Re: Reading Pipe - ASCII -> EBCDIC Conversion

hockchai Lim wrote:

Hey Simon,
care to explain a bit more about the different between these three prototype?
D proc1 pr
D data * const


D proc1 pr
D data * value


D proc1 pr
D data *

If I may jump in ...

* const: a pointer passed by reference. The called procedure is not expected to change the parameter. Coding CONST is a promise from the RPG programmer to the RPG compiler that the called procedure will not change the parameter. If the called procedure is an RPG procedure, the RPG compiler will enforce this.

* value: a pointer passed by value. If the called procedure changes the parameter, the caller will not see the change.

* : a pointer passed by reference. If the called procedure changes the parameter, the caller will see the change.

When a parameter is passed "by reference" means that the actual thing passed to the called procedure is a pointer to the parameter. The called procedure uses the pointer as a basing pointer under the covers. The called procedure has direct access to the storage in the caller. (For CONST parameters, it may be the storage of a compiler temporary in the caller rather than the actual parameter coded on the call.)

When a parameter is passed "by value", the actual thing passed to the called procedure is a copy of the parameter.

This can be confusing with pointer parameters. It's sometimes easier to understand with non-pointer types first, then with that understanding, mentally substitute a pointer.

Consider:
D proc pr
D constRefParm like(x) const
D refParm like(x)
D valueParm like(x) value

Say x is an integer. The called procedure gets passed a pointer, a ponter and an integer.

In the called procedure, _under_the_covers_, the procedure access its parameters like this (call the first pointer p1 and the second pointer p2):

D constRefParm s 10i 0 based(p1)
D refParm s 10i 0 based(p2)
D valueParm s 10i 0

Now, say x is a pointer. It's exactly the same except instead of 10i we have *.

D constRefParm s * based(p1)
D refParm s * based(p2)
D valueParm s *

The only difference between constRefParm and refParm is that the called procedure is not allowed to change constRefParm.