Using an allocatable string coarray in Fortran

Question

I'm writing some code using Fortran CoArrays (using caf and cafrun from OpenCoarrays) that requires having a coarray string of allocatable length. It can be exemplified by this code:

program CoarrayExample
   character(:), allocatable :: string[:]
   integer :: numImages, image, stringLength

   stringLength = 50
   allocate(character(stringLength) :: string[*])

   numImages = num_images()

   sync all

   do image = 1, numImages
      if (this_image() == image) then
         write(string, '(A,I0)') "Image ", this_image()
      end if
   end do

   sync all

   if (this_image() == 1) then
      do image = 1, numImages
         write(*,*) string[image]
      end do
   end if

   sync all

end program CoarrayExample

The code is expected to store a string copy for each image containing the text "Image <n>", where <n> is the number of the image. The first image is then expected to output the data to stdout.

The compiler has no trouble compiling (command: caf example.f90 -o example) but when running (command: cafrun -np 4 ./example), I get inconsistent behavior. Sometimes I only get empty lines at stdout, sometimes I get the strings but they are cut off, and some times I even get the error:

 Image 1                                           
--------------------------------------------------------------------------
Primary job  terminated normally, but 1 process returned
a non-zero exit code. Per user-direction, the job has been aborted.
--------------------------------------------------------------------------
--------------------------------------------------------------------------
mpiexec noticed that process rank 0 with PID 0 on node pop-os exited on signal 11 (Segmentation fault).
--------------------------------------------------------------------------
Error: Command:
   `/usr/bin/mpiexec -n 4 ./example`
failed to run.

All problems disappear when I remove the allocations. For example, this code works just fine:

program CoarrayExample
   character(len=50) :: string[*]
   integer :: numImages, image

   numImages = num_images()

   sync all

   do image = 1, numImages
      if (this_image() == image) then
         write(string, '(A,I0)') "Image ", this_image()
      end if
   end do

   sync all

   if (this_image() == 1) then
      do image = 1, numImages
         write(*,*) string[image]
      end do
   end if

   sync all

end program CoarrayExample

Any idea what the problem could be?

References

• Fortran: Arrays and Strings
• Allocatable CoArrays