[open-cobol-list] CALL USING ADDRESS OF, unsupported

[Tim Nixon]

I believe this is another "IBM Only" issue

ADDRESS OF is not supported on a call statement. See below for code example.

 

Background:

As is often the case, there is a need to set a pointer to the address of a working-storage variable. There is a defined procedure, SETPTR to do this that you call with the variable and the pointer:

i.e.: CALL SETPTR USING VARNAME PTR.

(all SETPTR does is SET ptr TO ADDRESS OF varname)

 

Assume there is a LINKAGE SECTION variable, lets call it LIN1. Also, assume a pointer PTR contains the address of some other variable.

In the code, we can change LIN1 to be an 'alias' for the other variable like this:

SET ADDRESS OF LIN1 TO PTR.

(after this, references to LIN1 are to where PTR pointed)

 

So far all this works, but here's where it gets interesting..

To set the LINKAGE SECTION variable's address to the address of a WORKING-STORAGE variable without using a pointer they use:

CALL SETPTR USING VARIABLE-NAME

ADDRESS OF LIN1.

The implication here is that this call to SETPTR can change the address of LIN1 in the same way it would change a pointers value.

It seems like it should work since "ADDRESS OF" should be compatible with "pointer to" the same item just like in “c”. Of course that is if ADDRESS OF is supported on a CALL.

 

 

As I see it there are 2 issues here.

1) This version of the compiler doesn't support ADDRESS OF on a call.

2) If ADDRESS OF is to be supported on a CALL, it must be implemented so that changes made to the address reflect on the original items address. When CALLing using a pointer, at the c-code level we're really passing the "address of" the pointer. In the same way, if we use ADDRESS OF and if we change where the pointer points we really should be changing the address of the original variable.

 

I dug through the compiler code as well as the C code resulting from trying this and it seems that to do allow ADDRESS OF on a CALL like this the code would need to:

-create a temp pointer that points to the actual item

-CALL using the address of this pointer

-upon return from the CALL reset the address of the original item to be the address stored in the pointer in case the address changed..

 

Any thoughts?

 

Tim Nixon