Passing a smaller array into a larger assumed size array

Question

I ran across something in someone else's code that seems like it should be undefined behavior in the fortran spec. There is an array defined with an explicit upper and lower bound

      ALLOCATE(arraysmall(10,5:10))

This array is passed into a subroutine with the an assumed size dummy argument an explicit index range to work over.

      SUBROUTINE foo(arraylarge, indexmin, indexmax)
      INTEGER, DIMENSION(10,1:20) :: arraylarge
      INTEGER, INTENT(in)         :: indexmin
      INTEGER, INTENT(in)         :: indexmax
      ...

!  Do work from indexmin to indexmax
      arraylarge(:,indexmin:indexmax) = ...

      END SUBROUTINE

When this subroutine then gets called.

      CALL foo(smallarray, 1, 5)

This seems like multiple things are wrong here that should have undefined behavior.

I would expect that passing smaller size array into a larger size assumed array would mean some of the indices inside the subroutine would be invalid memory locations.
While the smallarray and the size of the arraylarge(:,indexmin:indexmax) have the same have the same size, SIZE(small array, 2) .eq. SIZE(arraylarge(:,indexmin:indexmax),2), I would expect that should be accessing unallocated memory since smallarray isn't defined from indices 1:4. Unless it's doing something like arraylarge(:,1:5) = smallarray(:,5:10) in the background. But that would seem to me to be something that would be compiler dependent.

What should happen when a smaller array is passed into an larger assumed size array? It seems like this should cause a BAD_ACCESS condition but it isn't.

Vladimir F Vladimir F · Accepted Answer · 2016-09-26T20:53:33

"Unless it's doing something like arraylarge(:,1:5) = smallarray(:,5:10) in the background. But that would seem to me to be something that would be compiler dependent."

Indeed, that is happening and it is not compiler dependent. The array is passed as an array of shape (10,5) and it has lower bounds 1 and 1. Lower bounds of Fortran arrays are 1 unless declared otherwise. Only allocatable and pointer dummy arguments behave differently. So the array is seen in the subroutine as having shape (1:10,1:5).

That is why it is not immediately crashing. Still passing an array of shape (1:10,1:5) to a dummy argument of shape (1:10,1:20) is not standard conforming. In C they call it undefined behaviour, but Fortran does not use this term. Even if you don't access the elements after 5 bad things can still happen, if temporary array should be passed or similar.

Passing a smaller array into a larger assumed size array

1 Answers