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

orsini29

Hi all,

So as I have been experiencing this error for a little under a week, I have searched this forum for the answer, and have came across many possible solutions but none that have worked for me yet. I am currently trying to connect a .lib or .dll file to my project. My understanding is that I only need one of them, so whichever allows me to use the functions provided would be okay at this point.

Some specs:

I am programming the application in C++
My Qt version is Qt 6.2.2
I am on a Windows PC
I am using Qt Creator

Currently, I am experiencing the error

No rule to make target C:/Users/Sorsini/Documents/Qt ProtoTypes/Test/Test../../../../../../DLL/libFwlib32d.a needed by debug/Test.exe. Stop.

My current *.pro file is the following:

QT       += core gui

greaterThan(QT_MAJOR_VERSION, 4): QT += widgets

CONFIG += c++11

# You can make your code fail to compile if it uses deprecated APIs.
# In order to do so, uncomment the following line.
#DEFINES += QT_DISABLE_DEPRECATED_BEFORE=0x060000    # disables all the APIs deprecated before Qt 6.0.0

SOURCES += \
    main.cpp \
    mainwindow.cpp

HEADERS += \
    Fwlib32.h \
    mainwindow.h

FORMS += \
    mainwindow.ui

# Default rules for deployment.
qnx: target.path = /tmp/$${TARGET}/bin
else: unix:!android: target.path = /opt/$${TARGET}/bin
!isEmpty(target.path): INSTALLS += target


win32:CONFIG(release, debug|release): LIBS += -L$$PWD/../../../../../../DLL/ -lFwlib32
else:win32:CONFIG(debug, debug|release): LIBS += -L$$PWD/../../../../../../DLL/ -lFwlib32d

INCLUDEPATH += $$PWD/../../../../../../DLL
DEPENDPATH += $$PWD/../../../../../../DLL

win32-g++:CONFIG(release, debug|release): PRE_TARGETDEPS += $$PWD/../../../../../../DLL/libFwlib32.a
else:win32-g++:CONFIG(debug, debug|release): PRE_TARGETDEPS += $$PWD/../../../../../../DLL/libFwlib32d.a
else:win32:!win32-g++:CONFIG(release, debug|release): PRE_TARGETDEPS += $$PWD/../../../../../../DLL/Fwlib32.lib
else:win32:!win32-g++:CONFIG(debug, debug|release): PRE_TARGETDEPS += $$PWD/../../../../../../DLL/Fwlib32d.lib

On my C drive, I have a folder named DLL (I know this only contains .h and .lib files, I have been testing a plethora of things to make this work), which has the following files in it

Fwlib32.h
Fwlib32.lib

My original belief was that I would specify these with the INCLUDEPATH/LIBS variable as

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

Although when I took out the Qt generated 'Add to library' code, which starts at the CONFIG variable in the above .pro file, I received the following error:

undefined reference to `__imp_....`

Where the '...' implies the function name I was testing on.

I hope someone can spot what I am doing incorrectly, as I am new to Qt but have done many packaging with .dll's in Maven for Java, and I recall experiencing learning curve issues when beginning that, but I just cannot seem to figure out what I'm doing wrong here.

Thank you in advance, and if you need anymore information please let me know!

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!

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

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