Re: Checking for NULL field in C when using Record I/O - part 2

Hi Patrick,

I find the information provided in the manuals on this subject very poor.
Macros are not conveniently explained and there are no good examples.

I am doing “reverse engineering” and “trial and error”. For what I need I
have discovered that I have to adjust the _RFILE.out_null_map subfield.

Thank you.

Javier Mora

El vie, 21 feb 2025 a las 21:07, Patrik Schindler (<poc@xxxxxxxxxx>)
escribió:

Hello,

thanks to Javier Mora for reminding me about this topic. :-)

Thanks to Mark's answer from last October, I found some more breadcrumbs
to follow.

https://archive.midrange.com/c400-l/202410/msg00001.html

The PDF mentioned by Mark says:

When you read from a database file, the corresponding byte in the null
field map is indicated with a character '1' if the field is considered
null. This is specified in the null field map pointed to by the in_null_map
pointer.


I changed my CGI program to decide upon said character:

if (docs_data_nullfld.SUBTITLE == '0') {
do things if field is not NULL …

Unfortunately, this never matches, and hence the "field is not NULL" code
is never branched to.


Debugging. When I do a

printf("0x%0X\n", docs_data_nullfld.SUBTITLE);

Im getting a 0x1 when the field is NULL, and 0x2 when it's not. Neither in
ASCII nor in EBCDIC, numerals are encoded in character positions 0x1 or 0x2.

This leaves me greatly confused.


When I do a

if (docs_data_nullfld.SUBTITLE != '1') {
do things if field is not NULL …

branches to the correct code. Duh! So I basically got it to work.


I understand that the documentation mentions just the NULL case associated
with an '1', but not that a field not being NULL is actually '0'. My bad.

If someone can shed some light upon the seemingly bogus values being
output by my "hex" printf, I'm most interested!

Thanks!

:wq! PoC


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