QPluginLoader instance always returns null

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Greeting

I created plugin with Qt (5.6.2) and trying to load it but it returns null all the time. I checked several question and also tried the solutions but it didn't work for me.

Can you take a look of the following code and see whats wrong?

DeviceManager.hpp

Code:

#ifndef DEVICE_MANAGER_HPP
#define DEVICE_MANAGER_HPP
 
#include <QtCore>
#include <string>
 
using namespace std;
 
class DeviceManager
{
 
public:
    virtual ~DeviceManager() {}
 
    virtual bool initialize() = 0;
    virtual string getBrandName() = 0;
 
};
 
QT_BEGIN_NAMESPACE
Q_DECLARE_INTERFACE(DeviceManager, "com.some.address/1.0")
QT_END_NAMESPACE
 
#endif //DEVICE_MANAGER_HPP

DeviceManagerImpl.hpp

Code:

#ifndef DEVICE_MANAGER_IMPL_HPP
#define DEVICE_MANAGER_IMPL_HPP
 
#include "DeviceManager.hpp"
 
#include <string>
using namespace std;
 
class DeviceManagerImpl : public QObject, public DeviceManager
{
    Q_OBJECT
    Q_PLUGIN_METADATA(IID "com.some.address/1.0")
    Q_INTERFACES(DeviceManager)
 
public:
    DeviceManagerImpl();
 
    //Override Method
    bool initialize();          //Have implementation in cpp file
    string getBrandName();      //Have implementation in cpp file
 
private:
    ...
 
};
 
#endif //DEVICE_MANAGER_IMPL_HPP

Pro File

Code:

QT       += core gui sql
 
TARGET = Device-Manager
#TARGET = $$qtLibraryTarget(Device-Manager)
TEMPLATE = lib
CONFIG += plugin
 
 
SOURCES += \
    ...
 
HEADERS += \
    ...
 
DISTFILES += Device-Manager.json
 
unix {
    target.path = /usr/lib
    INSTALLS += target
}

And this is how i try to load the plugin in my main process.

Code:

QPluginLoader * pluginLoader = new QPluginLoader(pluginPath.c_str());
QObject * plugin = pluginLoader->instance();
 
if (plugin)
{
    deviceManager = qobject_cast<DeviceMAnager *>(plugin);
    return true;
}
else
{
    delete pluginLoader;
    return false;
}

Im using QT 5.6.2 and QT Creator and MinGW 32bit.

Thanks in advance.

12th December 2017, 01:06
ChrisW67

Re: QPluginLoader instance always returns null

QT_BEGIN_NAMESPACE and QT_END_NAMESPACE should not be needed in the header.

When loading, is plugin or deviceManager NULL? If plugin is NULL then the plugin DLL is either not found at all, or unable to load. If deviceManager is NULL (and plugin is not) then the loaded plugin does not contain a QObject handling the interface.
Are you certain the Device-Manager.dll is in the location identified pluginPath?
Are all of the external runtime dependencies of the plugin DLL met in the environment in which it is running? That is, if Device-Manager.dll depends on foolib.dll is that in the PATH?
12th December 2017, 05:14
meysam_hashemi

Re: QPluginLoader instance always returns null

Quote:

Originally Posted by ChrisW67

QT_BEGIN_NAMESPACE and QT_END_NAMESPACE should not be needed in the header.

When loading, is plugin or deviceManager NULL? If plugin is NULL then the plugin DLL is either not found at all, or unable to load. If deviceManager is NULL (and plugin is not) then the loaded plugin does not contain a QObject handling the interface.
Are you certain the Device-Manager.dll is in the location identified pluginPath?
Are all of the external runtime dependencies of the plugin DLL met in the environment in which it is running? That is, if Device-Manager.dll depends on foolib.dll is that in the PATH?

Thanks for reply. The plugin is in correct path and pluginLoader is not null but instance() return null. I think that one of run-time dependencies is missing or something. I'm breaking the code to several part and testing it to find out what exactly is wrong!
12th December 2017, 09:18
meysam_hashemi

Re: QPluginLoader instance always returns null
I found the reason but i have no idea why it cause the problem, Im using library that is linked to project but when i use its function, The instance returns null.
Code:

LIBS += $$(STANDARD_XFS_DIRECTORY)/LIB/MSXFS.lib INCLUDEPATH += $$(STANDARD_XFS_DIRECTORY)/INCLUDE
Can anyone tell me why it happens?
12th December 2017, 10:42
high_flyer

Re: QPluginLoader instance always returns null

mixed build mode perhaps? (release/debug)

Re: QPluginLoader instance always returns null

Quote:

Originally Posted by high_flyer

mixed build mode perhaps? (release/debug)

No, Both are in debug!
The thing is, I did something exactly like this before with CMakeList.txt and Visual Studio but its not working with MinGW and QT Creator . I'm missing something but i have no idea what it is !!!

Added after 18 minutes:

The following configs of library and plugin in `CMakeList.txt` work perfectly fine when i make the project with `Visual Studio`. (Made with CMake)
Code:

ADD_DEFINITIONS(${QT_DEFINITIONS}) ADD_DEFINITIONS(-DUNICODE -D_UNICODE) ADD_DEFINITIONS(-DQT_PLUGIN) ADD_DEFINITIONS(-DQT_SHARED) ADD_DEFINITIONS(-DQT_DLL) ADD_DEFINITIONS(-DQT_LARGEFILE_SUPPORT) ADD_DEFINITIONS(-DQT_THREAD_SUPPORT) INCLUDE_DIRECTORIES( ${STANDARD_XFS_DIRECTORY}/INCLUDE ) LINK_DIRECTORIES( ${STANDARD_XFS_DIRECTORY} ${STANDARD_XFS_DIRECTORY}/LIB ) SET(XFS_LIBS MSXFS xfs_conf SSIDLL ) TARGET_LINK_LIBRARIES(Device-Manager ${XFS_LIBS} )
And this is the configs of library and plugin in `.pro` file in `Qt Creator` with `MinGW` which is not working when i use the library.
Code:

QT -= gui QT += core sql TARGET = Device-Manager TEMPLATE = lib CONFIG += plugin CONFIG += c++11 DEFINES += DEVICEMANAGER_LIBRARY INCLUDEPATH += $$(STANDARD_XFS_DIRECTORY) INCLUDEPATH += $$(STANDARD_XFS_DIRECTORY)/INCLUDE message(Include : $${INCLUDEPATH}) LIBS += $$(STANDARD_XFS_DIRECTORY)/SSIDLL.lib LIBS += $$(STANDARD_XFS_DIRECTORY)/LIB/MSXFS.lib LIBS += $$(STANDARD_XFS_DIRECTORY)/LIB/xfs_conf.lib message(Lib : $${LIBS})
So i can say the code it self is fine and i'm definitely missing some configs in `.pro` file.

PS1: Plugin and the application that load the plugin, Both have `INCLUDEPATH` and `LIBS` of the library in their pro file.

PS2: The application and the plugin both are in debug mode

PS3: I dont get any compile error or linker error during compile.

12th December 2017, 15:16
high_flyer

Re: QPluginLoader instance always returns null

Quote:

No, Both are in debug!

do you have any other 3rd party dependencies which might be in release?

I'd try to build your project in release and try to see if it works, its an easy test worth trying.
12th December 2017, 16:26
meysam_hashemi

1 Attachment(s)

Re: QPluginLoader instance always returns null
Quote:

Originally Posted by high_flyer

do you have any other 3rd party dependencies which might be in release?

I'd try to build your project in release and try to see if it works, its an easy test worth trying.

No man i dont

I made simple console application and included my library header and lib file and called the function and this is the error i get when i run the application. So it must be it. But seriously why!!!

The Code:
Code:

LPWFSRESULT result = new WFSRESULT(); WFSFreeResult(result)
Attachment 12723

Re: QPluginLoader instance always returns null

Is this typo 'DeviceMAnager' really in the code or only in the code posted here?:

Code:

if (plugin)
{
    deviceManager = qobject_cast<DeviceMAnager *>(plugin);
    return true;
}

I'd add some defencive code here:

Code:

QPluginLoader * pluginLoader = new QPluginLoader(pluginPath.c_str());
QObject * plugin = pluginLoader->instance();
 
if (plugin)
{
    deviceManager = qobject_cast<DeviceMAnager *>(plugin);
    if(deviceManager) return true;
    return false;;
}
else
{
    delete pluginLoader;
    return false;
}

12th December 2017, 21:46
ChrisW67

Re: QPluginLoader instance always returns null

Your screenshot clearly shows error 0xc0000135, which means STATUS_DLL_NOT_FOUND. This goes back to:

Quote:

Originally Posted by me

Are all of the external runtime dependencies of the plugin DLL met in the environment in which it is running? That is, if Device-Manager.dll depends on foolib.dll is that in the PATH?

Is the MSXFS.DLL in the run time PATH of the process or in the same directory as the plugin DLL? Is everything the MSXFS.DLL depends on also in one of those locations?
13th December 2017, 12:22
meysam_hashemi

Re: QPluginLoader instance always returns null

Quote:

Originally Posted by ChrisW67

Your screenshot clearly shows error 0xc0000135, which means STATUS_DLL_NOT_FOUND. This goes back to:

Is the MSXFS.DLL in the run time PATH of the process or in the same directory as the plugin DLL? Is everything the MSXFS.DLL depends on also in one of those locations?

Yes.

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Well, I found the problem however its so silly. I run project over windows 10 32 bit and it work fine. Maybe something is not incompatible with 64 bit and that caused this mess.

Thanks everyone for all your solution and i hope your solutions help others problem.

Thanks alot!