Converting c-string to Fortran string

In the brave new world of c interoperability the standard requires that c-strings passed into a Fortran subroutine are interpreted as arrays of characters of length 1. So to use them in a Fortran program and array of characters has to be converted into a character string. So my question is what is the best way of doing this? At the moment I'm using the function below with example usage but I'm not convinced it is optimal. Thanks.

module cstr
contains
function c_to_f_string(s) result(str)
  use iso_c_binding
  character(kind=c_char,len=1), intent(in) :: s(*)
  character(len=:), allocatable :: str
  integer i, nchars
  i = 1
  do
     if (s(i) == c_null_char) exit
     i = i + 1
  end do
  nchars = i - 1  ! Exclude null character from Fortran string
  allocate(character(len=nchars) :: str)
  str = transfer(s(1:nchars), str)
end function c_to_f_string

subroutine pstr(s) bind(c,name='pstr')
  use iso_c_binding
  use cstr
  character(kind=c_char,len=1), intent(in) :: s(*)
  character(len=:), allocatable :: str
  integer i, nchars
  write(*,'(a)') c_to_f_string(s)
end subroutine pstr

extern "C" {
  void pstr(const char*);
};
int main(int argc, char** argv) {
  if (argc > 1) {
    pstr(argv[1]);
  } else {
    pstr("hello");
  }
  return 0;
}