[fpc-devel] TMySQLConnection.LoadField field length

Joost van der Sluis joost at cnoc.nl
Sun Jun 5 18:51:11 CEST 2005

Sorry for the late reply..

On Thu, 2005-05-19 at 19:03 +0200, Bram Kuijvenhoven wrote:
> After understanding what a VARCHAR is (namely some sort of shortstring
> with a fixed maximum length), I wanted first to know whether a newer
> MySQL server reports proper field lengths for fields like 'FORMAT
> (mydoublefield,3) AS myalias', so I switched to a 4.1 server.

If you store the string 'hello' in a varchar(1000) it only needs 9 bytes
of storage-space, while using char(1000) you would need 1000 bytes.
In memory, however, fpc indeed uses a shortstring.

> However this didn't work, because I needed a newer client also,
> because the older client didn't support the newer authentication
> protocol requested by the server (according to the error message I
> got).
> So I used a newer libMySQL.dll file and adjusted mysql4dyn to
> accepting this version. But now I got a authentication error for user
> ODBC at localhost. However I didn't connect as user ODBC?!?

That check isn't there for nothing. 

> The I figured out the TMySQLConnection object connected twice to the
> database, once in ConnectToServer and once in PrepareStatement. The
> second time it tries to fetch the Host, UserName, Password from the
> FMySQL:PMySQL field. But there seem to be nil values in there... and I
> guess MySQL somehow tries user ODBC because no username is given.

That's because they've changed the PMySQL-type. (Like they change
everything, always - just like they want)

> I also tried loading the newer dll statically, but then I had to
> comment out a lot more functions in mysql4 and mysql4_com than before,
> in particular some slave, master and the hash_password functions.
> So my findings are:
> - the fpc mysql headers contain way too much function headers, which
> causes 'entry point of function blablabla not found' error when
> statically loading the library. I think it's better only to try to
> import the functions which are also documented as part of the C API at
> http://dev.mysql.com/doc/mysql/en/c.html.
> - the fpc headers contain declarations of structs (records) that are
> not in the MySQL documentation. The only struct described there is the
> MYSQL_FIELD struct (or: TMySQL record), besides the other types of
> which no members are listed. Perhaps using the other structs isn't
> very safe and prone to change.
> - mysql4dyn only accepts a 4.0 library and no a 4.1 library; I hope
> this can be made more flexible...

> I understand the fpc team has become tired of mysql header changes and
> incompatibilities, but this problem does not seem to exist with e.g.
> the php mysql interface. Maybe the problems can be solved by only
> importing and functions that are also in the documentation. (This
> would require manual action instead of simply applying h2pas to
> mysql.h)

> Do you think this is an approach with a chance to success?

No. The idea is good though. I think I can use it with the problem of
what functions/procedures need to be put into the headers.

But, the largest problem is all those things which aren't documented. 
Example, in http://dev.mysql.com/doc/mysql/en/c-api-datatypes.html you
can find that 'field' can be one of the following: FIELD_TYPE_TINY,

But it could be that for MySQL 4.0 FIELD_TYPE_TINY is defined as 0,
while in 4.1 it's defined as 1. 

So for 4.0 it should be:

const FIELD_TYPE_TINY  : 0;

while for 4.1 it is:

const FIELD_TYPE_TINY  : 1;

And why they keep changing that sort of things, I really don't get. And
these differences you can only see in the .h's.

(I made this example up, but those differences do exist)

> MySQL is widely used and therefore it would be nice if it can be
> supported by fpc. (In fact, I've seen people asking things about mysql
> on the Lazarus mailing list several times)

Me too.

> After getting the mysql headers right and 'compatible', I could try to
> create a better TMySQLConnection.

I'm gonna throw out all non-documented procedures. (Well, they are
documented mostly, but elsewhere) And use diff on the .h's

Met vriendelijke groeten,

  Joost van der Sluis
  CNOC Informatiesystemen en Netwerken
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