CODEWITHSUNDEEP
c++cqt
Alexander Lyapin
Alexander Lyapin

Reputation: 347

Problem with linking C library to Qt project

I know this is a 100times discussed topic, but after number of attempts, I just can't find the solution since don't understand the situation - no errors. I am trying to connect dll "C" library to my project and receiving some crazy code in App output "...exited with code -1073741515" as well as empty console with Press to close this window...

So, here are mine: mylib.c:

#include "mylib.h"
int mysum(int a, int b){
    return a + b;
}

mylib.h:

#ifdef __cplusplus
extern "C" {
#endif
#define EXPORT __declspec(dllexport)

EXPORT int mysum(int, int);

#ifdef __cplusplus
}
#endif

testlib.pro:

QT -= gui

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

SOURCES += \
        main.cpp

LIBS += -L$$PWD/../../../../TestDLL/ -lmylib

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

HEADERS += \
    ../../../../TestDLL/mylib.h

main.cpp:

#include <QCoreApplication>
#include "mylib.h"
#include <QtDebug>

int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);
    qDebug()<<mysum(1,2);
    return a.exec();
}

I think, I am missing something with linkage in pro file, but can't get what. TestDLL is the folder with:

mylib.dll mylib.h mylib.c libmylib.a

to build the dll I've used:

gcc -c mylib.c
gcc -shared -o mylib.dll -Wl,--out-implib,libmylib.a mylib.o

Will be appreciated for some hints... Thank you

Upvotes: 1

Views: 168

Answers (2)

Alexander Lyapin
Alexander Lyapin

Reputation: 347

So, what I've changed following @R Sahu hints:

in mylib.h :

#ifdef BUILDING_DLL
#define EXPORT __declspec(dllexport)
#else
#define EXPORT __declspec(dllimport)
#endif

and for compiling dll:

gcc -c -DBUILDING_DLL mylib.c
gcc -shared -o mylib.dll mylib.o -Wl,--out-implib,libmylib.a

Plus in .pro file:

LIBS += "$$PWD/../../../../TestDLL/mylib.dll"

I believe, the line in .pro file is the most important. Now everything works fine.

Upvotes: 1

R Sahu
R Sahu

Reputation: 206557

You have to manipulate the compiler options such that:

  1. When building the library, you have

    #define EXPORT __declspec(dllexport)

  2. When using the library, you have

    #define EXPORT __declspec(dllimport)

One way to do this is to use:

#if defined(BUILD_DLL)
   #define EXPORT __declspec(dllexport)
#else
   #define EXPORT __declspec(dllimport)
#endif

And then use -DBUILD_DLL when building the DLL and leave it undefined when using the DLL.

Upvotes: 2

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