Creating a windows dll issues
lazarus / 34-Windows

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lainz · 2023-07-20 00:26

Hi, I'm facing some issues with memory when making a dll

Result of my function is a record, that's in memory until something
removes it?

Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i  
Taccesstoken = packed record    accesstoken: PChar;    userid: PChar; 
end;   
I create in the dll local variables

Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i function
somemethod: Taccesstoken; stdcall;var  accesstoken: ansistring =
''; ... accesstoken := Copy(somejson.asstring...,
1);result.accesstoken := PChar(accesstoken);
Then I call the method

Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i ...
 public  rec1 = Tacesstoken; ... rec1 := somemethodindll;
The problem is if I call other methods in the dll with the same PChar,
sometimes memory is lost or modified.

I think because the dll is no longer using it.

To avoid this problem I make a copy of the PChars in the target
application:

Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i
...public...atoken = ansistring;... atoken :=rec1.accesstoken;
My question is if there's a better way of doing this.

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KodeZwerg · 2023-07-20 05:12

AFAIK you need to allocate memory for the string, either in dll and
provide there a free method or in your app code by passing also the
length for the string.

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cdbc · 2023-07-20 05:28

Hi
I came to the same conclusion as @KodeZwerg...
You need something that's not refcounted, to base your pchar on...
e.g.:Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i function
somemethod: Taccesstoken; stdcall;var  accesstoken: ansistring =
''; ... accesstoken := Copy(somejson.asstring...,
1);result.accesstoken := StrNew(PChar(accesstoken)); // alloc new
mem// later, somewhere you free it again with
StrDispose(rec.accesstoken) EDIT: Come to think of it, it's important,
that you StrDispose in the same memory-manager as you StrNew it in,
because they rely on the length being written just before the pointer
itself. (...or You could just export StrDispose from the dll) 
Another possibility is, (if it's feasible) to use a shortstring
instead, it's a value-type: Code: Pascal  [Select][+][-]window.onload
= function(){var x1 = document.getElementById("main_content_section");
if (x1) { var x = document.getElementsByClassName("geshi");for (var i
= 0; i   Taccesstoken = packed record    accesstoken: shortstring;   
userid: PChar;  end; Just a couple of ideas 
Regards Benny

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Zvoni · 2023-07-20 06:54

Since it's windows:
https://www.freepascal.org/docs-html/rtl/sharemem/index.html

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440bx · 2023-07-20 12:34

Some generic advice that seems to apply in this case:

1. when using pointers, centralize memory management.  That is, either
the app or the dll is responsible for managing memory but _not_ both,
that is, do _not_ mix allocations and/or deallocations among them.

2. when using pointers,  ensure whatever space is needed is _clearly_
allocated by either the app or the dll (preferably the app, which
should subsequently and eventually free the memory block.)

corollary: if the memory management is partially done by the dll then
the dll should export functions for the app to control when the
allocation and de-allocation is done.

HTH.

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lainz · 2023-07-20 12:47

Thanks to all =)

I will use shortstring for now, but in the case I need longer strings,
I will use StrNew (in the dll) and StrDispose (in the application,
exported from the dll).

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KodeZwerg · 2023-07-20 12:56

Quote from: lainz on July 20, 2023, 02:47:38 pmThanks to all =)
You are welcome.

Quote from: lainz on July 20, 2023, 02:47:38 pmI will use StrNew (in
the dll) and StrDispose (in the application, exported from the dll).
Did you really read above messages?

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lainz · 2023-07-20 13:03

Quote from: KodeZwerg on July 20, 2023, 02:56:08 pmQuote from: lainz
on July 20, 2023, 02:47:38 pmI will use StrNew (in the dll) and
StrDispose (in the application, exported from the dll).
Did you really read above messages?

Yes I've readed. But I need to allocate in the dll better, since I
don't know the size of the string previously, and also is a result, I
don't know how to allocate the result of the record in my side... And
I can't free in the dll since I need to use the value in the
application. So I will free from the application with the StrDispose
from the dll. Like said here:

Quotecorollary: if the memory management is partially done by the dll
then the dll should export functions for the app to control when the
allocation and de-allocation is done.
Quote(...or You could just export StrDispose from the dll)
The allocation will be done in the function, and de allocation in the
method StrDispose exported.

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cdbc · 2023-07-20 16:38

Hi
You are most welcome, have fun
Regards Benny