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).