Issue linking .lib or .dll for usage in .cpp

[Christian Ehrlicher]

How do you export the functions? Are they plain c? If so did you add extern "C" around your include statement when you use the header.

[orsini29]

The functions are in the C header file supplied by the company, and I assume that was just added to the .cpp file as

#include "Fwlib32.h"

So following your comment, would the correct form be:

extern "C" {
   #include "Fwlib32.h"
}

Edit: I just added the above include statement to my mainwindow.cpp, and still received the undefined reference error for the function...Here's my mainwindow.cpp code, it's just barebones. I wanted to connect to the library without any other possible factors before integrating it into a more complex application.

#include "mainwindow.h"
#include "ui_mainwindow.h"

extern "C" {
    #include "Fwlib32.h"
}

MainWindow::MainWindow(QWidget *parent)
    : QMainWindow(parent)
    , ui(new Ui::MainWindow) {
    ui->setupUi(this);
    cnc_mdg_moniclear(1);        // <---- This is the line that is causing the error 
}

MainWindow::~MainWindow() {
    delete ui;
}

Thanks @Christian-Ehrlicher for helping out (:

Edit #2: Not sure if I mentioned this before, or if it matters, but it is a external header file, along with the external .dll and .lib files.

[Christian Ehrlicher]

When you really include Fwlib32.h with extern "C" everywhere then you should not get warnings about missing __imp_FOO functions when I'm correct.
Make sure to add the guard everywhere. Also please post the linker error with the full name of the function.

/edit: You're using MinGW? Right? Then try to link directly against the dll instead the lib - MinGW can't do anything with the static import lib for MSVC but has the ability to directly link against C dlls since some years iirc.

[orsini29]

@Christian-Ehrlicher

The full error(s) here:

debug/mainwindow.o: in function `MainWindow::MainWindow(QWidget*)':

C:\Users\Sorsini\Documents\Qt Prototypes\Test\build-Test-Desktop_Qt_6_2_3_MinGW_64_bit-Debug/../Test/mainwindow.cpp:13: undefined reference to `__imp_cnc_mdg_moniclear'

collect2.exe: error: ld returned 1 exit status

[Makefile.Debug:73: debug/Test.exe] Error 1

I went ahead and put the extern "C" {#include "Fwlib32.h"} around where I am including the .h file, and this is the only place I include it, which is in mainwindow.cpp.

Could this be an issue with how I am initializing the LIBS and INCLUDEPATH variables within the *.pro file?

[orsini29]

@Christian-Ehrlicher Yes I am using MinGW. So instead of in the .pro file having

win32:LIBS += C:/DLL/Fwlib32.lib

// Change it to the following

win32:LIBS += C:/DLL/Fwlib32.dll

Also, how would I verify that I am using MinGW? I am almost sure that I am, as when I go to the bottom where the errors are I right click and press 'Show Output', which takes me to the compile output, and I see this for my Makefile, making me believe I am using MinGW.

C:/Qt/Tools/mingw900_64/bin/mingw32-make -f Makefile.Debug

Edit: So I changed around my .pro file to mimic what you had said about the .dll instead of the .lib. When I did that, I receievd a new error which was

collect2.exe: error: ld returned 5 exit status

I saw on stackoverflow that adding this to the .pro file helped someone mitigate the issue

mingw {
        contains(QT_ARCH, x86_64): {
            LIBS+=-Wl,--no-gc-sections
        }
}

Which now I received a
Failed to start program. Path or permissions wrong?

I know that the path is there, and that the files I am looking for are as well there...

[Christian Ehrlicher]

@orsini29 said in Issue linking .lib or .dll for usage in .cpp:

x86_64

So you're using a 64Bit Qt? The library seems to be a 32bit one (when I interpret Fwlib32 correct) - you can't mix them.

[orsini29]

@Christian-Ehrlicher

I can not believe that slipped my mind... So if I download the 32 bit version, and then configure the .pro file the same as above, minus

mingw {
        contains(QT_ARCH, x86_64): {
            LIBS+=-Wl,--no-gc-sections
        }
}

You think that would be the solution?

[hskoglund]

Hi, I just tested for you :-) ir's pretty easy to find fwlib32 from FOCAS on google

Using a Qt version 6 then it's much more difficult to build for 32-bit dlls.
So instead I recommend you downgrade to Qt 5, I just tried Qt 5.15.2 and MinGW 8.1 32-bit version.
First, make an empty vanilla widgets app.
Add this line to your .pro file:

LIBS += C:/DLL/fwlib32.lib

Edit your mainwindow.cpp file to look like this:

#include "mainwindow.h"
#include "ui_mainwindow.h"

#include "C:/DLL/fwlib32.h"

MainWindow::MainWindow(QWidget *parent)
    : QMainWindow(parent)
    , ui(new Ui::MainWindow)
{
    ui->setupUi(this);
    cnc_mdg_moniclear(1);
}

MainWindow::~MainWindow()
{
    delete ui;
}

compile and build it in Release mode.
Then to test, make sure to copy the fwlib32.dll to the same directory as the .exe file.
Good luck!

[orsini29]

@hskoglund Awesome! I am going to try that now. Thanks so much, seriously. I have been banging my head against the wall on this over a week now. I couldn't find version 5.15.2, but I did find 5.0.3. Is just the MinGW the 32 bit version? I can't seem to find a Qt Creator in 32 bit version.... Thanks again :D

Edit: Nevermind...I found the 5.15.2 version and the MinGW 8.1 32 bit..I will be testing it out shortly! Thanks again @hskoglund

[SGaist]

Hi,

There's no need for Qt Creator's architecture to match the architecture of the Qt version you use for your project.

[orsini29]

@hskoglund

I just tested out your answer, and it worked like a charm. Thank you so much for your assistance! It is very appreciated :)

[orsini29]

This post is deleted!