I have recently upgraded from XE4 to XE8 and have come across a access violation when assigning a itemindex to a radio group. I was curious why in XE4 this works and XE8 it doesn't. All of the forms have been created when the main program starts up. This code works fine when debugging but when running as a standalone it throws the exception.
with TravelBookingForm do begin
try
rg1.itemindex:=tag-1;//not sure which causes the access violation
except
on E : Exception do
begin
showMessage(E.Message); //access violation message is shown
end;
rg1.tag := 0;
end;
end;
NB: I have omitted code that isn't relevent
So after more testing I can confirm that rg1 is not nil and that it is the item causing the access violation.
If rg1 is not nil and yet leads to an access violation then the most plausible explanation is that rg1 points to memory that has been freed. This matches the observation that the error occurs sometimes (outside debugger) and not others (under debugger).
So, your program is destroying the form at some point, and then later referring to the form.
Clearly that is an error in your program and you will need to make sure that you never refer to forms after they have been destroyed. Unfortunately the IDE encourages you to create all your forms once at startup, and hold references in global variables. This makes it all too easy to have stale references.
If you used the full debug version of FastMM then that tool would be able to warn you when you attempt to access memory that has been freed.
Related
My DES model was working well till I've added a new function (just like many others added before) and the model gave me all these error messages:
I wonder what do these messages mean and what made all of them appear suddenly. I was running the model just before it without any errors. Is there a way I could get my model back? :)
Thank you.
Edit 1:
Here is the newly created function: Newly created function
Given that I've ignored it, and made all the callings of it as comments, then the errors were still appearing. I've closed this model version and opened a backup then created the same function and the whole scenario happened again with the same 94 errors.
Is there something I need to change in the function itself?
The function checks the number of agents in certain parts of the model to stop delay blocks accordingly. That's done by adding variables that increment on exiting of agents and get the difference between them.
Typically this is caused by a syntax error in the new function that you created. See if you ignore this and then compile if it gets fixed.
Most of the time the compiler is smart enough to show you the error, but often if you replace a { with [ or you leave out a { the compiler get very confused and gives you 100s of errors.
If yous still cant find the issue maybe paste the code of your new function in the question.
I am trying to create an application with 2 forms in Delphi XE6. Depending on a ParamStr setting Form2 may or may not be shown before Form1.
In a quick test app both forms are created before Form1.Show is invoked - during which Form2 is shown or not
procedure TForm1.FormShow(Sender: TObject);
begin
if ParamStr(1) = 'foo' then
Form2.FooShow;
end;
procedure TForm2.FooShow;
begin
ShowModal;
end;
However in the "real" application I am seeing a different behaviour.
In this case Form1.Show is being called as soon as Application.CreateForm(TForm1, Form1) is called. This is before Form2 is being created, which is causing problems as Form2 doesn't exist when it is needed.
Any explanation why the behaviour would differ? Am I missing a setting buried somewhere in Project>Options
If your form is persisted with Visible set to True, then it will be shown as soon as it's created.
Setting the property to False should resolve your problem.
EDIT
PS: Just in case someone with a similar problem has their main form unexpectedly show even though Visible is set to False. This happens because by default the application will show the main form regardless of its Visible property in the call to Application.Run.
If so, the following question should help: How can I start Delphi application with the hidden main form?
EDIT2
For the sake of completeness, there are a couple other things that could cause a form to be shown as soon as it's created. However, these probably aren't applicable to this specific question.
Any code that explicitly shows the form when it's created (such as the OnCreate event) would obviously cause the form to be shown. However, one would hope that such actions don't lead to these kinds of questions.
An MDI child form can never be hidden. At best it can be minimised.
TIP
The quickest way to finding the answer to such questions is usually just a little bit of debugging.
Set a break-point in your FormShow method.
Go to Compiler Settings, and enable the option to build with debug DCU's. (You'll want to see the VCL code.)
Rebuild and run your application.
When you get to your break-point, open the Call Stack debug window.
Navigate the call-stack looking for the trigger.
In this case you should have found the following code in Forms.pas.
procedure TCustomForm.DoCreate;
begin
//...
if fsVisible in FFormState then Visible := True; //<-- The trigger
end;
And a little more investagation on fsVisible would reveal the root cause as: The Visible property is set to True.
That said, you don't want to be coding this way because you're creating dependencies via globals. This is error-prone; and your little experiment shows shows just one of many subtle things that can cause problems.
Rather avoid the globals with something like the following changes in your DPR:
begin
Application.Initialize;
ShowForms;
end;
Where ShowForms is implemented as:
procedure ShowForms;
var
LForm1: TForm1;
LForm2: TForm2;
begin
Application.CreateForm(TFrom1, LForm1);
Application.CreateForm(TFrom2, LForm2);
if (ParamStr(1) = 'foo') then
LForm2.Show
else
LForm1.Show;
end;
If you don't have any dependencies between the forms, the above will suffice.
If you do have a dependency, e.g. Form2 uses Form1: then you can explicitly pass a reference after creating the forms, bu before you start doing anything with them.
//Define a property on TForm2 E.g.
property MainForm: TForm1 read FMainForm write SetMainForm;
//Immediately after creating both forms tell form2 which to use as its main form.
LForm2.MainForm := LForm1;
I have 2 pieces of code :
It works normal
with ADOTemp do
begin
SQL.Clear;
SQL.Add('INSERT INTO documents');
SQL.Add('(document_date,fk_id_status,money_direction,');
SQL.Add('paid,addition,saving,fk_id_base,fk_id_user)');
SQL.Add('VALUES ');
SQL.Add('(CONVERT(DATE,GETDATE(),103),:pfk_id_status,:pmoney_direction,');
SQL.Add('0,0,0,'+IntToStr(p_id_base)+',:pfk_id_user)');
Parameters.ParamByName('pfk_id_status').Value := p_id_status;
Parameters.ParamByName('pmoney_direction').Value := p_money_direction;
// Parameters.ParamByName('p').Value := p_id_base;
Parameters.ParamByName('pfk_id_user').Value := fMain.ApplicationVariablers.user_id;
ExecSQL;
end;
It does not work and returns a terrible error
with ADOTemp do
begin
SQL.Clear;
SQL.Add('INSERT INTO documents');
SQL.Add('(document_date,fk_id_status,money_direction,');
SQL.Add('paid,addition,saving,fk_id_base,fk_id_user)');
SQL.Add('VALUES ');
SQL.Add('(CONVERT(DATE,GETDATE(),103),:pfk_id_status,:pmoney_direction,');
SQL.Add('0,0,0,:p,:pfk_id_user)');
Parameters.ParamByName('pfk_id_status').Value := p_id_status;
Parameters.ParamByName('pmoney_direction').Value := p_money_direction;
Parameters.ParamByName('p').Value := p_id_base;
Parameters.ParamByName('pfk_id_user').Value := fMain.ApplicationVariablers.user_id;
ExecSQL;
end;
Error on image, line 1917
It seems that this question is in reference to your earlier question: Delphi, error :"Access violation at address xxxxxxxx. Read of address yyyyyyyy", at AdoQuery.SQL.Text:='''
You had added and accepted an answer there along the lines seen in this question. The error message, an AV in msvcrt.dll is indicative of a serious programming error. Perhaps a heap corruption. Perhaps something else. The change you made that stops the error occurring does not really fix the problem. The problem will still be there, lying dormant. You just got (un)lucky that the change you made appeared to fix the problem.
Access violations are not always reproducible. That's just their nature. When you encounter one you need to understand why it happens. Simply using trial and error to re-organise code from a completely different location will never lead to the real solution.
What you should be doing is tracking down the real cause of the problem. Do that, fix it, and either version of the code above will work. This is probably not the answer you are wanting to get. You may very well not want to accept what I say. But, speaking from experience, until you recognise that you have a more serious problem, you will make no headway here.
Looking at your code in the bitmap...I'm pretty sure you access violation is because of what David just said...you have serious Memory problems in your code...Your not freeing your objects...that you are creating...Anytime you pass a nil reference to Owner in a TComponent Constructor...your telling the compiler...that you know what your doing and will free it when your finished with it...Matter of fact good practice is to always free objects that you declare and use exclusively in the scope of your method.
procedure TForm1.MyMethod;
var
a_MyComp: TMyComp;
begin
a_MyComp := TMyComp.Create(nil);
Try
//use my a_MyComp...
Finally
a_MyComp.Free;
End;
end;
Check out your code in CreateNewDocument...you'll notice that your not freeing your AdoTemp.
-Rick
Access violation exceptions (AVs) tell you that you have made a mistake in the memory access of your program. However, the way access violations work: they are unable to guarantee always detecting the error every time you make a mistake. (Sometimes you get un-lucky and no access violations are raised, but the mistake is still there causing other things to quietly go wrong inside your application.)
You can think of the memory available to your program being represented as below (where "." means the memory is not allocated to anything, and "A" means the memory is allocated to something within your program: e.g. object, local variable, parameter, machine code).
[.......AAA..AAAA.....AA......A...A...........AAA....A.......AA.........AAAAA]
Suppose you create some object; this will require memory to be allocated for the object itself. If the object in turn creates child objects, this will also be allocated in memory. (I'll use "O" and "C")
/-ref--\
[.......AAA..AAAAO....AAC.....A...A.....C.....AAA....A.......AA.........AAAAA]
\-ref------------------/
Note that within the memory allocated to O, it might hold references to its child objects.
Conversely to the above, whenever an object is destroyed, its memory is deallocated. Let us suppose you have made a mistake in your memory access, and something has destroyed one of O's child objects before O has finished using it.
/-ref--\
[.......AAA..AAAAO....AAC.....A...A...........AAA....A.......AA.........AAAAA]
\-ref------------------/
If O now tries to use its second child object, you will get an access violation. However, you might be un-lucky and not get an access violation showing your earlier mistake IF:
You destroy O without it trying to do anything to the second child object.
Or you first create a new object that happens to be allocated in the exact same place the child object was.
The second situation tends to be worse than the first, because every time C2 is used on the assumption that it is the correct child of O: unexpected results are produced and an incorrect values are written in memory. These incorrect values may be important data, or references to other objects (making the problem get worse over time).
So: Whenever yo do get an access violation, thank your lucky stars and hunt down the root cause of the problem.
NB! NB! I cannot stress the importance of the above enough.
Investigating your particular problem
First note, when an exception pauses in the debugger, the code usually points to the next line that would have been executed if not for the exception. (This doesn't affect anything here, because the previous line is also ADOTemp.SQL.Add; - the point is be prepared to consider the previous line as the cause of an exception - and test to confirm it!)
Useful tip: When an exception is thrown within Delphi/Third Party code, it can be useful to build with Debug DCU's or recompile Third Party source with debug information to get closer to the actual line raising the exception. (This can even be useful in situations like this where the error appears to be inside a Microsoft DLL.)
Looking at your screen-shot, the exception is thrown from a brand new instance of TADOQuery. Now there really isn't any sensible explanation for a brand new query to be throwing access violations when simply adding text to its SQL query. This strongly implies a corruption problem as described earlier.
One possibility would be another thread interfering with this one. (NOTE: even if your application isn't multi-threaded, the ADO objects do have built-in support for asynchronous operations.) However, I'm going to ignore that possibility for now because threading issues tend to be less consistent than you've implied this one is.
So, assuming you have a memory access problem somewhere else that is only manifesting here with a brand new TADOQuery (used correctly at least up to the point of the AV) - what other objects are interacting with this query to possibly cause corruption?
ADOTemp.Connection := fMain.ADOConnection;
There is a very strong possibility that commenting out the above line would also eliminate your access violation. So what are the possible problems:
fMain might have been destroyed prematurely and is now a dangling pointer meaning the code to return ADOConnection could do any of a number of unexpected things.
Any of the code backing fMain.ADOConnection may be referencing corrupted memory or dangling pointers; this includes the returned connection itself.
NOTE: One very common cause of invalid memory access is to have a function such as GetADOConnection that does not correctly initialise its Result. So in some cases it returns a 'random' address in memory, resulting in all sorts of unexpected behaviour when something tries to use the connection.
Is your ADOConnection created on a different thread, and being used by multiple threads?
PS: Don't forget to follow Rick's advice, and make sure you destroy the query when you've finished using it. Who knows, there might be an internal bug in ADO that when it runs out of a particular internal resource (due to queries not being destroyed), that causes ADO to start throwing AVs.
NOTE: You should be able to test my theory without altering functionality by simply changing the ADOTemp.Connection := ... line. Simply assign a connection string to the query instead of a connection object. However, if that solves (or more correctly stated: hides) the problem, please follow my advice, and hunt down the root cause of the AV.
I have a wxWidgets/GTK based application that works well - except for one installation on an Debian Squeeze ARM system. There it crashes when the user just activates the main window of it. To find the reason for that I added a signal handler to the application and use libunwind out of that signal handler to find the source for the crash. During a test that worked fine, when the software writes e.g. to address 0x0 libunwind correctly points me to the function where that happens.
But the results for the system where the crash appears unexpectedly are a bit strange, they seem to happen outside of my application. One crash comes from a function with no name (here libunwind returns an empty string), and one is caused by "malloc_usable_size", a system function which should never die this way.
So...what to do next? All ideas, suggestions or any other hints are welcome since I'm not sure how to contunue with that problem...
Check for buffer overrun or overwriting some memory unexpectedly for any structures, pointers, memory locations for items returned by library functions.
Check for invalid pointer frees in your code for the library allocated pointers that you are using.
May be using valgrind would also help.
I run into this regularly, and am just looking for best practice/approach. I have a database / datamodule-containing app, and want to fire up the database/datasets on startup w/o having "active at runtime" set to true at design time (database location varies). Also run a web "check for updates" routine when the app starts up.
Given TForm event sequences, and results from various trial and error, I'm currently using this approach:
I use a "Globals" record set up in the main form to store all global vars, have one element of that called Globals.AppInitialized (boolean), and set it to False in the Initialization section of the main form.
At the main form's OnShow event (all forms are created by then), I test Globals.AppInitialized; if it's false, I run my "Initialization" stuff, and then finish by setting Globals.AppInitialized := True.
This seems to work pretty well, but is it the best approach? Looking for insight from others' experience, ideas and opinions. TIA..
I generally always turn off auto creation of all forms EXCEPT for the main form and possibly the primary datamodule.
One trick that I learned you can do, is add your datamodule to your project, allow it to auto-create and create BEFORE your main form. Then, when your main form is created, the onCreate for the datamodule will have already been run.
If your application has some code to say, set the focus of a control (something you can't do on creation, since its "not visible yet") then create a user message and post it to the form in your oncreate. The message SHOULD (no guarantee) be processed as soon as the forms message loop is processed. For example:
const
wm_AppStarted = wm_User + 101;
type
Form1 = class(tForm)
:
procedure wmAppStarted(var Msg:tMessage); message wm_AppStarted;
end;
// in your oncreate event add the following, which should result in your wmAppStarted event firing.
PostMessage(handle,wm_AppStarted,0,0);
I can't think of a single time that this message was never processed, but the nature of the call is that it is added to the message queue, and if the queue is full then it is "dropped". Just be aware that edge case exists.
You may want to directly interfere with the project source (.dpr file) after the form creation calls and before the Application.Run. (Or even earlier in case.)
This is how I usually handle such initialization stuff:
...
Application.CreateForm(TMainForm, MainForm);
...
MainForm.ApplicationLoaded; // loads options, etc..
Application.Run;
...
I don't know if this is helpful, but some of my applications don't have any form auto created, i.e. they have no mainform in the IDE.
The first form created with the Application object as its owner will automatically become the mainform. Thus I only autocreate one datamodule as a loader and let this one decide which datamodules to create when and which forms to create in what order. This datamodule has a StartUp and ShutDown method, which are called as "brackets" around Application.Run in the dpr. The ShutDown method gives a little more control over the shutdown process.
This can be useful when you have designed different "mainforms" for different use cases of your application or you can use some configuration files to select different mainforms.
There actually isn't such a concept as a "global variable" in Delphi. All variables are scoped to the unit they are in and other units that use that unit.
Just make the AppInitialized and Initialization stuff as part of your data module. Basically have one class (or datamodule) to rule all your non-UI stuff (kind of like the One-Ring, except not all evil and such.)
Alternatively you can:
Call it from your splash screen.
Do it during log in
Run the "check for update" in a background thread - don't force them to update right now. Do it kind of like Firefox does.
I'm not sure I understand why you need the global variables? Nowadays I write ALL my Delphi apps without a single global variable. Even when I did use them, I never had more than a couple per application.
So maybe you need to first think why you actually need them.
I use a primary Data Module to check if the DB connection is OK and if it doesn't, show a custom component form to setup the db connection and then loads the main form:
Application.CreateForm(TDmMain, DmMain);
if DmMain.isDBConnected then
begin
Application.CreateForm(TDmVisualUtils, DmVisualUtils);
Application.CreateForm(TfrmMain, frmMain);
end;
Application.Run;
One trick I use is to place a TTimer on the main form, set the time to something like 300ms, and perform any initialization (db login, network file copies, etc). Starting the application brings up the main form immediately and allows any initialization 'stuff' to happen. Users don't startup multiple instances thinking "Oh..I didn't dbl-click...I'll do it again.."