RE: Hex '00' to Hex '40'?

Hello Buck,

You are quite correct in your belief that X'00' and NULL indicators are 
unrelated.  The 
confusion arises primarily from C and C programmers use of X'00' as a 
null-terminator or 
memset()ing a field or structure to X'00'.

The database uses a null-byte map to indicate whether a field is NULL or not.  
There is one bit 
for each null-capable field (hence the restriction on the maximum length of a 
record in a 
null-capable file).  Database does not change the current value of the field 
when the field is 
set NULL or NOT NULL, it only sets the appropriate bit in the null byte map.  
Each bit is 
represented in your program as an indicator byte.

The only valid way of determining whether a null-capable field contains a value 
is to first 
test the null indicator for that field.  That is why processing null-capable 
input-only files 
in RPG/400 is painful.  The database presents default values for NULL fields 
(if you compile 
with ALWNULL(*YES) ) so you can never tell programmatically whether the field 
contains blanks 
or is NULL (assuming the field default value has not been overridden).  RPG IV 
provides access 
to the null byte map via the %NULLIND BIF.

Regards,
Simon Coulter.

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//--- forwarded letter -------------------------------------------------------
> X-Mailer: Internet Mail Service (5.5.2448.0)
> Date: Tue, 29 Jun 99 15:25:40 -0400
> From: "Buck Calabro" <mcalabro@commsoft.net>
> To: "'MIDRANGE-L@midrange.com'" <MIDRANGE-L@midrange.com>
> Reply-To: MIDRANGE-L@midrange.com
> Subject: RE: Hex '00' to Hex '40'?

> 
> 
This one may be a bit tricky.
> 
> The contents of the field and the NULL indicator are unrelated to each
> other.  WS-LAST-DIFF-SEQ-IND may have a value of zero and be NOT NULL or it
> may contain a value of 100 and be NULL.  Even if the value of the field is
> x'00', I do not believe this affects the null indicator in any way.  I ran a
> quick (ugly) program to test this theory; I'll post it to embarrass myself
> in front of the real SQL folks on the list! :-)

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