opendir() in RPG IV

[Jon.Paris@xxxxxxxxxx]

 >> This failed.  When I switched to value instead of const it worked.  This
 >> isn't the first time I've been confused between VALUE and CONST.
 >> Looking at the C prototype, it specifies CONST CHAR *DIRNAME.  Why do I
 >> need to use VALUE in RPG?

There are two basic methods for passing parameters to a procedure - by reference
(i.e. a pointer is passed not the actual data) and by value (a copy of the
actual data is passed).  For *PGM objects only pass by reference is supported.
C functions on the other hand _only_ support passing by value.  The fact that
until ILE there was no mechanism on the AS/400 to pass parameters by Value has a
lot to do with the fact that C performance on the system was terrible under the
OPM model - parameter passing had to be "faked-out" !!.

OK - that's the basics.  When calling a C function you should always use the
VALUE keyword.  When you specify CONST (or nothing at all) you are passing the
parameter by reference (not what C wants) the CONST keyword simply means that
since the parameter will be treated read-only by the procedure you are calling,
the compiler can generate a temporary field when needed (e.g. if an expression
is passed as a parameter or if the size/type of the data item doesn't match
exactly) .  That's all it means - it doesn't change the way the parameter is
passed.

It does seem that _sometimes_ using CONST (or nothing) will work to a C routine
when C expects a pointer (since that is what will be passed) but it doesn't
always work and I'm not sure why/when it won't.  Simple answer is _always_ pass
by value then you can't get into trouble.

Oh yes, one other thing - in C the keyword CONST means the value of the
parameter can't be changed - but the parameter is still expected to be passed by
value.

Hope this helps explain it.


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