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Windows API: WinInet callback linked local vs standlone mode
2003-05-31 -- Alexey Solovjev
 
Hello,

> Another standalone vs local link problem....
> ...
> When linked local in c6ea4-1, the callback seems to be called ok until i
> open a status window...event:openwindow is posted then the program just
> goes away with no further event.or explanation of itself at all...
> just gone.
>
> If anyone is doing a wininet callback, I would greatly appreciate it if
> you could try it linked local.

Well, let me explain that happening.

EXEs and DLLs are about identical. There are only few differences, most of
them are not significant. But one is principal. Every EXE and DLL has the
entry point. The manner how Windows calls to this entry point is very
different for EXE and DLL:

- the entry point of the EXE is called once only on the start of process.
- the entry point of the DLL is called on the loading and unloading this
  DLL to and from the process virtual space AND on start and finish of
  every thread (except one that is active at loading and unloading time).

This means that Windows does not "inform" the EXE when thread is started
or closed. The Clarion RTL knows about this Windows behavior and handles
it. Threaded data declared in the EXE is initialized on the start of new
thread and it is cleaned up on the thread closing. The RTL also detects
threads started, for example, by dialogs started from the program and
initializes all required internal data.

The case in your program is the most difficult case: the new thread is
started out of the program by Internet stuff and calls to passed callback
function within this new thread. If the program is linked in the standalone
mode, there are no problems: the RTL is a DLL and Windows calls to its
entry point that allows to initialize all threaded data. If the program
is linked in local mode, Windows does not inform it about new thread. So,
the callback is the first code in the program that is executed by the
external thread. Hence, first attempt to access threaded data declared
within the program or within RTL causes the fault.

This is not a defect of Clarion, this is consequence of Windows behavior.
We can handle case of such callbacks in one-piece EXEs too but do not
consider that this is reasonable - because of the cost of solution: every
program regardless the mode it has been linked would be need to perform
extra code on every call to the most frequently used operation in the RTL.
This is too high cost for handling very rare situation that is not handled
by Windows. On the other hand, there are several alternative solutions:

- to make the program in the standalone mode
- if you don't want to ship multiple DLLs with your program (RTL, drivers),
  it can be made it not one-piece but as two pieces: the locally linked DLL
  and the small EXE that does the only thing - calls to specific function
  from the DLL.

Follow next simple steps for the latter variant:

- In the program's project set Target Type to DLL and Run-Time Library to
  Local
- Move code from the program's CODE section to a procedure, e.g.

From:

  CODE
  

To:

  MAP
    EntryPoint
  END

  CODE

EntryPoint PROCEDURE
  CODE
  

- Add defined procedure's external name (EntryPoint@F here) to the EXPORTS
  list in DLL's EXP file together with at least following names defined in
  the RTL:
  __checkversion
  __sysstart
  __sysinit
  _exit
  Cla$code
  Cla$init
  Wsl$Closedown

- Make the DLL
- In the project for the starter EXE set Target Type to EXE and Run-Time
  Library to External, add the LIB file for the DLL built on the previous
  step to the project
- The source of the starter EXE can be as follows:

        PROGRAM

        MAP
          MODULE('')
            EntryPoint()
          END
        END

  CODE
  EntryPoint

- Make the EXE

>  in a program that works fine in c55 linked standalone and local and in
> c6ea4-1 linked standalone.

The program you've provided corrupts memory in C55 if it is linked the local
mode. It is accidental that it not GPFs: if WININET would call to callback
in unfortunate time, your program would fail.

Alexey Solovjev

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