Getting Result from Database using DLL library

[fiazhnd]

I am trying to access a value from sqlite3 database through dll library, but getting an error "raised exception class 'External: SIGSEGV'. "

Can anyone help me 

Code: Pascal  [Select][+][-]

1. unit Unit1;
2.  
3. {$mode objfpc}{$H+}
4.  
5. interface
6.  
7. uses
8.   Classes, SysUtils, Forms, Controls, Graphics, Dialogs, StdCtrls, ExtCtrls, Buttons, strutils, DynLibs;
9.  
10. type
11.   { TForm1 }
12.   TForm1 = class(TForm)
13.     BtnCodeBySerial: TButton;
14.     EditRCode: TEdit;
15.     EditVSerial: TEdit;
16.     procedure BtnCodeBySerialClick(Sender: TObject);
17.   private
18.     { private declarations }
19.  
20.   public
21.  
22.   end;
23.  
24. var
25.   Form1: TForm1;
26.  
27. implementation
28.  
29. type
30. TMyDLL = function (var RevFRCode: String): String; stdcall;
31.  
32. {$R *.lfm}
33.  
34. { TForm1 }
35.  
36. procedure TForm1.BtnCodeBySerialClick(Sender: TObject);
37. var
38. RadioCode: String;
39. MyHandle: TLibHandle;
40. MyDLLFUNC: TMyDLL;
41. begin
42.    RadioCode:= EditVSerial.Text;
43.    MyHandle := SafeLoadLibrary('drofrc.dll');
44.    if MyHandle <> 0 then
45.      begin
46.       MyDLLFUNC := TMyDLL(GetProcedureAddress(MyHandle, 'FRCBySerial'));
47.       if Assigned (MyDLLFUNC) then
48.       begin
49.       MyDLLFUNC(RadioCode);
50.       end;
51.       end
52.       else
53.       Begin
54.       ShowMessage('Function is not Found');
55.       end;
56.       FreeLibrary(MyHandle);
57.       EditRCode.Text:= Radiocode;
58. end;
59. end.
60.  

Library

Code: Pascal  [Select][+][-]

1. library drofrc;
2.  
3. {$mode objfpc}{$H+}
4.  
5. uses
6.   LCLType, StdCtrls, Classes, SysUtils, sqlite3conn, sqldb, db, FileUtil, Forms, Controls, Interfaces, Windows;
7.  
8. var
9.     SQLite3Connection1: TSQLite3Connection;
10.     SQLQuery1: TSQLQuery;
11.     SQLTransaction1: TSQLTransaction;
12.  
13. function FRCBySerial(FRCode: String): String; stdcall;
14. var
15.     LResult: String;
16. begin
17.   Result:= '';
18.   //Creating Control
19.   SQLite3Connection1:= TSQLite3Connection.Create(Nil);
20.   SQLTransaction1:= TSQLTransaction.Create(Nil);
21.   SQLQuery1:= TSQLQuery.Create(Nil);
22.  
23.   SQLite3Connection1.DatabaseName:= 'newWE.db';
24.   SQLite3Connection1.Transaction:= SQLTransaction1;
25.   SQLTransaction1.Database:= SQLite3Connection1;
26.  
27.   SQLQuery1.Database:= SQLite3Connection1;
28.   SQLQuery1.Transaction:= SQLTransaction1;
29.   SQLQuery1.Close;
30.   SQLQuery1.SQL.Text:= ('Select RCode From FordRCBSerial Where VSerial = ' + FRCode);
31.   SQLite3Connection1.Connected:= True;
32.   SQLTransaction1.Active:= True;
33.   SQLQuery1.Open;
34.  
35.   LResult := SQLQuery1.Fields[0].AsString;
36.   SQLite3Connection1.Open;
37.   Result:= LResult;
38. end;
39.  
40. exports FRCBySerial;
41.  
42. begin
43. end.
44.  

[Cyrax]

Don't use String type across dynamic libraries, they are managed and can cause issues. Use PChar instead.

[Leledumbo]

Don't use String type across dynamic libraries, they are managed and can cause issues. Use PChar instead.

Adding to it: don't just cast the volatile string instance to pchar if it's meant to be still accessible in the (DLL) caller after the callee (DLL function) ends, it will point to invalid memory address and eventually the same SIGSEGV will be raised. Make a copy in the heap or make the interface provides fillable caller allocated space as parameter.

[fiazhnd]

I am trying to understand your instruction but it beyond my understanding can you please give me an example of you advice so next time when i write dll will keep in mind that things. 

Don't use String type across dynamic libraries, they are managed and can cause issues. Use PChar instead.

Adding to it: don't just cast the volatile string instance to pchar if it's meant to be still accessible in the (DLL) caller after the callee (DLL function) ends, it will point to invalid memory address and eventually the same SIGSEGV will be raised. Make a copy in the heap or make the interface provides fillable caller allocated space as parameter.

[Phil]

I am trying to understand your instruction but it beyond my understanding can you please give me an example of you advice

Maybe look at the NDFD library example code here for how to use a buffer to return string results from a dynamic library:

https://macpgmr.github.io/MacXPlatform/PascalDynLibs_2.html

[fiazhnd]

I amend the code as per instruction, not facing any error now but i am not receiving any result after calling the function. any suggestion. 

Code: Pascal  [Select][+][-]

1. unit Unit1;
2.  
3. {$mode objfpc}{$H+}
4.  
5. interface
6.  
7. uses
8.   Classes, SysUtils, Forms, Controls, Graphics, Dialogs, StdCtrls, ExtCtrls, Buttons, strutils, DynLibs;
9.  
10. type
11.   { TForm1 }
12.   TForm1 = class(TForm)
13.     BtnCodeBySerial: TButton;
14.     EditRCode: TEdit;
15.     EditVSerial: TEdit;
16.     procedure BtnCodeBySerialClick(Sender: TObject);
17.   private
18.     { private declarations }
19.  
20.   public
21.  
22.   end;
23.  
24. var
25.   Form1: TForm1;
26.  
27. implementation
28.  
29. type
30. TMyDLL = function (FRadioCode: PChar): PChar; stdcall;   //Importing Dynmic Function
31.  
32. {$R *.lfm}
33.  
34. { TForm1 }
35.  
36. procedure TForm1.BtnCodeBySerialClick(Sender: TObject);
37. var
38. RadioCode: PChar;
39. MyHandle: TLibHandle;
40. MyDLLFUNC: TMyDLL;
41. begin
42.    RadioCode:= PChar(EditVSerial.Text);
43.    MyHandle := LoadLibrary('drofrc.dll');
44.    if MyHandle <> 0 then
45.      begin
46.       MyDLLFUNC := TMyDLL(GetProcAddress(MyHandle, 'FRCBySerial'));
47.       if Assigned (MyDLLFUNC) then
48.       begin
49.       MyDLLFUNC(RadioCode); //call the function
50.       EditRCode.Text:= Radiocode;
51.       end
52.       else
53.       ShowMessage('"FRCBySerial" Function is not Found');
54.       FreeLibrary(MyHandle);
55.       end else begin
56.       ShowMessage('drofrc.dll not found / not loaded') ;
57.       end;
58. end;          
59.  

Library

Code: Pascal  [Select][+][-]

1. library drofrc;
2.  
3. {$mode objfpc}{$H+}
4.  
5. uses
6.   LCLType, StdCtrls, Classes, SysUtils, sqlite3conn, sqldb, db, FileUtil, Forms,
7.   Controls, Interfaces, Windows;
8.  
9. var
10.     SQLite3Connection1: TSQLite3Connection;
11.     SQLQuery1: TSQLQuery;
12.     SQLTransaction1: TSQLTransaction;
13.  
14. function FRCBySerial(RadioCode: PChar): PChar; stdcall;
15. begin
16.   Result:= '';
17.   //Creating Control
18.   SQLite3Connection1:= TSQLite3Connection.Create(Nil);
19.   SQLTransaction1:= TSQLTransaction.Create(Nil);
20.   SQLQuery1:= TSQLQuery.Create(Nil);
21.  
22.   SQLite3Connection1.DatabaseName:= 'newdbWE.db';
23.   SQLite3Connection1.Transaction:= SQLTransaction1;
24.   SQLTransaction1.Database:= SQLite3Connection1;
25.  
26.   SQLQuery1.Database:= SQLite3Connection1;
27.   SQLQuery1.Transaction:= SQLTransaction1;
28.   SQLQuery1.Clear;
29.   SQLQuery1.Close;
30.   SQLQuery1.SQL.Text:= ('Select RCode From RCBSerial Where VSerial = ' + RadioCode);
31.   SQLite3Connection1.Connected:= True;
32.   SQLTransaction1.Active:= True;
33.   SQLQuery1.Open;
34.  
35.   Result:= PChar(SQLQuery1.Fields[0].AsString);
36.  
37.   SQLite3Connection1.Open;
38. end;
39.  
40. exports FRCBySerial;
41.  
42. begin
43. end.
44.  

[jamie]

debugging DLL's is a messy thing, what I do is insert "Assertions" in the code for testing ..

In your case, you must test If the function is getting called! If the function is making it at the end, and is the
results set propertly before exiting the function ?

 It could also mean you may need to set a standard STRING to the result value and not pass a pointer back
from the AsString function, cause it could become invalid...

 Set the results of your DB actions in a string from the DLL and then pass that string back using PCHAR(MyresultString)..

make sure that string is not inside the function so it'll still be valid when the function exits.

[fiazhnd]

Seriously your paragraph 2 and 3 confuse me a lot. btw i check no Value came back (Error raise on call the function)

Can give me a favor to amend my code above, it will help me to understand your point??? please

debugging DLL's is a messy thing, what I do is insert "Assertions" in the code for testing ..

In your case, you must test If the function is getting called! If the function is making it at the end, and is the
results set propertly before exiting the function ?

 It could also mean you may need to set a standard STRING to the result value and not pass a pointer back
from the AsString function, cause it could become invalid...

 Set the results of your DB actions in a string from the DLL and then pass that string back using PCHAR(MyresultString)..

make sure that string is not inside the function so it'll still be valid when the function exits.

[jamie]

what I was conveying is using a Pointer (Pchar) to return the AsString may not work...

AsString is a function and yes it returns a string, if it supported but may be cleaned up when your function exits.

Create a String in your Variable section of the DLL, not inside the function of the DLL..
The code in the DLL function
 MyGlobalString := MySQL,,,,AsString;

Result := Pchar(MyGlobalString);

Make sure the string you return via the Pchar is not inside the function..


This way the meaning of the string will remain..

[engkin]

1-Put your function FRCBySerial in your application and call it there before you move it back into a dll.

2-Make sure your have sqlite dll - sqlite3.dll or whatever its name - in your application folder.

3-You have no way to find out if there is an error. I suggest you change your function into something like:

Code: Pascal  [Select][+][-]

1. library drofrc;
2.  
3. {$mode objfpc}{$H+}
4.  
5. uses
6.   sqlite3conn, sqldb, sysutils;
7.  
8. var
9.     SQLite3Connection1: TSQLite3Connection;
10.     SQLQuery1: TSQLQuery;
11.     SQLTransaction1: TSQLTransaction;
12.     globalStr: string;
13.  
14. function FRCBySerial(RadioCode: PChar; out Res: PChar): boolean; stdcall;
15. begin
16.   Result:= False; { Assume an error }
17.   //Creating Control
18.   try
19.   SQLite3Connection1:= TSQLite3Connection.Create(Nil);
20.     try
21.       SQLTransaction1:= TSQLTransaction.Create(SQLite3Connection1);
22.       SQLQuery1:= TSQLQuery.Create(SQLite3Connection1);
23.  
24.       SQLite3Connection1.DatabaseName:= 'newdbWE.db';
25.       SQLTransaction1.Database:= SQLite3Connection1;
26.       SQLQuery1.Transaction:= SQLTransaction1;
27.  
28.       SQLQuery1.SQL.Text:= ('Select RCode From RCBSerial Where VSerial = ' + RadioCode);
29.       SQLQuery1.Open;
30.  
31.       globalStr := SQLQuery1.Fields[0].AsString;
32.     finally
33.       SQLite3Connection1.Free;
34.     end;
35.     Res := pchar(globalStr);
36.     Result := True;
37.   except on e : Exception do
38.     ShowException(e, ExceptAddr); { or whatever }
39.   end;
40. end;
41.  
42. exports FRCBySerial;
43.  
44. begin
45. end.

if it does not return true then you know there is an error.

Test project:

Code: Pascal  [Select][+][-]

1. program project1;
2.  
3. {$mode objfpc}{$H+}
4.  
5. function FRCBySerial(RadioCode: PChar): PChar; stdcall; external 'drofrc';
6.  
7. begin
8.   WriteLn(FRCBySerial(pchar('''test''')));
9.   ReadLn;
10. end.

[Phil]

Create a String in your Variable section of the DLL, not inside the function of the DLL..

No, set up a buffer in the _calling_ code, then pass the buffer and the _size_ of the buffer to the library function. The function then fills the buffer with the return string value, using the buffer size to guard against buffer overflows. This is the only safe way of doing it. See many examples in the Win API. Or take a look at the NDFD library, its wrapper class and examples that I referred to above.

[engkin]

Create a String in your Variable section of the DLL, not inside the function of the DLL..

No, set up a buffer in the _calling_ code, then pass the buffer and the _size_ of the buffer to the library function. The function then fills the buffer with the return string value, using the buffer size to guard against buffer overflows.

I lost you here. How would a buffer overflow happen if the string is a global variable in the dll?

[Phil]

I lost you here. How would a buffer overflow happen if the string is a global variable in the dll?

The buffer overflow could occur with the buffer. You're confusing two different approaches.

In general, you never use global variables in dynamic libraries. Think of a threaded app using the library. The library could be called simultaneously by multiple threads or processes.

Follow the NDFD example like a cookbook and you'll be okay. Otherwise, you're just wandering around in the dark.

[engkin]

I lost you here. How would a buffer overflow happen if the string is a global variable in the dll?

The buffer overflow could occur with the buffer. You're confusing two different approaches.

I see. you mean the programmer needs to be careful and consider not to write buffer overflow vulnerable code using size and buffer method.

And if the buffer is not big enough, get the needed buffer size and try again. Finally, copy the buffer to a string.

So the function should include two arguments: buffer and size  and should return the actual size. Right?

In general, you never use global variables in dynamic libraries. Think of a threaded app using the library. The library could be called simultaneously by multiple threads or processes.

The question is not about thread-safe code, but to answer your concern:
For multiple threads, it is possible to change var to threadvar.
For different processes their is no problem when using global variables.

Follow the NDFD example like a cookbook and you'll be okay. Otherwise, you're just wandering around in the dark.

Thanks for the advice.

[balazsszekely]

@fiazhnd
Use callback functions. I attach a simple project, you can adapt it to your needs. The idea is to pass to your library a function, which will report back the error(if any). With this method you can send even managed types to your library.