c_loc() with dynamic arrays

Question

I am Fortran beginner and I am trying to adopt some ifort code for compilation with gfortran.

I have problem with the c_loc() function, which in ifort seems to accept dynamic arrays but with gfortran compilation stops with error:

Error: Argument 'septr1' to 'c_loc' at (1) must be an associated scalar POINTER

So does anyone knows how to adapt the following ifort code for compilation with gfortran?

integer(c_int), dimension(:), pointer  :: septr1=>null()
type(c_PTR) :: septr

allocate (septr1(10))
septr1 = 33
septr = c_loc(septr1)

Answer 1

This appears to be a requirement from the old Fortran 2003 and relaxed in Fortran 2008. More recent gfortran (5+) accepts this.

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You can get the location of the start of the array, the value with offset 0 in C.

septr = c_loc(septr1(1))

or generally not 1 but lbound(septr1).

See the requirements for c_loc argument in Metcalf, Reid and Cohen or in Fortran standard.

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It is generally much better to pass the array by reference in normal Fortran way and not to construct an explicit c pointer. For example:

 call some_c_function(n,A)

where some_c_function has Fortran interface

 interface
  subroutine some_c_function(n,A) bind(C)
   use iso_c_binding,only: c_int,c_float    !you can use also import here

   integer(c_int),value      :: n
   real(c_float),dimension(n):: A
  end subroutine some_c_function
 end interface

for C prototype

 void some_c_function(int n, float* A)  //(hope so, I am not so good in C).

Answer 2

ifort developers say that standards compliance checking of c_loc, including its more restricted usage than the legacy LOC(), is not required by the provisions of the Fortran standard. Possibly in view of the wider scope for useful application of this intrinsic function in current standards, and the need to restrict the scope of uses which need to be tested, most compilers will reject the non-standard usage.