Segfault when passing data using MPI Windows (MPI_Put) - parallel-processing

Question

I am trying to figure out how to use MPI to work with matrices.
I have a 3x6 matrix filled with zeros and am running code with 3 threads. 0 is the main one, 1 writes to the first row of the matrix in columns from 1 to 3 ones, and 2 stream writes to the second row in columns 4-6 of two.
I pass these formed parts to the main thread (at 0), I get the correct result, but after that a memory error is output to the console.
I can't figure out what I'm doing wrong. Can you please tell me what is my mistake?
program test
Use mpi
Implicit None
integer :: process_Rank, size_Of_Cluster, ierror = 0, win, size_s, n = 6
integer:: i , j
integer:: start, target_count = 9
integer :: mtx(3,6)
integer(kind = MPI_ADDRESS_KIND) :: nbytes = 4
!input matrix
do i = 1,3
do j =1,6
mtx(i,j) = 0
end do
end do
Call mpi_sizeof( mtx, size_s, ierror ) !Get the size of a matrix element
call MPI_INIT(ierror)
call MPI_COMM_SIZE(MPI_COMM_WORLD, size_Of_Cluster, ierror)
call MPI_COMM_RANK(MPI_COMM_WORLD, process_Rank, ierror)
!create windows
if(process_Rank == 0) then
call MPI_WIN_CREATE(mtx, size_s *6 * 3 * nbytes, 1, MPI_INFO_NULL, MPI_COMM_WORLD, win, ierror)
else
call MPI_WIN_CREATE(mtx, size_s * 6* 3*nbytes,1, MPI_INFO_NULL, MPI_COMM_WORLD, win, ierror)
end if
CALL MPI_Win_fence(0,win,ierror)
if(process_Rank == 1) then
!fill 3 columns of the first row with ones
start = 0
do i = 0,3
mtx(process_Rank,i+start) = process_Rank
end do
CALL MPI_PUT(mtx, size_s*3*6, MPI_INTEGER, 0, start * nbytes, target_count, MPI_INTEGER, win, ierror)
!print mtx
print *, process_Rank, ' put = '
do i = 1,3
print *, ''
do j = 1,3
write(*,fmt='(g0)', advance = 'no') mtx(i,j)
write(*,fmt='(g0)', advance = 'no') ' '
end do
end do
end if
CALL MPI_Win_fence(0, win,ierror)
if(process_Rank == 2) then
!fill the last 3 columns of the second row with twos
start = 3
do i = 1,3
mtx(process_Rank,i+start) = process_Rank
end do
CALL MPI_PUT(mtx(1:3,4:6), size_s* 3 *6, MPI_INTEGER, 0, 3 * 3 * nbytes, target_count, MPI_INTEGER, win, ierror)
!print mtx
print *, process_Rank, ' put = '
do i = 1,3
print *, ''
do j = 4,6
write(*,fmt='(g0)', advance = 'no') mtx(i,j)
write(*,fmt='(g0)', advance = 'no') ' '
end do
end do
end if
CALL MPI_Win_fence(0, win,ierror)
! print result
if(process_Rank == 0) then
print *, 'result = '
do i = 1,3
print *, ''
do j = 1,6
write(*,fmt='(g0)', advance = 'no') mtx(i,j)
write(*,fmt='(g0)', advance = 'no') ' '
end do
end do
end if
CALL MPI_Win_fence(0, win,ierror)
CALL MPI_WIN_FREE(win, ierror)
call MPI_FINALIZE(ierror)
end program test
Console:
1 put =
1 1 1
0 0 0
0 0 0
2 put =
0 0 0
2 2 2
0 0 0
result =
1 1 1 0 0 0
0 0 0 2 2 2
0 0 0 0 0 0
Program received signal SIGSEGV: Segmentation fault - invalid memory reference.
Backtrace for this error:
#0 0x7fd4447bcd01 in ???
#1 0x7fd4447bbed5 in ???
#2 0x7fd4445f020f in ???
--------------------------------------------------------------------------
Primary job terminated normally, but 1 process returned
a non-zero exit code. Per user-direction, the job has been aborted.
--------------------------------------------------------------------------
--------------------------------------------------------------------------
mpirun noticed that process rank 0 with PID 0 on node alm-VirtualBox exited on signal 11 (Segmentation fault).
--------------------------------------------------------------------------

Answer 1

If you use the -fcheck=all, which Ian Bush suggested to you in the first comment under your question, you will get the reason for the error immediately and you do not have to wait many hours for feedback on the internet. I got:
At line 38 of file mpi_wins.f90 Fortran runtime error:
Index '0' of dimension 2 of array 'mtx' below lower bound of 1
Error termination. Backtrace:
#0 0x7f7ed3e75640 in ???
#1 0x7f7ed3e76185 in ???
#2 0x7f7ed3e7652a in ???
#3 0x4010e4 in test
at /home/lada/f/testy/stackoverflow/mpi_wins.f90:38
#4 0x401e78 in main
at /home/lada/f/testy/stackoverflow/mpi_wins.f90:3
You are indexing your mtx array using the process rank, but the array is defined to start from 1.
integer :: mtx(3,6)
However, MPI ranks start from 0, not from 1.
Also notice that the backtrace now contains a better code location thanks to the -g compiler option.