Can An Allocatable Intent(inout) Argument Be Optional?

Question

I have problem trying to define a subroutine, whose argument contains an allocatable, optional, intent(inout) variable shown below. The code compiles fine, but get runtime error of "Segmentation fault - invalid memory reference".

Subroutine test_routine.f90

SUBROUTINE test_routine(A,B)

IMPLICIT NONE

    REAL,ALLOCATABLE,INTENT(IN)               :: A(:,:)
    REAL,ALLOCATABLE,OPTIONAL,INTENT(INOUT)   :: B(:,:)

    B = A
    B(:,:) = 1

END SUBROUTINE

This subroutine is packed in a module, and is called in Main.

Module test_module.f90

MODULE test_module

IMPLICIT NONE

    INTERFACE test_routine
        MODULE PROCEDURE test_routine
    END INTERFACE

END MODULE test_module

Main test_main.f90

PROGRAM main

    USE test_module

IMPLICIT NONE

    REAL,ALLOCATABLE    :: A(:,:),B(:,:)

    ALLOCATE(A(6,6))
    ALLOCATE(B(6,6))
    A(:,:) = 0
    CALL test_routine(A,B)    ! WORKS FINE
    CALL test_routine(A)      ! DOESN'T WORK!

END PROGRAM main

Then I tried to assign another variable op_B, to make up for B, which doesn't actually exist if the main routine doesn't pass it in. However the following code still doesn't work.

SUBROUTINE test_routine(A,B)

IMPLICIT NONE

    REAL,ALLOCATABLE,INTENT(IN)    :: A(:,:)
    REAL,ALLOCATABLE,OPTIONAL,INTENT(INOUT)   :: B(:,:)
    REAL,ALLOCATABLE               :: op_B(:,:)

    IF(.NOT. PRESENT(B)) THEN
        ALLOCATE(op_B(SIZE(A,1),SIZE(A,2)))
        B = op_B
    END IF
    B = A
    B(:,:) = 1

END SUBROUTINE

By the way, I also tried using fixed size array, but still doesn't help. I wonder if it's impossible to do such things?

score 2 · Accepted Answer · answered Aug 15 '17 at 03:17

Dummy arguments can be optional, allocatable and intent(inout).

But you are not permitted to define or reference an optional dummy argument, bar passing it to PRESENT or associating it with another optional argument. If 'B' is not present, you cannot execute 'B = A'.

score 0 · Answer 2 · answered Aug 15 '17 at 18:01

Based on the answer by @lanH, the only way that I currently figure out is to give an alias to every optional dummy argument. This version works now.

SUBROUTINE test_routine(A,B)

IMPLICIT NONE

    REAL,ALLOCATABLE,INTENT(IN)    :: A(:,:)
    REAL,ALLOCATABLE,OPTIONAL,INTENT(INOUT)   :: B(:,:)
    REAL,ALLOCATABLE               :: op_B(:,:)

    ALLOCATE(op_B(SIZE(A,1),SIZE(A,2)))
    op_B = A
    op_B(:,:) = 1

    IF(PRESENT(B)) THEN
        B = op_B
    END IF

    DEALLOCATE(op_B)

END SUBROUTINE

Can An Allocatable Intent(inout) Argument Be Optional?

2 Answers2