FreeTDS Development Group

["Lowden, James K" <LowdenJK AT bernstein.com>]

> From: James Vanns [mailto:jimv AT canterbury.ac.uk]
> Sent: January 19, 2004 1:34 PM
> 
> OK, so shoot me but I'm confused by the TDS error handling/catching. 

It's only software, James.  In the history of man, no one's been shot over
software.  ;-)

> For example, if I send a query to the server with tds_query_submit and
> that query fails (incorrect table name say) what would the return code
> be? 

The return code would indicate success, unless the query were not accepted
by the server.  How things turned out for the query would be the subject of
the server's response.  An incorrect table name would engender a message
packet from the server, which would be routed to tds_process_msg().  

> if the SQL command succeeds but returns no rows what then? 

Well, the query "create table #t (t int)" is not expected to return rows,
and doesn't, and is not an error.  The response packet from the server can
indicate whether rows will be forthcoming, but in any case the
absence/presence of a resultset packet settles the matter.  

> I
> realise that the err_handler of the TDSCONTEXT structure can 
> print error
> messages but what if I actually want to handle the error by using some
> code?

I'm not sure I understand what you mean.  Your handler is "some code", and
it can do anything it wants, be it print messages or whatever, eh?  

I think you're asking: I send a query, the query has a problem, the problem
is identified not to my mainline code but to my message handler; how does my
message handler communicate that error back to the mainline code in good
time?  

The answer is, basically, set a flag that your mainline code will inspect.
There's no race condition: the server will send its error packets ahead of
any data, so your row-processing logic won't be invoked before your message
handler.  

--jkl


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