Difference between automatic arrays on the heap and allocatable arrays

7

I totally agree. The intrinsic support of arrays is a nice feature of Fortran, and it should not be a feature that can easily lead to crashes.

For example, the following code will crash with a segmentation fault on Linux or Mac when i = 10 if compiled with ifort without any options. In today’s applications, a matrix of size 1024-by-1024 is really not that big. I would be surprised if a new user of Fortran is not surprised by this behavior.

Code:

module s_mod
implicit none
private
public :: sf_auto

contains

function sf_auto(n) result(s)
integer, intent(in) :: n
real(kind(0.0D0)) :: s
real(kind(0.0D0)) :: x_auto(n, n)
x_auto = 1.0D0
s = sum(x_auto)
end function sf_auto

end module s_mod

program test_mem
use :: s_mod, only:sf_auto
implicit none
integer :: i

do i = 1, 30
    write (*, *) 2**i, sf_auto(2**i)
end do

end program test_mem

Result:

$ uname -a && ifort --version && ifort test.f90 && ./a.out
Linux zP 5.15.0-56-generic #62-Ubuntu SMP Tue Nov 22 19:54:14 UTC 2022 x86_64 x86_64 x86_64 GNU/Linux
ifort (IFORT) 2021.8.0 20221119
Copyright (C) 1985-2022 Intel Corporation.  All rights reserved.

           2   4.00000000000000     
           4   16.0000000000000     
           8   64.0000000000000     
          16   256.000000000000     
          32   1024.00000000000     
          64   4096.00000000000     
         128   16384.0000000000     
         256   65536.0000000000     
         512   262144.000000000     
forrtl: severe (174): SIGSEGV, segmentation fault occurred
Image              PC                Routine            Line        Source             
libc.so.6          00007F8B6540A520  Unknown               Unknown  Unknown
a.out              0000000000404275  Unknown               Unknown  Unknown
a.out              000000000040415D  Unknown               Unknown  Unknown
libc.so.6          00007F8B653F1D90  Unknown               Unknown  Unknown
libc.so.6          00007F8B653F1E40  __libc_start_main     Unknown  Unknown
a.out              0000000000404075  Unknown               Unknown  Unknown