Re: Blobs: How to tell when I retrieved a null blob

>From: Jacob Salomon <jake@apparel.net> >Date: Thu, 13 Feb 1997 10:17:37 GMT >X-Informix-List-Id: <news.33861> > >2 questions, after verbose explanation. > >I am writing a small program to insert & retrieve blobs. Right now it's >very simple: When a user wants to insert a blob, they type the name of >a .gif file in a field in the form. My program LOCATEs the blob >variable in the file and does the insert. > >If the user omits the file name, I still do the insert but first I >locate the blob variable in /dev/null. I have confirmed that when I >locate in /dev/null, the onstat -d shows no fewer free blob pages. So >this works, though I'm sure there is a better way. My first question is: >1. Is there something I can do in 4GL to specify a null location, so > that my code won't even bother opening /dev/null? (For that matter, > are the 4GL library functions smart enough to recognize /dev/null and > skip opening it already?) Yes, but be very careful. DEFINE blob BYTE LOCATE blob IN FILE LET blob = NULL After you've set the blob to NULL, it stays NULL permanently (there was a bug entered about this some time ago; I forget the number). I think you have to re-locate the blob to unset the null-ness pointer, or drop into C (ESQL/C) code passing the blob (by reference, not value!) as an argument, and therefore using poplocator() in the C code, and fiddle the indicator portion of the loc_t structure you are given. >Next is the issue of how I recognize a null blob when I read it back in. >For now, I have hard coded the location before fetching the blob i.e. >Before fetching it I code: > LOCATE blobvar IN FILE "yutz.blob" > FETCH blobcurs INTO blobvar > >2. How can I recognize when the blob data is null? In ESQL I could look > at the length member variable of the locator structure (I think) You could look at the length, but that will only tell you whether you have an empty blob; an empty blob is not null. You should look at the indicator field -- that tells you whether the blob is null or not. Similarly, a zero-length VARCHAR field is different from a null VARCHAR at the database storage level -- the zero-length field is stored as '\\0', the null is stored as '\\001', '\\0'. > but what do I look at in 4GL to see a 0-length? BTW, when I do the > retrieval, I can see that the file "yutz.blob" is indeed 0 bytes > long. I don't see anything in the sqlca that would warn me. IF blobvar IS NULL THEN ... END IF Sometimes, it pays to try the obvious:-) Yours, Jonathan Leffler (johnl@informix.com) #include <disclaimer.h>