Understanding how to link dynamic / static library with Qt and MinGW

[giorgik]

Hello everyone, I have a bit confused about how to write in the project file .pro for QtCreator to link a dynamic and static library. I use the MinGW compiler (making a Windows).
Suppose you have the following libraries:
1 - Dynamic: vettore.dll
2 - static: matrice.a
Suppose also that the two libraries are in the directory:
1 - C :/ dynamic dynamic library vettore.dll
2 - C :/ static for static library matrice.a

The project file is called test.pro. How should I write it for him to link the libraries to my program that do the test named main.cpp ?

[amleto]

LIBS += -Lc:/dynamic -Lc:/static -lvettore -lmatrice

http://doc.qt.digia.com/qt/qmake-project-files.html

for windows you will need a .lib to go with the .dll for linking, I believe

[giorgik]

Thank you. To obtain a dynamic library. Lib and. Dll with QtCreator, how should I do? I can write a very simple example (like Hello World) ?

[amleto]

using the creator gui:
new project -> c++ library -> Type= Shared library

This gives me:
pro file

QT       -= core gui
 
TARGET = helloworld_dll
TEMPLATE = lib
 
DEFINES += HELLOWORLD_DLL_LIBRARY
 
SOURCES += helloworld_dll.cpp
 
HEADERS += helloworld_dll.h\
        helloworld_dll_global.h

QT       -= core gui

TARGET = helloworld_dll
TEMPLATE = lib

DEFINES += HELLOWORLD_DLL_LIBRARY

SOURCES += helloworld_dll.cpp

HEADERS += helloworld_dll.h\
        helloworld_dll_global.h

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header

#ifndef HELLOWORLD_DLL_H
#define HELLOWORLD_DLL_H
 
#include "helloworld_dll_global.h"
 
class HELLOWORLD_DLLSHARED_EXPORT Helloworld_dll {
public:
    Helloworld_dll();
 
    void print();
};
 
#endif // HELLOWORLD_DLL_H

#ifndef HELLOWORLD_DLL_H
#define HELLOWORLD_DLL_H

#include "helloworld_dll_global.h"

class HELLOWORLD_DLLSHARED_EXPORT Helloworld_dll {
public:
    Helloworld_dll();

    void print();
};

#endif // HELLOWORLD_DLL_H

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source

#include "helloworld_dll.h"
 
#include <iostream>
Helloworld_dll::Helloworld_dll()
{
}
 
void Helloworld_dll::print()
{
    std::cout << "hi from dll\n";
}

#include "helloworld_dll.h"

#include <iostream>
Helloworld_dll::Helloworld_dll()
{
}

void Helloworld_dll::print()
{
    std::cout << "hi from dll\n";
}

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dll defines

#ifndef HELLOWORLD_DLL_GLOBAL_H
#define HELLOWORLD_DLL_GLOBAL_H
 
#include <QtCore/qglobal.h>
 
#if defined(HELLOWORLD_DLL_LIBRARY)
#  define HELLOWORLD_DLLSHARED_EXPORT Q_DECL_EXPORT
#else
#  define HELLOWORLD_DLLSHARED_EXPORT Q_DECL_IMPORT
#endif
 
#endif // HELLOWORLD_DLL_GLOBAL_H

#ifndef HELLOWORLD_DLL_GLOBAL_H
#define HELLOWORLD_DLL_GLOBAL_H

#include <QtCore/qglobal.h>

#if defined(HELLOWORLD_DLL_LIBRARY)
#  define HELLOWORLD_DLLSHARED_EXPORT Q_DECL_EXPORT
#else
#  define HELLOWORLD_DLLSHARED_EXPORT Q_DECL_IMPORT
#endif

#endif // HELLOWORLD_DLL_GLOBAL_H

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[giorgik]

thanks amleto, for the sample code. I discovered, however, that it is not enough that automatically creates the wizard, you have in the project file (.pro file), add this line:

CONFIG += dll

CONFIG += dll

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so the compiler does not find the function main (being a library, there is no main).

[ChrisW67]

No you don't. The linker will only expect to find a main() if the template type is app. Amletos' example builds cleanly and correctly with MingW.

When you build a "lib" project with you will get a shared library by default on Linux and Windows. All that adding "dll" to CONFIG does is request a shared library again.
If you build the project with the MingW tools there will be a resulting helloworld_dll.dll file and a libhelloworld_dll.a file.
If you build the same project with the Microsoft compiler you will get helloworld_dll.dll and helloworld_dll.lib.
The linkers use information from the .a or .lib file to arrange linking to the the .dll at run time. Only the .dll file is required at run time.

[giorgik]

Thanks
ChrisW67