Linking against DLL and calling function in DLL at run-time

kahlenberg

Hi,
There are lots of topics about linking DLLs but I couldn't find proper answer to my question. Maybe I am missing something.
So, in order to link to a external, 3rd party library there are some ways according to my understanding:

Static linking at compile-time. Either with wizard in QtCreator or manually add or extend INCLUDEPATH, LIBS variables in .pro file. Include header file of DLL in source code and call functions.
Dynamic linking at run-time (which I want). Using QLibrary, load the DLL from a valid path and include header file of DLL in source code. Get functions in DLL (one-by-one?) with resolve function of QLibrary (Sort of wrapping).

Is this correct?

I have a USB chip ( MCP2221) from Microchip and want to use in Qt project. They provide DLL and LIB files and header file.
How can I load library at run-time. I think I don't need to change my .pro file since it will not be static linking.
How can I use this function?

//mcp2221_dll_um.h
int Mcp2221_GetLibraryVersion(wchar_t* version);

I try to make exactly same thing in the end of this page https://wiki.qt.io/How_to_link_to_a_dll . But important information is missing.
Thanks.

aha_1980

Hi @kahlenberg

as you said you want to load the library at runtime, have a look at the QLibrary documentation page.

I've adopted the example from there a bit, so for your case it could look like this:

QLibrary myLib("mcp2221");

typedef int (*fp_Mcp2221_GetLibraryVersion)(wchar_t*);

fp_Mcp2221_GetLibraryVersion myFunction = 
  (fp_Mcp2221_GetLibraryVersion)
  myLib.resolve("Mcp2221_GetLibraryVersion");

if (Mcp2221_GetLibraryVersion) {
    int result = Mcp2221_GetLibraryVersion("whatever");
}

Disclaimer: I've not tested this.

SGaist

Hi,

For static linking, you need static libraries. Since you have a .dll, you have a shared library.
That's rather dynamic loading, there's not linking done

Out of curiosity, since you have both the .lib file and header, why not link to the library directly ?

kahlenberg

@aha_1980
That is what I am exactly doing, but I get error. Here is my code:

    typedef int (__stdcall *Fp_GetLibraryVersion)(wchar_t *); // with or without __stdcall
	
	if(QLibrary::isLibrary(MCP2221LibPath)) { // a valid path
        MCP2221Lib.setFileName(MCP2221LibPath); 
        MCP2221Lib.load();
        if(MCP2221Lib.isLoaded())
        {
            wchar_t *libver;
            QString libVersion;

            Fp_GetLibraryVersion Resolved_GetLibraryVersion = (Fp_GetLibraryVersion) MCP2221Lib.resolve(QString("Mcp2221_GetLibraryVersion").toLatin1().data()); // with or without toLatin1().data()
            qDebug() << MCP2221Lib.errorString(); // Error string comes from here

            if (Resolved_GetLibraryVersion)
            {
                qDebug() << Resolved_GetLibraryVersion(libver);

                libVersion = QString::fromWCharArray(libver);
                qDebug() << libVersion;
            }
        }
        else
            qDebug() << "Error " << MCP2221Lib.errorString() << "\n";
    } else
        qDebug() << "Not a library\n";

I am getting:

Cannot resolve symbol "Mcp2221_GetLibraryVersion" in C:\Projects\SW\test\MCP2221_DLL(v2.2)\unmanaged\dll\mcp2221_dll_um_x86.dll: The specified procedure could not be found.

@SGaist
I just want to try whether or not it is possible, but I think I will link it at compile-time.

JonB

@kahlenberg said in Linking against DLL and calling function in DLL at run-time:

Mcp2221_GetLibraryVersion

One thing: is Mcp2221_GetLibraryVersion actually exported by your library file? What is its export signature?

aha_1980

@kahlenberg said in Linking against DLL and calling function in DLL at run-time:

I just want to try whether or not it is possible, but I think I will link it at compile-time.

I'd say either both variants work or none of them.

Is your program 32 bit too as the library seems to be 32 bit?

kahlenberg

@JonB
This is the header file of library:

#ifdef __cplusplus
extern "C"{
#endif


    //for projects importing the .lib, use the MCP2221_LIB preprocessor definition
#ifndef MCP2221_LIB
	#ifdef MCP2221_DLL_UM_EXPORTS
		#define MCP2221_DLL_UM_API __declspec(dllexport)
	#else
		#define MCP2221_DLL_UM_API __declspec(dllimport)
	#endif
#else 
	#define MCP2221_DLL_UM_API
#endif

#define CALLING_CONVENTION __stdcall
//...
MCP2221_DLL_UM_API int CALLING_CONVENTION Mcp2221_GetLibraryVersion(wchar_t *version);
//...
#ifdef __cplusplus
}
#endif

@aha_1980
Both variants are not working. I also tried to link as static, it gives linker error "undefined reference to `_imp__Mcp2221_GetLibraryVersion@4'". I am using MinGW_5.3.0 32 bit and library is also 32 bit.

I created a console application as test project and added library via wizard.
Right-click on project->Add Library->External Library->Linkage:Static, uncheck "Add d suffix for debug version->Finish.

Here is .pro file:

QT -= gui

CONFIG += c++11 console
CONFIG -= app_bundle

# The following define makes your compiler emit warnings if you use
# any feature of Qt which as been marked deprecated (the exact warnings
# depend on your compiler). Please consult the documentation of the
# deprecated API in order to know how to port your code away from it.
DEFINES += QT_DEPRECATED_WARNINGS

# You can also make your code fail to compile if you use deprecated APIs.
# In order to do so, uncomment the following line.
# You can also select to disable deprecated APIs only up to a certain version of Qt.
#DEFINES += QT_DISABLE_DEPRECATED_BEFORE=0x060000    # disables all the APIs deprecated before Qt 6.0.0

SOURCES += main.cpp

unix|win32: LIBS += -L$$PWD/'../test/MCP2221_DLL(v2.2)/unmanaged/lib/' -lmcp2221_dll_um_x86

INCLUDEPATH += $$PWD/'../test/MCP2221_DLL(v2.2)/unmanaged/lib'
DEPENDPATH += $$PWD/'../test/MCP2221_DLL(v2.2)/unmanaged/lib'

win32:!win32-g++: PRE_TARGETDEPS += $$PWD/'../test/MCP2221_DLL(v2.2)/unmanaged/lib/mcp2221_dll_um_x86.lib'
else:unix|win32-g++: PRE_TARGETDEPS += $$PWD/'../test/MCP2221_DLL(v2.2)/unmanaged/lib/libmcp2221_dll_um_x86.a'

First try to build, I am getting linker error:

:-1: error: No rule to make target 'C:/Projects/SW/libraryTest/../test/MCP2221_DLL(v2.2)/unmanaged/lib/libmcp2221_dll_um_x86.a', needed by 'debug\libraryTest.exe'.  Stop.

I don't have an libmcp2221_dll_um_x86.a file. Here is the directory structure:

C:\Projects\SW\test\MCP2221_DLL(v2.2)\unmanaged\dll
C:\Projects\SW\test\MCP2221_DLL(v2.2)\unmanaged\dll\mcp2221_dll_um.h
C:\Projects\SW\test\MCP2221_DLL(v2.2)\unmanaged\dll\mcp2221_dll_um_x64.dll
C:\Projects\SW\test\MCP2221_DLL(v2.2)\unmanaged\dll\mcp2221_dll_um_x64.lib
C:\Projects\SW\test\MCP2221_DLL(v2.2)\unmanaged\dll\mcp2221_dll_um_x86.dll
C:\Projects\SW\test\MCP2221_DLL(v2.2)\unmanaged\dll\mcp2221_dll_um_x86.lib
C:\Projects\SW\test\MCP2221_DLL(v2.2)\unmanaged\lib
C:\Projects\SW\test\MCP2221_DLL(v2.2)\unmanaged\lib\mcp2221_dll_um.h
C:\Projects\SW\test\MCP2221_DLL(v2.2)\unmanaged\lib\mcp2221_dll_um_x64.lib
C:\Projects\SW\test\MCP2221_DLL(v2.2)\unmanaged\lib\mcp2221_dll_um_x86.lib

When I delete the line in .pro file

else:unix|win32-g++: PRE_TARGETDEPS += $$PWD/'../test/MCP2221_DLL(v2.2)/unmanaged/lib/libmcp2221_dll_um_x86.a'

..and try to build, I get linker error:

C:\Projects\SW\libraryTest\main.cpp:9: error: undefined reference to `_imp__Mcp2221_GetLibraryVersion@4'

Here is the main.cpp file:

#include <QCoreApplication>
#include <QDebug>
#include "mcp2221_dll_um.h"

int main(int argc, char *argv[])
{
    wchar_t libver[100];
    QCoreApplication a(argc, argv);
    qDebug() << Mcp2221_GetLibraryVersion(libver);

    return a.exec();
}

aha_1980

@kahlenberg said in Linking against DLL and calling function in DLL at run-time:

Both variants are not working. I also tried to link as static, it gives linker error "undefined reference to `_imp__Mcp2221_GetLibraryVersion@4'". I am using MinGW_5.3.0 32 bit and library is also 32 bit.

You need a linker include file (*.a) for MinGW. In your directory you only have MSVC linker include files. So you can google how to create an linker include file for MinGW or ask the library vendor. Otherwise you will not get any step forward in this direction.

Nevertheless, loading with QLibrary works independent of the Compiler. If you cannot resolve the function, then either it is not exported, or you got the name wrong or the library is not in a valid format (like 32bit vs. 64bit issue).

JonB

@kahlenberg said in Linking against DLL and calling function in DLL at run-time:

//for projects importing the .lib, use the MCP2221_LIB preprocessor definition
#ifndef MCP2221_LIB
#ifdef MCP2221_DLL_UM_EXPORTS
#define MCP2221_DLL_UM_API __declspec(dllexport)
...
MCP2221_DLL_UM_API int CALLING_CONVENTION Mcp2221_GetLibraryVersion(wchar_t *version);

I think you ought just make sure your compilation (when you compiled that DLL) is following this __declspec(dllexport) route, else you'll get nowhere?

jsulm

@kahlenberg Is the DLL built using same compiler?

kahlenberg

I solved the problem. The problem was, I was using wrong library file :). I used library file (.lib) in dll/ directory not in lib/ directory. It worked. Maybe I can update wiki at the end of https://wiki.qt.io/How_to_link_to_a_dll .

@jsulm No, I downloded them from microchip website.

Thanks for all answers.