When calling the Shared Libraries (DLL) created by QT6, the main program will crash

pedisChen

I created a DLL file with QT6, which uses the QTCPSocket network component. When I call the function that uses QT components in DLL with QLibrary, the main program will crash, such as connect_to_server() function.

I want to know how to implement QT components such as QTCPSocket or UI components such as Widget in the DLL file, so that I can use the DLL file on other platforms (VC or the others)

Thanks!

J.Hilk

You probably have neglected to provide the dll with the dll's it itself needs. You'll have to run win deploy qt on your dll just as you would with a normal application

pedisChen

@J-Hilk Thank you for your relpy. I tried, like this
C:\Qt\Tools\QtDesignStudio\qt6_design_studio_reduced_version\bin>windeployqt6.exe D:\documents\zxin\dll\build-myDLL-Desktop_Qt_6_5_3_MinGW_64_bit-Release\release\myDLL.dll
Then I added the generated file to my project directory, but it's still the same.
My dll file was built with QT6, and then I created another project with QT6 and tried to call it

pedisChen

@J-Hilk I think it is because the dll lacks of an QT event loop mechanism. so I added an init function in dll,and need to call the init function first , defined like this:
QCoreApplication a(argc, argv);
return a.exec();

This worked, but it would always block the main program. Then I planned to use QThread to create a new process in the dll to run a.exec(), but it failed and the new process task could not run.I dont know why...

Pl45m4

@pedisChen said in When calling the Shared Libraries (DLL) created by QT6, the main program will crash:

My dll file was built with QT6, and then I created another project with QT6 and tried to call it

You don't "call" DLLs, you link them.

Then I added the generated file to my project directory, but it's still the same.

This is not what @J-Hilk was talking about.
If you export/deploy your qt widget library, you need to ship it with all the other libs your library needs.

@pedisChen said in When calling the Shared Libraries (DLL) created by QT6, the main program will crash:

I think it is because the dll lacks of an QT event loop mechanism. so I added an init function in dll,and need to call the init function first , defined like this:

QCoreApplication a(argc, argv);
return a.exec();

A library doesn't necessarily need that.
A Qt library can be a QWidget, just a colletion of functions or something else...
None of these need their own QCoreApplication, when used in other Qt apps.
(They do in general in order to work, but at least not their own. The main loop from the app to which your library is linked to, is fine).

See this example:
If you look at the code, there is no main.cpp or additional QApplication involved.
But one is needed to use the library widget in your main app.

https://doc.qt.io/qt-6/qtdesigner-customwidgetplugin-example.html
- Code: https://code.qt.io/cgit/qt/qttools.git/tree/examples/designer/customwidgetplugin?h=6.8

Edit:

@pedisChen said in When calling the Shared Libraries (DLL) created by QT6, the main program will crash:

This worked, but it would always block the main program. Then I planned to use QThread to create a new process in the dll to run a.exec(), but it failed and the new process task could not run.I dont know why...

In what way do you use your library? Or how do you intend to use it in some application?
How does your library work? Do you export a single widget, which uses QTCPSocket in your library?

Another observation:

If you don't have an event loop in your main application (when no Qt involved in main app), you obviously can't create a QThread which then should host the QApplication instance... QThread is a wrapper for std::thread and is a QObject itself. Therefore it also needs a QApplication to work properly.
Use a plain std::thread instead.

pedisChen

@Pl45m4 said in When calling the Shared Libraries (DLL) created by QT6, the main program will crash:

If you export/deploy your qt widget library, you need to ship it with all the other libs your library needs.

Because I was linking the dll file use another qt project
so I copied all the files(ship it all?) generated by windeployqt6.exe to the project dlll linking dir

I use QTCPSocket in the dll for TCP connection, and then the main program crashes for unknown reasons

dll code like this:
A.cpp

uint8_t mySocket::Connect()
{
    if(this->ip.isEmpty()) return CONNECT_ADDR_FAIL;
    if(this->port == 0) return CONNECT_ADDR_FAIL;

    /* device connect */
    if(_s.state() != QAbstractSocket::ConnectedState)
        _s.connectToHost(this->ip, this->port);
    return 0;
}

B.cpp

uint8_t tcp_connect(void *mySocketHandle)
{
    return ((mySockeDLL_t)mySocketHandle)->Connect();
}

B.h

#ifdef __cplusplus
extern "C"
{
#endif //__cplusplus
....
uint8_t DLL_EXPORT tcp_connect(void * mySocketHandle);

.....
#ifdef __cplusplus
}
#endif //__cplusplus

in another project, use QLibrary to link dlls
code like this

    QLibrary library("myDLL");

    if (library.load())  
    {

         ......
        typedef uint8_t (* tcp_connect)(void * mySocketHandle);
        tcp_connect f2 = (tcp_connect)library.resolve("tcp_connect");
        f2(mySocketHandle);
         .......
    }
    library.unload();

after run .exe
23:00:52: D:\Code\QT_PROJECT\libraryTest\importDLL\build-importDLL-Desktop_Qt_6_5_3_MinGW_64_bit-Release\release\importDLL.exe crashed.

Christian Ehrlicher

Apart from wtf are you doing here:

f2(mySocketHandle);

Does mySocketHandle really contains the pointer to your class instance?

pedisChen

@Christian-Ehrlicher yes，it is a pointer，maybe the var name is confusing，but the tcp socket connected successfully and after some second the main program crash down

JonB

@pedisChen Run under debugger and see whether stack trace gives you a hint.

Christian Ehrlicher

tbh - wrapping a c++ class in a c api is...

Why all this strange stuff instead simply linking against the library like everyone else is doing it?

pedisChen

@Christian-Ehrlicher because Idont know how to use Qlibrary to resolve a class object and I want it to be compatible for use in code written in C.

Christian Ehrlicher

because Idont know how to use Qlibrary to resolve a class object

https://doc.qt.io/qt-6/plugins-howto.html#the-low-level-api-extending-qt-applications

and I want it to be compatible for use in code written in C.

Since you need a running Q(Core)Application somewhere this will not work out.

pedisChen

@Christian-Ehrlicher said in When calling the Shared Libraries (DLL) created by QT6, the main program will crash:

Since you need a running Q(Core)Application somewhere this will not work out.

Can I still use QTcpSocket for TCP communication? I want the generated DLL to be linkable by C code.

pedisChen

@Christian-Ehrlicher Can I create a init function in dll running in another thread that running QCoreApplication?

Christian Ehrlicher

@pedisChen said in When calling the Shared Libraries (DLL) created by QT6, the main program will crash:

Can I still use QTcpSocket for TCP communication? I want the generated DLL to be linkable by C code.

You need a running Q(Core)Application as we already told you.

Can I create a init function in dll running in another thread that running QCoreApplication?

You can, but you will get in trouble when the objects are not in that thread. It's not worth the trouble.

pedisChen

@Christian-Ehrlicher said in When calling the Shared Libraries (DLL) created by QT6, the main program will crash:

You can, but you will get in trouble when the objects are not in that thread. It's not worth the trouble.

But if it's in the same thread, QCoreApplication will block the program from running. How should I solve this？

Pl45m4

@pedisChen said in When calling the Shared Libraries (DLL) created by QT6, the main program will crash:

But if it's in the same thread, QCoreApplication will block the program from running. How should I solve this？

Like I've said above, create a new std::thread for your Q(Core)Application.

pedisChen

@Pl45m4 Thank you for your reply. I tried. here is my example code:

Version A

void start_app(void)
{
    int argc = 0;
    char*argv[] = {nullptr};
    QCoreApplication a(argc, argv);

    a.exec();
}

void init_dll()
{
    std::thread th(start_app);
    h.detach();
}

I use QLibrary to resolve "init_dll" function and run it.This time the main program is not blocked, no crash down occured .But the signal slot did not work, and QTCPSocket cannot be used.

Version B

void start_app(void)
{
    int argc = 0;
    char*argv[] = {nullptr};
    QCoreApplication a(argc, argv);

    a.exec();
}

void init_dll()
{
    start_app();
}

when I call "init_dll" function in DLLs，the main program will be blocked from running, but the QTCPSocket and signal slots function is workable.

How can I modify the DLL code so it doesn't block the main program's execution and still allows using QTcpSocket and other Qt components that depend on the event loop(QCoreApplication)?