When connecting to Oracle DB I get External: SIGSEGV error - freepascal

Question

I'll say how I reproduce the problem on lazarus.
I have a form and a datamodule using zeos to enstablish a connection with a local oracle db.
The problem born when I put some code to interlocute with the db.
Here is an example:
OracleMng.ZQuery1.SQL.Clear;
That is exactly the line going in error.
Here is the full code of the form:
unit form1;
{$mode objfpc}{$H+}
interface
uses
Classes, SysUtils, Forms, Controls, Graphics, Dialogs, DBGrids, StdCtrls,
datamodule2;
type
{ TLogin }
TLogin = class(TForm)
Button1: TButton;
DBGrid1: TDBGrid;
procedure Button1Click(Sender: TObject);
private
public
end;
var
Login: TLogin;
implementation
{$R *.lfm}
{ TLogin }
procedure TLogin.Button1Click(Sender: TObject);
begin
OracleMng.ZQuery1.SQL.Clear;
end;
end.
Here is the code of the datamodule:
unit datamodule2;
{$mode objfpc}{$H+}
interface
uses
Classes, SysUtils, DB, ZConnection, ZDataset, ZSqlMonitor;
type
{ TOracleMng }
TOracleMng = class(TDataModule)
DataSource1: TDataSource;
ZConnection1: TZConnection;
ZQuery1: TZQuery;
private
public
end;
var
OracleMng: TOracleMng;
implementation
{$R *.lfm}
{ TOracleMng }
end.
I'm trying
if (OracleMng <> Nil) and (OracleMng.Zquery1 <> Nil) then OracleMng.ZQuery1.SQL.add('select * from help');
if (OracleMng <> Nil) and (OracleMng.Zquery1 <> Nil) then OracleMng.ZQuery1.ExecSQL;
dbgrid1.refresh;
I have no more errors but the DBGrid1 is not filled.
This is my project lpr file:
program project1;
{$mode objfpc}{$H+}
uses
{$IFDEF UNIX}{$IFDEF UseCThreads}
cthreads,
{$ENDIF}{$ENDIF}
Interfaces, // this includes the LCL widgetset
Forms, zcomponent, datamodule2, form1
{ you can add units after this };
{$R *.res}
begin
RequireDerivedFormResource:=True;
Application.Scaled:=True;
Application.Initialize;
Application.CreateForm(TLogin, Login);
Application.Run;
end.

Answer 1

The fact that the change I suggested in my comment, namely
if (OracleMng <> Nil) and (OracleMng.Zquery1 <> Nil) then
OracleMng.ZQuery1.SQL.Clear
evidently stopped you getting the SIGSEGV error suggests that your DataModule and
form are being created in the wrong order, i.e. form first. Check this out by going to
Project | View Source in the IDE. If you see something like
program MyProgram;
[...]
begin
Application.Initialize;
Application.MainFormOnTaskbar := True;
Application.CreateForm(TForm1, Form1);
Application.CreateForm(TDataModule1, DataModule1);
Application.Run;
end.
they are in the wrong order, so swap the two CreateForm lines
Application.CreateForm(TDataModule1, DataModule1);
Application.CreateForm(TForm1, Form1);
With that change, you should no longer need the
if (OracleMng <> Nil) and (OracleMng.Zquery1 <> Nil) then`
Next thing: You seem to be confused about when to use
ZQuery1.ExecSQL
and
ZQuery1.Open
Open is intended for when the SQL statement you are using produces a result set, that is
a collection of records which can be viewed in a TDBGrid. The most usual way to do this
is to use a SELECT statement as in
ZQuery1.SQL.Text := 'select * from MyTable';
ZQuery1.Open;
ExecQuery is intended for use where your SQL statement performs some operation on the database
which does not involve SELECTing records. The most common SQL statements which need ExecSQL are
UPDATE
INSERT
DELETE
though there are others, for example statements which execute stored procedures on the SQL Server
(note that some stored procedures return result sets and so need Open, rather than ExecSQL).
Note that ExecSQL will clear out any records which are in the dataset (ZQuery1) so after
you need to do Open again using a suitable SQL statement
var
S : String;
begin
S := 'update MyTable set number = number +1 where id = 5';
ZQuery.SQL.Text := S;
ZQuery1.ExecSQL; // no records shown in DBGrid1 from here
S := 'select * from MyTable';
ZQuery.SQL.Text := S;
ZQuery1.Open; // records shown in DBGrid1 again
end;
Note that I do
S := 'select * from MyTable';
ZQuery.SQL.Text := S;
instead of
ZQuery1.SQL.Clear;
ZQuery1.SQL.Add('select * from myTable');
The reason for this is that it's much easier to see the whole SQL statement in the debugger by
inspecting the variable S than inspecting the ZQuery1.SQL.Text property and much easier to
see any syntax errors.
You should always Close a dataset that you've Opened once you have finished working with it as it ensures what the data on disk is up to date. if the last SQL operation was ExecSQL, you don't need to close the dataset.
If you set the query's Text property the way I do, with ZQuery1.SQL.Text, you don't need to uses Clear. In any case, it is only equivalent to doing ZQuery1.SQL.Text := '' and it does not affect the state of the dataset - it only does anything when you call ExecSQL or Open.