C/Fortran memory interop

carray.c

#include <stdio.h>
#include <stdlib.h>

void allocate_from_c(double **arr, int *n) {
  *arr = (double*)malloc(5 * sizeof(double));
  *n = 5;

  double *x = *arr;

  for (int i = 0; i < *n; i++) {
    x[i] = (double)i;
  }

  printf("from c: %p\n", *arr);
}

void deallocate_from_c(double **arr) {
  free((void*)*arr);
}

read_carray.f90

program read_carray
  use iso_c_binding

  implicit none

  interface allocate_from_c
    subroutine allocate_from_c(arr, n) bind(c)
      use iso_c_binding

      type(c_ptr), intent(out) :: arr
      integer(c_int), intent(out) :: n
    end subroutine allocate_from_c
  end interface allocate_from_c

  interface deallocate_from_c
    subroutine deallocate_from_c(arr) bind(c)
      use iso_c_binding

      type(c_ptr), intent(in) :: arr
    end subroutine deallocate_from_c
  end interface deallocate_from_c

  type(c_ptr) :: arr
  integer :: n
  double precision, dimension(:), pointer :: darr

  print *, "Hello world"

  call allocate_from_c(arr, n)

  print *, "Allocated array length", n
  print *, arr

  call c_f_pointer(arr, darr, [n])

  print *, darr

  call deallocate_from_c(arr)

end program read_carray