Confusion on using USING/CHANGING in PERFORMS and FORMS

Question

I got a little confused with PERFORM statements.
As far as I understood, the FORM definition is the one that ultimately defines which variable is going to be handled by value and which by reference (Using either USING, USING VALUE(...) or CHANGING (which is a particularity to deal with recursive functions). Thus, why does the PERFORM statement also accepts the CHANGING option ?
Should I worry with the way the PERFORM statement is written or can I simply imply from it the values of the parameters passed, in a given moment of execution ?
Thanks
Avraham

Answer 1

Hi
The USING and CHANGING additions in the FORM statement define the formal parameters of a subroutine. The sequence of the additions is fixed. Each addition can be followed by a list of any number of formal parameters. When you call a subroutine, you must fill all formal parameters with the values from the actual parameters. At the end of the subroutine, the formal parameters are passed back to the corresponding actual parameters.
Within a subroutine, formal parameters behave like dynamic local data. You can use them in the same way as normal local data objects that you would declare with the DATA statement. They mask global data objects with the same name. The value of the parameters at the start of the subroutine is the value passed from the corresponding actual parameter.
Parameters Passed by Reference
You list these parameters after USING or CHANGING without the VALUE addition:
FORM <subr> USING    ... <pi> [TYPE <t>|LIKE <f>] ...
            CHANGING ... <pi> [TYPE <t>|LIKE <f>] ...
The formal parameter occupies no memory of its own. During a subroutine call, only the address of the actual parameter is transferred to the formal parameter. The subroutine works with the field from the calling program. If the value of the formal parameter changes, the contents of the actual parameter in the calling program also change.
For calling by reference, USING and CHANGING are equivalent. For documentation purposes, you should use USING for input parameters which are not changed in the subroutine, and CHANGING for output parameters which are changed in the subroutine.
To avoid the value of an actual parameter being changed automatically, you must pass it by value.
Input Parameters That Pass Values
You list these parameters after USING with the VALUE addition:
FORM <subr> USING    ... VALUE(<pi>) [TYPE <t>|LIKE <f>] ...
The formal parameter occupies its own memory space. When you call the subroutine, the value of the actual parameter is passed to the formal parameter. If the value of the formal parameter changes, this has no effect on the actual parameter.
Output Parameters That Pass Values
You list these parameters after CHANGING with the VALUE addition:
FORM <subr> CHANGING ... VALUE(<pi>) [TYPE <t>|LIKE <f>] ...
The formal parameter occupies its own memory space. When you call the subroutine, the value of the actual parameter is passed to the formal parameter. If the subroutine concludes successfully, that is, when the ENDFORM statement occurs, or when the subroutine is terminated through a CHECK or EXIT statement, the current value of the formal parameter is copied into the actual parameter.
If the subroutine terminates prematurely due to an error message, no value is passed. It only makes sense to terminate a subroutine through an error message in the PAI processing of a screen, that is, in a PAI module, in the AT SELECTION-SCREEN event, or after an interactive list event.
reward if useful
Regards
Pavan