Strange bind_param issue
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# the string at that address is no longer valid.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.
use DBI;
use strict;
use warnings;
use Data::Dumper;
my $h = DBI-connect('dbi:ODBC:baugi','sa','easysoft', {RaiseError = 1});
eval {
$h-do(q/drop table mje/);
};
$h-do(q/create table mje (a int)/);
my $s = $h-prepare(q/insert into mje values(?)/);
{
my $a = 99;
$s-bind_param(1, $a);
$a = 66;
}
$s-execute;
print Dumper($h-selectall_arrayref(q/select * from mje/));
outputs:
$VAR1 = [
[
99
]
];
Martin
--
Martin J. Evans
Easysoft Limited
http://www.easysoft.com
Re: Strange bind_param issue
On Wed, Aug 15, 2012 at 04:14:52PM +0100, 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.
The driver should take a copy of the value when bind_param is called.
Perhaps stored by the ParamValues attribute.
(bind_param_inout takes a reference instead and binds at execute.)
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); }
The driver needs to make a copy and ensure that 'get magic' is called.
SvPV will call get magic but won't make a copy. Since you need an SV for
the ParamValues attribute maybe newSVsv() followed by SvPOK_only_UTF8()
on the copy would be a suitable incantation.
Tim.
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
c...@cam.ac.ukTel: +44 1223 334506, Fax: +44 1223 334679
Re: Strange bind_param issue
On Wed, Aug 15, 2012 at 06:02:35PM +0100, Charles Jardine wrote: $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. Without any method attributes in the bind_param call the driver is free to 'do something reasonable' which typically means treat the value as a string (or a number if it knows the field is numric). I think this is a case of user error. It would be reasonable for a driver to complain if passed an object that doesn't have string/numeric overloading, but this one does. Tim.
Re: Strange bind_param issue
On 15/08/2012 18:41, Tim Bunce wrote: On Wed, Aug 15, 2012 at 06:02:35PM +0100, Charles Jardine wrote: $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. Without any method attributes in the bind_param call the driver is free to 'do something reasonable' which typically means treat the value as a string (or a number if it knows the field is numric). I think this is a case of user error. It would be reasonable for a driver to complain if passed an object that doesn't have string/numeric overloading, but this one does. All points taken so thanks for that but in this case although the object does have a stringification overload as far as I can see it was not used i.e., it was bind_param(1, $obj) not bind_param(1, $obj) as I received the code. If you use the latter, DBD::ODBC works but the point I'm still uncertain about is that IF DBD::ODBC has not made a copy of the the scalar passed to bind_param then how is it working. I will look further but as far as I can see DBD::ODBC is simply looking at the pv on the passed scalar and passing that pointer to SQLBindParameter and by the time SQLExecute is called Perl may have removed the now unreferenced scalar passed to bind_param. Having said that, I must surely be wrong on how DBD::ODBC does this or my other example which deliberately passes a scalar which a) is changed after the bind_param call and b) goes out of scope before the execute call and yet inserts the value in the scalar passed to bind_param at the time bind_param is called (that is why I added that example). Hopefully, this explains why this confused me a little. I did not write the original bind_param code and need to work my way through it. Martin
