将未分配的可分配变量作为可分配的虚拟参数传递 [英] Passing unallocated allocatable variable as an allocatable dummy argument

问题描述

在下面的程序中，一个可分配的变量x被传递给子程序test_sub而没有被分配.在test_sub中分配对应的可分配虚拟变量x_sub:

In the following program, an allocatable variable x is passed to a subroutine test_sub without being allocated. The corresponding allocatable dummy variable x_sub is allocated in test_sub:

module test_mod

    implicit none

    contains

        subroutine test_sub(x_sub)

            implicit none
            real, allocatable :: x_sub(:)

          ! Execution !

!           return
            allocate(x_sub(1))
!           deallocate(x_sub)

        end subroutine test_sub

end module test_mod

program test

    use test_mod, only: test_sub
    implicit none
    real, allocatable :: x(:)

  ! Execution !

    print*, 'Before call to sub', allocated(x)
    call test_sub(x)
    print*, 'After call to sub', allocated(x)

end program test

当控制权返回给主程序时，分配x:

When control is returned to the main program, x is allocated:

 Before call to sub F
 After call to sub T

如果test_sub在分配x_sub前返回，或者x_sub在返回前被释放，则x不被分配:

If test_sub returns before allocating x_sub, or if x_sub is deallocated before returning, x is not allocated:

 Before call to sub F
 After call to sub F

我使用 gfortran 4.4.7 和 ifort 19.0 观察到这种行为.

I observe this behavior using both gfortran 4.4.7 and ifort 19.0.

我的问题是这是否是标准行为.我本来希望在尝试传递 x 而不分配它时会出现段错误.我担心这种行为可能会导致大型程序出现意外结果.

My question is whether this is standard behavior. I would have expected to get a segfault when trying to pass x without it being allocated. I'm worried this behavior might lead to unexpected results in larger programs.

推荐答案

您可以使用 intent 声明来控制例程中变量的分配行为:

You can use an intent declaration to control the allocation behavior of your variable inside the routine:

1. real, allocatable, intent(inout) :: x(:) 等价于你的情况.

• 分配状态不是先验的，可以用allocated(x)进行测试；
• 分配状态可以使用allocate/deallocate
改变

• Allocation status is not known a priori, it can be tested with allocated(x);
• Allocation status can change using allocate/deallocate

如果real, allocatable,intent(in) :: x(:)，则分配状态不能在里面改变；

if real, allocatable, intent(in) :: x(:), then the allocation status cannot change inside of it;

• 可以使用 allocated(x)
测试分配状态
• 无法修改

如果 real, allocatable,intent(out) :: x(:)，那么变量 x 总是初始化为非分配，不管它是什么调用例程之前的状态，因此您可以总是以 allocate(x(n)) 开始您的子例程.

if real, allocatable, intent(out) :: x(:), then variable x is always initialized as non-allocated, regardless of its former status before calling the routine, so you can always begin your subroutine with allocate(x(n)).

我相信您正在使用选项 1)，但您想要 3).

I believe you're using option 1) but you want 3).

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