RE: BASE 7.0.x@Windows_64 - Channel Access Context in shared library - "ca_context_destroy bug"

[Freddie Akeroyd - UKRI STFC via Tech-talk <tech-talk at aps.anl.gov>]

Hi Carsten,

 

Glad to hear you now have a working solution on Windows.

 

When dlclose() is called on Linux it decrements an internal reference counter on the library, and when the counter reaches zero the library is unloaded. So if you were to call dlopen(“path to my library”) from within your library to create an additional reference to it, then when LabVIEW calls dlclose() the library should remain loaded as the reference count will still be above zero. There appears to be a RTLD_NODELETE flag that has been available since glibc 2.2 and can be passed to dlopen() to keep the library loaded after a dlclose(), it may be worth adding that too if the flag is available, but I would have thought a single dlopen() to increase the reference count would be enough.

 

The reference counting principle also applies on windows, so the GET_MODULE_HANDLE_EX_FLAG_PIN flag may not be needed as the call to GetModuleHandleEx() will have increased the internal reference count on the library by one.

 

If you wanted to have an (optional for the user) way to unload channel access without terminating labview then the following might work:

-          In your library call GetModuleHandleEx() in DLL_PROCESS_ATTACH to increase the reference count as currently, but without GET_MODULE_HANDLE_EX_FLAG_PIN. This additional module reference should be stored in a global variable.

-          In the disconnect.vi call a separate function in your DLL that calls ca_context_destroy() directly (as Peter describes below) but which also then calls FreeLibrary() on the previous stored GetModuleHandleEx () reference.     

 

Regards,

 

Freddie

 

From: Carsten Winkler <carsten.winkler at helmholtz-berlin.de>
Sent: 25 May 2020 09:51
To: Heesterman, Peter J <Peter.Heesterman at ukaea.uk>; Johnson, Andrew N. <anj at anl.gov>; Akeroyd, Freddie (STFC,RAL,ISIS) <freddie.akeroyd at stfc.ac.uk>; tech-talk at aps.anl.gov
Subject: Re: BASE 7.0.x@Windows_64 - Channel Access Context in shared library - "ca_context_destroy bug"

 

Hi there,

up to now "caLabDisconnect.vi" is optional. The success of the CA Lab is the simple black box concept. You can use "caLabGet.vi" whenever you want without any separate "connect.vi" nor "disconnect.vi". I would like to retain this concept. Therefore, the additional function with "ca_task_exit()" is no solution in the CA Lab case.

Thanks to the hint from Michael and the friendly request from Freddi, I followed up the concept GetModuleHandleEx() with GET_MODULE_HANDLE_EX_FLAG_PIN. And this seems to be the key.
Now I can use my old code without any deadlocks. LabVIEW can open and close VIs with CA Lab repeatedly without problems. But LabVIEW must now be closed completely to get rid of CA Lab and Channel Access. I don't think that should be a problem.

Is there a similar solution (prevent unload library) for Linux?

Best regards
Carsten

Am 21.05.20 um 11:56 schrieb Heesterman, Peter J:

Hi Carsten,

 

See https://stackoverflow.com/questions/35828004/waitforsingleobject-for-thread-object-does-not-work-in-dll-unload

 

Basically calling ca_context_destroy()  from within DLLMain() is not an option, as it will cause deadlock.

 

It will need to be explicitly invoked - prior to DLL unload -  from within application code.

This might be from within your caLabDisconnect.vi?

 

If this is not an option, a more complex approach (as described in the article) involving 2 DLLs is possible.

But this more complex and I haven’t tested that, to date.

 

I changed your test code like below, and it exits normally.

 

All the best,

 

Peter.

 

#include <windows.h>

#include <iostream>

#include <epicsExit.h>

 

#include "myCaLib.h"

 

int main()

{

       HINSTANCE hinstLib;

       BOOL fFreeResult;

       hinstLib = LoadLibrary(TEXT("myCaLib.dll"));

       if (hinstLib == NULL) {

              std::cout << "myCaLib.dll not found" << std::endl;

              return 1;

       }

       std::cout << "library loaded; hit ENTER to unload" << std::endl;

       getchar();

       myCaLibClosing();

       fFreeResult = FreeLibrary(hinstLib);

       std::cout << "bye" << std::endl;

       return 0;

}

 

// myCaLib.cpp : Defines the exported functions for the DLL application.

//

 

#include <iostream>

#include <cadef.h>

#include <windows.h>

 

#define epicsExportSharedSymbols

 

#include "myCaLib.h"

 

void myCaLibClosing()

{

       ca_task_exit();

}

 

BOOLEAN WINAPI DllMain(HINSTANCE hDllHandle, DWORD nReason, LPVOID

       Reserved) {

       switch (nReason)

       {

       case DLL_PROCESS_ATTACH: {

              int32_t iResult =

                     ca_context_create(ca_enable_preemptive_callback);

              if (iResult != ECA_NORMAL) {

                     std::cout << "Error: Could not create any instance of Channel Access." << std::endl;

                     return 0;

              }

              std::cout << "New instance of Channel Access context." << std::endl;

              std::cout << ca_context_status(ca_current_context(), 1) << std::endl;

              break;

       }

       case DLL_PROCESS_DETACH: {

              std::cout << "detach process ... " << std::endl;

              break;

       }

       }

       return 1;

}

 

 

From: Heesterman, Peter J
Sent: 19 May 2020 10:40
To: Johnson, Andrew N. <anj at anl.gov>; Carsten Winkler <carsten.winkler at helmholtz-berlin.de>
Subject: RE: BASE 7.0.x@Windows_64 - Channel Access Context in shared library - "ca_context_destroy bug"

 

Hi Andrew, Carsten,

 

I may be able to help with this, if that would be beneficial.

 

I myself use caLab quite extensively.

 

Cheers,

 

Peter.

 

From: Tech-talk <tech-talk-bounces at aps.anl.gov> On Behalf Of Johnson, Andrew N. via Tech-talk
Sent: 18 May 2020 18:57
To: Carsten Winkler <carsten.winkler at helmholtz-berlin.de>
Cc: EPICS tech-talk <tech-talk at aps.anl.gov>
Subject: Re: BASE 7.0.x@Windows_64 - Channel Access Context in shared library - "ca_context_destroy bug"

 

Hi Carsten,

 

I’m not aware of anything that has actually been added to libCom or libca recently to prevent a completely shutdown; are you saying that you used to be able to unload the CA library and you can’t now? I don’t believe this has ever been possible (which is probably what Michael is also going by), but I might be wrong – please let us know what version of Base you were using if and when you could do that.

 

If it’s your requirement that has changed we would be happy to advise someone how to work out what needs to be modified to allow the complete unloading that you’re looking for, but I can’t promise that we’ll have core developer effort available to do the actual investigation and any development needed (we all manage our own time and have our own priorities controlled by our employers, so I can’t speak for the others).

 

- Andrew

 

 

On May 18, 2020, at 12:21 PM, Carsten Winkler via Tech-talk <tech-talk at aps.anl.gov> wrote:

thank you for your answer. This are bad news for our community. Many users are waiting for an EPICS Base upgrade for the Open Source software CaLab (interface beween LabVIEW and EPICS).

Best reganrds
Carsten

Am 18.05.2020 um 18:27 schrieb Michael Davidsaver:

libca and especially libCom can't be completely shutdown, and therefore

can't be safely unloaded.

On 5/18/20 8:34 AM, Carsten Winkler via Tech-talk wrote:

Hi,

for a project I need to load and unload the Channel Access context in a

shared library. Loading is no problem but unloading. It always fails

during ca_context_destroy() function.

Here is my demo code:

main.cpp (most simple case for a bigger project where it's necessary to

load and unload different shared libraries at runtime without

terminating main application)

----------------------------------------------------------

#include <windows.h>

#include <iostream>

int main()

{

    HINSTANCE hinstLib;

    BOOL fFreeResult;

    hinstLib = LoadLibrary(TEXT("myCaLib.dll"));

    if (hinstLib == NULL) {

        std::cout << "myCaLib.dll not found" << std::endl;

        return 1;

    }

    std::cout << "library loaded; hit ENTER to unload" << std::endl;

    getchar();

    fFreeResult = FreeLibrary(hinstLib);

    std::cout << "bye" << std::endl;

    return 0;

}

_______________________________________________________________________

myCaLib.cpp (most simple shared library with no functions except

creating and releasing the Channel Access context)

--------------------------------------------------------------

#include <iostream>

#include <cadef.h>

#include <windows.h>

BOOLEAN WINAPI DllMain(HINSTANCE hDllHandle, DWORD nReason, LPVOID

Reserved) {

    switch (nReason)

    {

        case DLL_PROCESS_ATTACH: {

            int32_t iResult =

ca_context_create(ca_enable_preemptive_callback);

            if (iResult != ECA_NORMAL) {

                std::cout << "Error: Could not create any instance of

Channel Access." << std::endl;

                return 0;

            }

            std::cout << "New instance of Channel Access context." <<

std::endl;

            std::cout << ca_context_status(ca_current_context(), 1) <<

std::endl;

            break;

        }

        case DLL_PROCESS_DETACH: {

            std::cout << "detach process ... " << std::endl;

            ca_context_destroy();

            break;

        }

    }

    return 1;

}

_______________________________________________________________________

build at command line at Windows 7 64 bit

--------------------------------------------------------------

%comspec% /k "C:\Program Files (x86)\Microsoft Visual

Studio\2019\Community\VC\Auxiliary\Build\vcvars64.bat"

set base=C:\base-7.0.x            <== directory of nightly build or

stable EPICS BASE 7.0 (built with "EPICS_HOST_ARCH=windows-x64")

cd C:\test_case\                       <== directory of our test sources

main.cpp and myCaLib.cpp

cl -nologo -FC -D__STDC__=0 -D_CRT_SECURE_NO_DEPRECATE

-D_CRT_NONSTDC_NO_DEPRECATE -Ox -GL -Oy- -W3 -MD -EHsc -DEPICS_BUILD_DLL

-DEPICS_CALL_DLL -I%base%/include/compiler/msvc

-I%base%/include/os/WIN32 -I%base%/include -c main.cpp myCaLib.cpp

link -nologo -subsystem:windows -dll -LTCG -incremental:no -opt:ref

-release -version:7.0 -MACHINE:X64 -libpath:%base%\lib\windows-x64

-out:myCaLib.dll -implib:myCaLib.lib myCaLib.obj ca.lib Com.lib

link -nologo -LTCG -incremental:no -opt:ref -release -version:7.0

-MACHINE:X64 -out:test.exe main.obj

set path=%base%\bin\windows-x64;%path%

test.exe

_______________________________________________________________________

test.exe will freeze because it hangs on ca_context_destroy()

I need any solution for this case, please.

Best regards

Carsten

________________________________

Helmholtz-Zentrum Berlin für Materialien und Energie GmbH

Mitglied der Hermann von Helmholtz-Gemeinschaft Deutscher Forschungszentren e.V.

Aufsichtsrat: Vorsitzender Dr. Volkmar Dietz, stv. Vorsitzende Dr. Jutta Koch-Unterseher

Geschäftsführung: Prof. Dr. Bernd Rech (Sprecher), Prof. Dr. Jan Lüning, Thomas Frederking

Sitz Berlin, AG Charlottenburg, 89 HRB 5583

Postadresse:

Hahn-Meitner-Platz 1

D-14109 Berlin

 

---

Helmholtz-Zentrum Berlin für Materialien und Energie GmbH

Mitglied der Hermann von Helmholtz-Gemeinschaft Deutscher Forschungszentren e.V.

Aufsichtsrat: Vorsitzender Dr. Volkmar Dietz, stv. Vorsitzende Dr. Jutta Koch-Unterseher
Geschäftsführung: Prof. Dr. Bernd Rech (Sprecher), Prof. Dr. Jan Lüning, Thomas Frederking

Sitz Berlin, AG Charlottenburg, 89 HRB 5583

Postadresse:
Hahn-Meitner-Platz 1
D-14109 Berlin

 

-- 

Complexity comes for free, simplicity you have to work for.

 

 

---

Helmholtz-Zentrum Berlin für Materialien und Energie GmbH

Mitglied der Hermann von Helmholtz-Gemeinschaft Deutscher Forschungszentren e.V.

Aufsichtsrat: Vorsitzender Dr. Volkmar Dietz, stv. Vorsitzende Dr. Jutta Koch-Unterseher
Geschäftsführung: Prof. Dr. Bernd Rech (Sprecher), Prof. Dr. Jan Lüning, Thomas Frederking

Sitz Berlin, AG Charlottenburg, 89 HRB 5583

Postadresse:
Hahn-Meitner-Platz 1
D-14109 Berlin

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