Problem with linking with homemade dll

[Synk]

Hi

I'm trying to create dll and link with simple Qt application.
It looks that I have created dll in good way.
I has extern "C" and definition for import export of classes and functions in dll.
Is very any good tutorial how to make dll in qt and use with new Qt application?
I have found some examples in Internet but I would like to see good example of let's say dll with one object and application which uses this object.
Do you know any good tutorials about it?

Regards,
Pawel

[wysota]

You don't need a tutorial, just add a LIBS+=-lnameofthelibrary to your qmake project and rerun qmake.

[auba]

This is windows related stuff, as you don't need these cryptic export definitions on linux (unix?) systems. There you even can overwrite the malloc function to debug memory allocation...

.pro file:

TARGET              = mydll
TEMPLATE            = lib                                                      
 
CONFIG              += dll
 
win32 {
    DEFINES         += MYDLL_EXPORTS _MBCS _USRDLL
#    would be better to have an rc file to add some descriptions shown in the explorer
#    but without win, I do not have an example :p
#    RC_FILE         = MYDLL.rc
}
SOURCES          += mydll.cpp
HEADERS          += mydll.h

TARGET              = mydll
TEMPLATE            = lib                                                      

CONFIG              += dll

win32 {
    DEFINES         += MYDLL_EXPORTS _MBCS _USRDLL
#    would be better to have an rc file to add some descriptions shown in the explorer
#    but without win, I do not have an example :p
#    RC_FILE         = MYDLL.rc
}
SOURCES          += mydll.cpp
HEADERS          += mydll.h

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mydll.h:

#ifndef             MYDLL_H
#define             MYDLL_H
// to know the OS...
#include            <QString>
    #if( defined Q_OS_WIN32 )
        #ifdef  MYDLL_EXPORTS
            #define     MYDLL_API       __declspec(dllexport)
        #else
            #define     MYDLL_API       __declspec(dllimport)
        #endif
    #else
        #define         MYDLL_API
    #endif
#endif

#ifndef             MYDLL_H
#define             MYDLL_H
// to know the OS...
#include            <QString>
    #if( defined Q_OS_WIN32 )
        #ifdef  MYDLL_EXPORTS
            #define     MYDLL_API       __declspec(dllexport)
        #else
            #define     MYDLL_API       __declspec(dllimport)
        #endif
    #else
        #define         MYDLL_API
    #endif
#endif

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mydll.cpp:

#include            "mydll.h"                                     
#ifdef      Q_OS_WIN32
#include    <windows.h>
BOOL APIENTRY DllMain( HANDLE hModule,
                       DWORD  ul_reason_for_call,
                       LPVOID lpReserved
                     )
{
    switch (ul_reason_for_call)
    {
        case DLL_PROCESS_ATTACH:
        case DLL_THREAD_ATTACH:
        case DLL_THREAD_DETACH:
        case DLL_PROCESS_DETACH:
            break;
    }
    return TRUE;
}
#endif

#include            "mydll.h"                                     
#ifdef      Q_OS_WIN32
#include    <windows.h>
BOOL APIENTRY DllMain( HANDLE hModule,
                       DWORD  ul_reason_for_call,
                       LPVOID lpReserved
                     )
{
    switch (ul_reason_for_call)
    {
        case DLL_PROCESS_ATTACH:
        case DLL_THREAD_ATTACH:
        case DLL_THREAD_DETACH:
        case DLL_PROCESS_DETACH:
            break;
    }
    return TRUE;
}
#endif

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Any other header class:

#ifndef EXTENDER_H
   #define EXTENDER_H
   #include "mydll.h"
   class MYDLL_API TMyDllExtender
    {
    };
    #endif

#ifndef EXTENDER_H
   #define EXTENDER_H
   #include "mydll.h"
   class MYDLL_API TMyDllExtender
    {
    };
    #endif

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Program which uses this class:

#include "extender.h"
 
void main() 
{
  TMyDllExtender ext;
  ...
}

#include "extender.h"

void main() 
{
  TMyDllExtender ext;
  ...
}

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pro file of this program:

LIBS += -lmydll

LIBS += -lmydll

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[Synk]

Hi,

Thank you for useful reply. I finally compiled example successfully.
I have mad some modifications, but finally I have working dll.

I have question on defines in project file:

DEFINES         += MYDLL_EXPORTS _MBCS _USRDLL

DEFINES         += MYDLL_EXPORTS _MBCS _USRDLL

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I know what MYDLL_EXPORTS mean, but I don't know what _MBCS and _USRDLL mean?

Could you also explain code from mydll.cpp?

#ifdef      Q_OS_WIN32
#include    <windows.h>
BOOL APIENTRY DllMain( HANDLE hModule,
                       DWORD  ul_reason_for_call,
                       LPVOID lpReserved
                     )
{
    switch (ul_reason_for_call)
    {
        case DLL_PROCESS_ATTACH:
        case DLL_THREAD_ATTACH:
        case DLL_THREAD_DETACH:
        case DLL_PROCESS_DETACH:
            break;
    }
    return TRUE;
}
#endif

#ifdef      Q_OS_WIN32
#include    <windows.h>
BOOL APIENTRY DllMain( HANDLE hModule,
                       DWORD  ul_reason_for_call,
                       LPVOID lpReserved
                     )
{
    switch (ul_reason_for_call)
    {
        case DLL_PROCESS_ATTACH:
        case DLL_THREAD_ATTACH:
        case DLL_THREAD_DETACH:
        case DLL_PROCESS_DETACH:
            break;
    }
    return TRUE;
}
#endif

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I know that it is windows specific code, but I dont know the meaning.

Thank you for help.
Paweł

[auba]

I don't know what _MBCS and _USRDLL mean?

Me either I think these both defines where once set in a Visual Project File, and I needed them somehow. Maybe you can cut or just ignore them

Could you also explain code from mydll.cpp?

Once again, this is a copy of a Visual Studio Generated DLL project. When I dont have it, I get (or got) unresolved externals. You can add some qDebugs and see, when your dll is loaded or when a process attaches to/detaches from it. Has something to do with the loading mechanism. Maybe you can even trigger how many times it is loaded when used by two program (I think I needed it to find some problems when using a static library with

extern MyClass myclass;

extern MyClass myclass;

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which was linked against the main program and such a dll. Quiz: How many instances of MyClass are generated? If more than one, which one uses the main app and which the dll?

Resume: avoid those externals