Re: copy file member using SEPT calls

[David McKenzie <davemck@xxxxxxxxxx>]

Hi Leif,

Regarding pointers, I think the word "null" just means "tag bit not set"
without regard to what's in the 16 bytes of storage.  So you can get a null ptr
by just not setting it (either with INIT or a pointer-setting instruction) to
point to anything.  For example:

     DCL SPCPTR ASPP;
     CMPPTRT(B) ASPP,*/EQ(ITSNULL);

This code will take the branch (assuming something else didn't set ASPP).

If you do GENOPT(*LIST) on an RPGIII pgm that has files, you see:

     DCL DD .NULLCL CHAR(1) INIT(X'FF')/*NO CONTROL LIST*/;
     DCL SPCPTR .CO01001;
     DCL OL .OL01001(.F01UFCB,.P01001,.CO01001);
     SETSPP .CO01001,.NULLCL /*NO CONTROL LIST*/;

.CO01001 corresponds to Gene's ?NULL.  It's not really a null ptr.  It's a
NON-null ptr pointing to a byte of X'FF'.  The IBM I/O pgms are explicitly
coded to regard a ptr pointing to X'FF' to mean "no control list was
provided".  Evidently, from Gene's later post, setting ?NULL or .CO01001 to be
_actually_ null gets the same effect.

--Dave

Leif Svalgaard wrote:

>         [Leif Svalgaard]  One learns something new everyday from this list.
> > DCL DD     NULL      AUTO CHAR(1) INIT(X"FF");
> > DCL SPCPTR ?NULL     AUTO INIT(NULL);
> > DCL OL     IOLIST1   (?UFCB1, ?TYPE1, ?NULL) ARG;
> >
>         [Leif Svalgaard]  Gene, could you explain the trick you use to init
>         a pointer to null?  I found that DCL SPCPTR .NULL INIT(*) doesn't
>         work and had to use CPYBWP .NULL, * instead. I thought that a
>         null pointer was all zeroes. Not FF ?????

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