (countOld==-huge(0_IK) .or. nint(countRate)==0_IK .or. countMax==0_IK) then
write(output_unit,"(A)") "Error occurred. There is no processor clock."
error stop
end if
countRate = 1._RK / countRate
do
call system_clock( count=countNew )
if (countNew==countMax) then
write(output_unit,"(A)") "Error occurred. Maximum processor clock count reached."
error stop
end if
if ( real(countNew-countOld,kind=RK) * countRate > seconds ) exit
cycle
end do
end subroutine sleep
end module sleep_mod
program main
use sleep_mod, only: output_unit, sleep, RK
implicit none
write(output_unit,"(A)") "Sleep for 5 second."
call execute_command_line(" ") ! flush stdout
call sleep(seconds = 5._RK)
write(output_unit,"(A)") "Wake up."
end program main
You can test it online here: https://www.tutorialspoint.com/compile_fortran_online.php