OpenMP segmentation error with uncalled program subroutine [duplicate] - fortran

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Why Segmentation fault is happening in this openmp code?
(2 answers)
Closed 2 years ago.
I have a very strange error when I enable OpenMP in my compilation options. I have pinned it down to a call to a module subroutine using a dynamic sized array from my main program subroutine. Here is a simplified example:
module arr_mod
contains
subroutine add2_mod(arr)
integer, dimension(:) :: arr
integer i, n
n = size(arr)
do i=1,n
arr(i) = arr(i)+2
enddo
end subroutine
end module
PROGRAM TEST_OMP
use arr_mod
integer, dimension(2000000) :: array
array = 0
write(*,*) array(1)
contains
subroutine add2()
! Note that this subroutine is not even called in the main program...
! When the next line is commented, the program runs.
call add2_mod(array)
end subroutine
END PROGRAM TEST_OMP
When I compile and run this program without OpenMP, it runs fine:
$ gfortran -o test_omp test_omp.f90
$ ./test_omp
0
But when I use OpenMP, the program immediately segfaults:
$ gfortran -o test_omp test_omp.f90 -fopenmp
$ ./test_omp
[1] 10291 segmentation fault ./test_omp
If I remove the program subroutine (or simply comment the add2_mod call), it works fine even with OpenMP. It still works fine even if I call the add2_mod subroutine from the main program directly.
It also works when compiling with optimizations (tested with -O3), and when setting unlimited stack with ulimit -s unlimited.
As far as I can tell, it works fine with Intel Fortran (tested on version 17, with no specific flags other than -qopenmp).

Answer 1

As noted in the gfortran documentation:
-fopenmp implies -frecursive, i.e., all local arrays will be allocated on the stack. When porting existing code to OpenMP, this may lead to
surprising results, especially to segmentation faults if the stacksize
is limited.
To overcome this limitation, you can force the array on the heap with the allocatable specifier:
module arr_mod
contains
subroutine add2_mod(arr)
integer, dimension(:) :: arr
integer i, n
n = size(arr)
do i=1,n
arr(i) = arr(i)+2
enddo
end subroutine
end module
PROGRAM TEST_OMP
use arr_mod
integer, dimension(:), allocatable :: array ! array is allocated on the heap
allocate(array(2000000))
array = 0
write(*,*) array(1)
contains
subroutine add2()
call add2_mod(array)
end subroutine
END PROGRAM TEST_OMP
Which will work fine with the -fopenmp flag.