Re: Strange bind_param issue

On 15/08/12 16:14, Martin J. Evans wrote:
> I've just been given an rt
> https://rt.cpan.org/Ticket/Display.html?id=78838 and am at a loss to
> explain exactly what is happening. I wonder if anyone can help?
>
> Some background:
>
> DBI says for bind_param:
>
> "The bind_param method takes a copy of $bind_value and associates it
> (binds it) with a placeholder"
>
> As far as I am aware DBD::ODBC does not copy the scalar given to it - so
> perhaps DBI does this. The problem I'm seeing in the provided example is
> the pointer passed to ODBC's SQLBindParameter at the time bind_param is
> called no longer points to a valid string when execute is called.
> However, changing the call to bind_param to pass $obj as "$obj" appears
> to fix the problem. Can anyone say if DBD::ODBC should work with either
> example and explain what might be happening here:
>
> use DBI;
> my $dbh =
> DBI->connect("dbi:ODBC:DSN=xxx;UID=xx;PWD=yy;MARS_Connection=No;");
> my $obj = new Object();
> my $sql = q(SELECT ? AS result);
> my $sth = $dbh->prepare($sql);
>
> # interesting - is the following use of $obj really as a string?
> # in the call to bind_param here, DBD::ODBC binds the pv ptr of the scalar
> # it is given in a SQLBindParameter call but by the time execute is called
> # the string at that address is no longer valid. I kind of expect that as
> # what to_s returns is only required in the bind_param statement and yet
> # DBI says "bind_param takes a copy".
> # However if the following is changed to "$obj" it works
> $sth->bind_param(1, $obj);
> $sth->execute();
>
> while (my $row = $sth->fetchrow_hashref()) {
>    print $row->{'result'}, "\n";
> }
>
> package Object;
> use overload '""' => 'to_s';
> sub new() { bless { }, shift };
> sub to_s() { my $self = shift; ref($self); }
>
> Output when using $obj:
>
> value passed to DBD::ODBC's bind_param = "Object"
> pointer at execute time seems to point to rubbish
> output of script: 8�k�8
>
> When "$obj" passed to bind_param
>
> value passed to DBD::ODBC's bind_param = "Object"
> pointer at execute time points to "Object"
> output of script: Object
>
> As a quick test I did the following and it seems to work so I guess
> there is something about the above Perl I don't get.

$obj is not a string. It is an object of a class which has a
stringify operator. "$obj" is a string, because "..." stringifies.

It is not at all clear how the DBI should take a copy of an object.

I think this is a case of user error.

-- 
Charles Jardine - Computing Service, University of Cambridge
[email protected]    Tel: +44 1223 334506, Fax: +44 1223 334679