What happens when I pass an array to a function/subroutine?

Question

I had never thought about this before, but lately I've been worried about something. In Fortran90(95), say I create a really big array

Integer :: X(1000000)

and then I write a function that takes this array as an argument. When I pass the array to the function (as in myfunc(X)) what exactly happens during run time?

Does the entire array get passed by value and a new copy constructed inside the function?(costly)
Or does the compiler simply pass some sort of reference or pointer to the array?(cheap)

Do the dimension of the array or the declaration of the function make a difference?

Edoardo Pirovano · Accepted Answer · 2011-12-02 14:59:02Z

2

In Fortran 90 , as in most other programming languages, arrays are passed by reference (technically, this is often a reference to the first item of the array). In Fortran 90, non-array values are also usually passed by reference. So, you needn't worry about the size of the parameters you pass, since they won't be copied but will, instead, be passed simply by reference.

answered Dec 2, 2011 at 14:59

Edoardo Pirovano

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FrenchKheldar · Accepted Answer · 2011-12-06 09:27:16Z

1

The one thing you don't want to do is something like:

INTEGER :: X(1:1000,1:1000,1:1000)
CALL myRoutine(X(2:999,2:999,2:999))

where myRoutine cannot operate on the bounds of the array for some reason. It cannot pass the reference to the slice of the array since it not contiguous in memory. So it creates a temporary array and copies the values from X. Needless to say this is very slow. But you shouldn't have that issue with 1D array, even when specifying slices, as they are still contiguous in memory.

answered Dec 6, 2011 at 9:27

FrenchKheldar

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