Qt Creator中的共享库与mingw给出了未定义的引用错误

Question

Resolved. When I add my library as a normal internal library everything is fine. What I had failed to do was use the MY_LIBRARY_EXPORT macro on some free functions defined in a namespace. Somehow I had manipulated the library import in such a way to get those functions working, but it broke in other ways as described here.

I have not been able to use a shared library using the Qt Creator wizard. I have tried with dynamic linkage, and have failed, and cannot use as a static lib either. In both cases I get an 'undefined reference to ` IMP _*' error.

Here are my pro files. They are both in the same SUB_DIR parent project and the paths are correct.

Relevant parts of the library pro file:

TARGET = Prospec
TEMPLATE = lib
CONFIG += staticlib    
QMAKE_CXXFLAGS += -std=c++11    
DEFINES += PROSPEC_LIBRARY

The entirety of the user .pro file. (ProspecTest is a unit test project for Prospec).

#-------------------------------------------------
#
# Project created by QtCreator 2013-12-12T15:04:30
#
#-------------------------------------------------

QT       += testlib    
QT       -= gui

TARGET = prospectest
CONFIG   += console
CONFIG   -= app_bundle

TEMPLATE = app

QMAKE_CXXFLAGS += -std=c++11

SOURCES += prospectest.cpp \
    mltest.cpp \
    convertertest.cpp \
    numericitemtest.cpp
DEFINES += SRCDIR=\\\"$$PWD/\\\"

HEADERS += \
    utilities.h \
    mltest.h \
    convertertest.h \
    numericitemtest.h

win32:CONFIG(release, debug|release): LIBS += -L$$PWD/../Prospec/release/ -lProspec
else:win32:CONFIG(debug, debug|release): LIBS += -L$$PWD/../Prospec/debug/ -lProspec

INCLUDEPATH += $$PWD/../Prospec/debug
DEPENDPATH += $$PWD/../Prospec/debug

win32:CONFIG(release, debug|release): PRE_TARGETDEPS += $$PWD/../Prospec/release/libProspec.a
else:win32:CONFIG(debug, debug|release): PRE_TARGETDEPS += $$PWD/../Prospec/debug/libProspec.a

# Boost
INCLUDEPATH += C:/boost/boost_1_55_0/
LIBS += "-LC:/boost/boost_1_55_0/stage/lib/"

EDIT: the undefined reference errors happen only where I invoke functionality in a source file. For instance, if I construct a library object I get the error, but if I move that objects constructor into a header file, then compile is OK. Right now I'm not sure if the problem is due to the Qt environment, or me not understanding shared libraries in general.

Answer 1

The error you describe points to a linker error. It happens when a necessary object/lib file is not specified. The most likely scenario is this: you include a header and use a function from that header, but don't specify in which library that function is defined.

Small example:

//test.c
#include <winsock.h>
int main() { gethostbyname("localhost"); }

'undefined reference' linker error:

>gcc test.c
cczICEqq.o:test.c:(.text+0x1e): undefined refer ence to `gethostbyname@4'
collect2.exe: error: ld returned 1 exit status

In this case you would have to specify libwsock32.a like this:

gcc test.c -lwsock32

Also, note that a libsomething.a is not the only place where a reference can be resolved. You might have missed an object file.

Unfortunately without more details I am not able to tell you the exact problem.

Answer 2

You probably forgot to export your symbols, as described here on MSDN and in various questions on this website. It's a pain in general, and there are a few ways to handle this, none much better than the other.

This makes it so that the DLL and import library actually contain a reference to the symbols you compiled into it. You can compare it partially to GCC's symbol visibility, although that only works in Linux, and GCC also supports the stuff described on MSDN for Windows.

Answer 3

Had the same problem just now. Build was working fine on macOS/OSX but not on Windows with the same codebase. Thanks to rubenvb , who pointed me in the right direction. Here the full solution, how to do it for Qt:

Use Q_DECL_EXPORT on functions when building the lib. And use Q_DECL_IMPORT when using the functions from the lib.

Example how to do this here: https://wiki.qt.io/How_to_create_a_library_with_Qt_and_use_it_in_an_application#Creating_a_shared_library

Solved the problem in my case, hope it helps!