coarray fortran array doesn't get updated - fortran

Question

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

Answer 1

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