如何在Fortran公共块中使用可分配的字符？

Question

在Fortran 77中，我试图在一个公共块中包含一个可分配的字符。但是编译器始终会抱怨

  

错误：COMMON属性与（1）处的ALLOCATABLE属性冲突

我的代码是

      subroutine word()
          character (len = :), allocatable :: a
          common /a/ a
          a = 'word'
      end subroutine word

      program main
          character (len = :), allocatable :: a
          common /a/ a
          call word()
          print *, a
      end program main

我看到的唯一使之起作用的方法是不使用allocatable

      subroutine word()
          character * 4 :: a
          common /a/ a
          a = 'word'
      end subroutine word

      program main
          character * 4 :: a
          common /a/ a
          call word()
          print *, a
      end program main

但这会很麻烦，因为我必须提前知道一个字符的长度。

如何使allocatable在common块中工作？