coarray fortran array doesn't get updated

I'm just learning to use coarry with Fortran. I have a very simple program. I have an array of length 9 that I want to distribute over 3 processes, do some calculations, and merge them back to a single array (basic MPI_scatter/MPI_gather like problem).

program name
   implicit none
   integer:: arr(9), local(3)[*]
   integer:: i, j, k, iz

   arr = [(i, i = 1, 9)]

   local(:)[1] = arr(1:3)
   local(:)[2] = arr(4:6)
   local(:)[3] = arr(7:9)

   iz = this_image()
   local = local*iz

   sync all
    
   if(iz == 1) then
      arr(1:3) = local(:)[1]
      arr(4:6) = local(:)[2]
      arr(7:9) = local(:)[3]
      write(*,'(*(i3))')arr
   endif

end program name

I'm compiling this with

gfortran -fcorray=lib -lcaf_mpi

and executing with this

mpirun -np 3 ./a.out

This should a print output like this

 1 2 3 8 10 12 21 24 27

But, If I run the executable multiple times (without recompilation) sometimes it shows multiple results, like 1 2 3 4 5 6 21 24 27 etc. that is the values do not get updated with the calculation.

What I'm doing wrong here? How to fix this problem

Solution

Welcome to the wonderful world of shared memory programming!

You have a race condition. It is possible that one of the images is so far behind the others that it resets the (remote) data after the other image performs local = local*iz. Here's a way to fix it, being extra careful that only 1 process ever sets up a given part of the data:

ijb@ijb-Latitude-5410:~/work/stack$ cat caf.f90
program name
   implicit none
   integer:: arr(9), local(3)[*]
   integer:: i, j, k, iz

   iz = this_image()

   ! Only one image should ever write to a given memory location
   ! between synchronisation points. In a real code each different image
   ! would set up different parts of the array
   If( iz == 1 ) Then
      arr = [(i, i = 1, 9)]
      local(:)[1] = arr(1:3)
      local(:)[2] = arr(4:6)
      local(:)[3] = arr(7:9)
   End If

   ! Make sure the array is fully set up before you use it
   sync all

   local = local*iz

   sync all
    
   if(iz == 1) then
      arr(1:3) = local(:)[1]
      arr(4:6) = local(:)[2]
      arr(7:9) = local(:)[3]
      write(*,'(*(i3))')arr
   endif

end program name
ijb@ijb-Latitude-5410:~/work/stack$ mpif90 -std=f2018 -fcheck=all -Wall -Wextra -O -g -fcoarray=lib  caf.f90 -lcaf_openmpi
caf.f90:4:17:

    4 |    integer:: i, j, k, iz
      |                 1
Warning: Unused variable ‘j’ declared at (1) [-Wunused-variable]
caf.f90:4:20:

    4 |    integer:: i, j, k, iz
      |                    1
Warning: Unused variable ‘k’ declared at (1) [-Wunused-variable]
ijb@ijb-Latitude-5410:~/work/stack$ mpirun -np 3 ./a.out
  1  2  3  8 10 12 21 24 27
ijb@ijb-Latitude-5410:~/work/stack$ mpirun -np 3 ./a.out
  1  2  3  8 10 12 21 24 27
ijb@ijb-Latitude-5410:~/work/stack$ mpirun -np 3 ./a.out
  1  2  3  8 10 12 21 24 27
ijb@ijb-Latitude-5410:~/work/stack$ mpirun -np 3 ./a.out
  1  2  3  8 10 12 21 24 27
ijb@ijb-Latitude-5410:~/work/stack$ mpirun -np 3 ./a.out
  1  2  3  8 10 12 21 24 27
ijb@ijb-Latitude-5410:~/work/stack$ mpirun -np 3 ./a.out
  1  2  3  8 10 12 21 24 27
ijb@ijb-Latitude-5410:~/work/stack$ mpirun -np 3 ./a.out
  1  2  3  8 10 12 21 24 27
ijb@ijb-Latitude-5410:~/work/stack$ mpirun -np 3 ./a.out
  1  2  3  8 10 12 21 24 27
ijb@ijb-Latitude-5410:~/work/stack$ mpirun -np 3 ./a.out
  1  2  3  8 10 12 21 24 27
ijb@ijb-Latitude-5410:~/work/stack$ mpirun -np 3 ./a.out
  1  2  3  8 10 12 21 24 27
ijb@ijb-Latitude-5410:~/work/stack$ mpirun -np 3 ./a.out
  1  2  3  8 10 12 21 24 27
ijb@ijb-Latitude-5410:~/work/stack$ mpirun -np 3 ./a.out
  1  2  3  8 10 12 21 24 27
ijb@ijb-Latitude-5410:~/work/stack$ mpirun -np 3 ./a.out
  1  2  3  8 10 12 21 24 27
ijb@ijb-Latitude-5410:~/work/stack$ mpirun -np 3 ./a.out
  1  2  3  8 10 12 21 24 27
ijb@ijb-Latitude-5410:~/work/stack$ mpirun -np 3 ./a.out
  1  2  3  8 10 12 21 24 27
ijb@ijb-Latitude-5410:~/work/stack$ mpirun -np 3 ./a.out
  1  2  3  8 10 12 21 24 27
ijb@ijb-Latitude-5410:~/work/stack$ mpirun -np 3 ./a.out
  1  2  3  8 10 12 21 24 27
ijb@ijb-Latitude-5410:~/work/stack$ mpirun -np 3 ./a.out
  1  2  3  8 10 12 21 24 27
ijb@ijb-Latitude-5410:~/work/stack$ mpirun -np 3 ./a.out
  1  2  3  8 10 12 21 24 27
ijb@ijb-Latitude-5410:~/work/stack$ mpirun -np 3 ./a.out
  1  2  3  8 10 12 21 24 27
ijb@ijb-Latitude-5410:~/work/stack$ mpirun -np 3 ./a.out
  1  2  3  8 10 12 21 24 27
ijb@ijb-Latitude-5410:~/work/stack$ mpirun -np 3 ./a.out
  1  2  3  8 10 12 21 24 27