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program pario
use mpi
use, intrinsic :: iso_fortran_env, only : error_unit, output_unit
implicit none
integer, parameter :: datasize = 64, writer_id = 0
integer :: rc, my_id, ntasks, localsize, i
integer, dimension(:), allocatable :: localvector
integer, dimension(datasize) :: fullvector
call mpi_init(rc)
call mpi_comm_size(mpi_comm_world, ntasks, rc)
call mpi_comm_rank(mpi_comm_world, my_id, rc)
if (ntasks > 64) then
write(error_unit, *) 'Maximum number of tasks is 64!'
call mpi_abort(MPI_COMM_WORLD, -1, rc)
end if
if (mod(datasize, ntasks) /= 0) then
write(error_unit,*) 'Datasize (64) should be divisible by number of tasks'
call mpi_abort(MPI_COMM_WORLD, -1, rc)
end if
localsize = datasize / ntasks
allocate(localvector(localsize))
localvector = [(i + my_id * localsize, i=1,localsize)]
call many_writers()
deallocate(localvector)
call mpi_finalize(rc)
contains
subroutine single_writer()
implicit none
call mpi_gather(localvector, localsize, mpi_integer, fullvector, &
& localsize, mpi_integer, writer_id, mpi_comm_world, rc)
if (my_id == writer_id) then
open(10, file='singlewriter.dat', status='replace', form='unformatted', &
& access='stream')
write(10, pos=1) fullvector
close (10)
write(output_unit,'(A,I0,A)') 'Wrote ', size(fullvector), &
& ' elements to file singlewriter.dat'
end if
end subroutine single_writer
subroutine many_writers()
implicit none
character(len=85) :: filename
write(filename, '(A,I0,A)') 'manywriters-', my_id, '.dat'
open(my_id+10, file=filename, status='replace', form='unformatted', &
& access='stream')
write(my_id+10, pos=1) localvector
close (my_id+10)
write(output_unit,'(A,I0,A,A)') 'Wrote ', size(localvector), &
& ' elements to file ', filename
end subroutine many_writers
end program pario