Linking to external shared library.

Chrisw01

Hello,

i am using Qt 6.3.1 with Qt Creator 7 on Linux. When i try to call a function in my added C Shared Library i get undefined reference <function>. The linker shows the -L and the l<shared name> in the compile output. There are no errors if I comment out the shared C function . I have moved the shared library and its symbolic links to a library folder and re-ran ldconfig -v and it shows it finds the shared library. When I run ldd on the generated binary it does not list my shared library in the output. Any ideas?

Thanks..

Axel Spoerl

The most common reason for such behaviour is that you link to another (version of the) library. It can also be a dependency of that shared library to another one, but you’d see that in the include statement already.
Please paste your .pro / CMakeLists.txt, compiler command and output. In Qt creator the latter is in the compiler tab.

Chrisw01

@Axel-Spoerl Hello and thank you for replying...

Here is requested snippets

QT       += core gui

greaterThan(QT_MAJOR_VERSION, 4): QT += widgets

CONFIG += c++17

# 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 \
    bbsmonitor.cpp

HEADERS += \
    bbsmonitor.h

FORMS += \
    bbsmonitor.ui

INCLUDEPATH += /home/chris/sourceCode/gitCode/falkenbbs/include

# 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$$PWD/../../../gitCode/falkenbbs/shared/release/ -lfalken
else:win32:CONFIG(debug, debug|release): LIBS += -L$$PWD/../../../gitCode/falkenbbs/shared/debug/ -lfalken
else:unix: LIBS += -L$$PWD/../../../gitCode/falkenbbs/shared/ -lfalken

DEPENDPATH += $$PWD/../../../gitCode/falkenbbs/shared

And the compiled output

07:32:34: Running steps for project bbsMonitor...
07:32:34: Configuration unchanged, skipping qmake step.
07:32:34: Starting: "/usr/bin/make" -j2
/home/chris/Qt/6.3.1/gcc_64/libexec/uic ../bbsMonitor/bbsmonitor.ui -o ui_bbsmonitor.h
g++ -c -pipe -g -std=gnu++1z -Wall -Wextra -D_REENTRANT -fPIC -DQT_QML_DEBUG -DQT_WIDGETS_LIB -DQT_GUI_LIB -DQT_CORE_LIB -I../bbsMonitor -I. -I../../../gitCode/falkenbbs/include -I../../../../Qt/6.3.1/gcc_64/include -I../../../../Qt/6.3.1/gcc_64/include/QtWidgets -I../../../../Qt/6.3.1/gcc_64/include/QtGui -I../../../../Qt/6.3.1/gcc_64/include/QtCore -I. -I. -I../../../../Qt/6.3.1/gcc_64/mkspecs/linux-g++ -o main.o ../bbsMonitor/main.cpp
g++ -pipe -g -std=gnu++1z -Wall -Wextra -dM -E -o moc_predefs.h ../../../../Qt/6.3.1/gcc_64/mkspecs/features/data/dummy.cpp
g++ -c -pipe -g -std=gnu++1z -Wall -Wextra -D_REENTRANT -fPIC -DQT_QML_DEBUG -DQT_WIDGETS_LIB -DQT_GUI_LIB -DQT_CORE_LIB -I../bbsMonitor -I. -I../../../gitCode/falkenbbs/include -I../../../../Qt/6.3.1/gcc_64/include -I../../../../Qt/6.3.1/gcc_64/include/QtWidgets -I../../../../Qt/6.3.1/gcc_64/include/QtGui -I../../../../Qt/6.3.1/gcc_64/include/QtCore -I. -I. -I../../../../Qt/6.3.1/gcc_64/mkspecs/linux-g++ -o bbsmonitor.o ../bbsMonitor/bbsmonitor.cpp
/home/chris/Qt/6.3.1/gcc_64/libexec/moc -DQT_QML_DEBUG -DQT_WIDGETS_LIB -DQT_GUI_LIB -DQT_CORE_LIB --include /home/chris/sourceCode/qtCode/falkenbbs/build-bbsMonitor-Desktop_Qt_6_3_1_GCC_64bit-Debug/moc_predefs.h -I/home/chris/Qt/6.3.1/gcc_64/mkspecs/linux-g++ -I/home/chris/sourceCode/qtCode/falkenbbs/bbsMonitor -I/home/chris/sourceCode/gitCode/falkenbbs/include -I/home/chris/Qt/6.3.1/gcc_64/include -I/home/chris/Qt/6.3.1/gcc_64/include/QtWidgets -I/home/chris/Qt/6.3.1/gcc_64/include/QtGui -I/home/chris/Qt/6.3.1/gcc_64/include/QtCore -I. -I/usr/include/c++/9 -I/usr/include/x86_64-linux-gnu/c++/9 -I/usr/include/c++/9/backward -I/usr/lib/gcc/x86_64-linux-gnu/9/include -I/usr/local/include -I/usr/include/x86_64-linux-gnu -I/usr/include ../bbsMonitor/bbsmonitor.h -o moc_bbsmonitor.cpp
g++ -c -pipe -g -std=gnu++1z -Wall -Wextra -D_REENTRANT -fPIC -DQT_QML_DEBUG -DQT_WIDGETS_LIB -DQT_GUI_LIB -DQT_CORE_LIB -I../bbsMonitor -I. -I../../../gitCode/falkenbbs/include -I../../../../Qt/6.3.1/gcc_64/include -I../../../../Qt/6.3.1/gcc_64/include/QtWidgets -I../../../../Qt/6.3.1/gcc_64/include/QtGui -I../../../../Qt/6.3.1/gcc_64/include/QtCore -I. -I. -I../../../../Qt/6.3.1/gcc_64/mkspecs/linux-g++ -o moc_bbsmonitor.o moc_bbsmonitor.cpp
g++ -Wl,-rpath,/home/chris/Qt/6.3.1/gcc_64/lib -Wl,-rpath-link,/home/chris/Qt/6.3.1/gcc_64/lib -o bbsMonitor  main.o bbsmonitor.o moc_bbsmonitor.o   -L/home/chris/sourceCode/qtCode/falkenbbs/bbsMonitor/../../../gitCode/falkenbbs/shared/ -lfalken /home/chris/Qt/6.3.1/gcc_64/lib/libQt6Widgets.so /home/chris/Qt/6.3.1/gcc_64/lib/libQt6Gui.so /home/chris/Qt/6.3.1/gcc_64/lib/libQt6Core.so -lpthread -lGL   
07:32:38: The process "/usr/bin/make" exited normally.
07:32:38: Elapsed time: 00:04.

And when you run ldd against the binary this is the output...

	linux-vdso.so.1 (0x00007ffec90f4000)
	libQt6Widgets.so.6 => /home/chris/Qt/6.3.1/gcc_64/lib/libQt6Widgets.so.6 (0x00007f1e5f0c3000)
	libQt6Core.so.6 => /home/chris/Qt/6.3.1/gcc_64/lib/libQt6Core.so.6 (0x00007f1e5ea60000)
	libstdc++.so.6 => /lib/x86_64-linux-gnu/libstdc++.so.6 (0x00007f1e5e86a000)
	libgcc_s.so.1 => /lib/x86_64-linux-gnu/libgcc_s.so.1 (0x00007f1e5e84f000)
	libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007f1e5e65d000)
	libQt6Gui.so.6 => /home/chris/Qt/6.3.1/gcc_64/lib/libQt6Gui.so.6 (0x00007f1e5dcda000)
	libGL.so.1 => /lib/x86_64-linux-gnu/libGL.so.1 (0x00007f1e5dc50000)
	libxkbcommon.so.0 => /lib/x86_64-linux-gnu/libxkbcommon.so.0 (0x00007f1e5dc0e000)
	libpthread.so.0 => /lib/x86_64-linux-gnu/libpthread.so.0 (0x00007f1e5dbeb000)
	libm.so.6 => /lib/x86_64-linux-gnu/libm.so.6 (0x00007f1e5da9c000)
	libicui18n.so.56 => /home/chris/Qt/6.3.1/gcc_64/lib/libicui18n.so.56 (0x00007f1e5d603000)
	libicuuc.so.56 => /home/chris/Qt/6.3.1/gcc_64/lib/libicuuc.so.56 (0x00007f1e5d24b000)
	libicudata.so.56 => /home/chris/Qt/6.3.1/gcc_64/lib/libicudata.so.56 (0x00007f1e5b866000)
	libdl.so.2 => /lib/x86_64-linux-gnu/libdl.so.2 (0x00007f1e5b860000)
	libglib-2.0.so.0 => /lib/x86_64-linux-gnu/libglib-2.0.so.0 (0x00007f1e5b737000)
	libz.so.1 => /lib/x86_64-linux-gnu/libz.so.1 (0x00007f1e5b71b000)
	libgthread-2.0.so.0 => /lib/x86_64-linux-gnu/libgthread-2.0.so.0 (0x00007f1e5b716000)
	librt.so.1 => /lib/x86_64-linux-gnu/librt.so.1 (0x00007f1e5b70c000)
	/lib64/ld-linux-x86-64.so.2 (0x00007f1e5f7f5000)
	libEGL.so.1 => /lib/x86_64-linux-gnu/libEGL.so.1 (0x00007f1e5b6f5000)
	libfontconfig.so.1 => /lib/x86_64-linux-gnu/libfontconfig.so.1 (0x00007f1e5b6ae000)
	libX11.so.6 => /lib/x86_64-linux-gnu/libX11.so.6 (0x00007f1e5b571000)
	libQt6DBus.so.6 => /home/chris/Qt/6.3.1/gcc_64/lib/libQt6DBus.so.6 (0x00007f1e5b4a1000)
	libfreetype.so.6 => /lib/x86_64-linux-gnu/libfreetype.so.6 (0x00007f1e5b3e2000)
	libGLdispatch.so.0 => /lib/x86_64-linux-gnu/libGLdispatch.so.0 (0x00007f1e5b328000)
	libGLX.so.0 => /lib/x86_64-linux-gnu/libGLX.so.0 (0x00007f1e5b2f4000)
	libpcre.so.3 => /lib/x86_64-linux-gnu/libpcre.so.3 (0x00007f1e5b281000)
	libexpat.so.1 => /lib/x86_64-linux-gnu/libexpat.so.1 (0x00007f1e5b253000)
	libuuid.so.1 => /lib/x86_64-linux-gnu/libuuid.so.1 (0x00007f1e5b24a000)
	libxcb.so.1 => /lib/x86_64-linux-gnu/libxcb.so.1 (0x00007f1e5b220000)
	libdbus-1.so.3 => /lib/x86_64-linux-gnu/libdbus-1.so.3 (0x00007f1e5b1cd000)
	libpng16.so.16 => /lib/x86_64-linux-gnu/libpng16.so.16 (0x00007f1e5b195000)
	libXau.so.6 => /lib/x86_64-linux-gnu/libXau.so.6 (0x00007f1e5b18f000)
	libXdmcp.so.6 => /lib/x86_64-linux-gnu/libXdmcp.so.6 (0x00007f1e5b187000)
	libsystemd.so.0 => /lib/x86_64-linux-gnu/libsystemd.so.0 (0x00007f1e5b0d8000)
	libbsd.so.0 => /lib/x86_64-linux-gnu/libbsd.so.0 (0x00007f1e5b0bc000)
	liblzma.so.5 => /lib/x86_64-linux-gnu/liblzma.so.5 (0x00007f1e5b093000)
	liblz4.so.1 => /lib/x86_64-linux-gnu/liblz4.so.1 (0x00007f1e5b072000)
	libgcrypt.so.20 => /lib/x86_64-linux-gnu/libgcrypt.so.20 (0x00007f1e5af54000)
	libgpg-error.so.0 => /lib/x86_64-linux-gnu/libgpg-error.so.0 (0x00007f1e5af31000)

As you can see libfalken is not linked to the binary. Even if I was missing a symbolic link it would still show the library with a Not Found error.

I've tried with CMake and it showed the library being linked in compiler tab but same outcome so I switched to QMake and added the library via the Add Library tab. If I put a function in my code from the library it will error out.

Thanks...

Christian Ehrlicher

Then you maybe don't use any function from this library so the linker doesn't need to add it. qmake pass the correct flags to g++/the linker so it's not a qmake (or cmake) problem.

Chrisw01

When I try to use a function from that library it fails to compile

09:40:55: Running steps for project bbsMonitor...
09:40:55: Configuration unchanged, skipping qmake step.
09:40:55: Starting: "/usr/bin/make" -j2
g++ -c -pipe -g -std=gnu++1z -Wall -Wextra -D_REENTRANT -fPIC -DQT_QML_DEBUG -DQT_WIDGETS_LIB -DQT_GUI_LIB -DQT_CORE_LIB -I../bbsMonitor -I. -I../../../gitCode/falkenbbs/include -I../../../../Qt/6.3.1/gcc_64/include -I../../../../Qt/6.3.1/gcc_64/include/QtWidgets -I../../../../Qt/6.3.1/gcc_64/include/QtGui -I../../../../Qt/6.3.1/gcc_64/include/QtCore -I. -I. -I../../../../Qt/6.3.1/gcc_64/mkspecs/linux-g++ -o bbsmonitor.o ../bbsMonitor/bbsmonitor.cpp
g++ -Wl,-rpath,/home/chris/Qt/6.3.1/gcc_64/lib -Wl,-rpath-link,/home/chris/Qt/6.3.1/gcc_64/lib -o bbsMonitor  main.o bbsmonitor.o moc_bbsmonitor.o   -L/home/chris/sourceCode/qtCode/falkenbbs/bbsMonitor/../../../gitCode/falkenbbs/shared/ -lfalken /home/chris/Qt/6.3.1/gcc_64/lib/libQt6Widgets.so /home/chris/Qt/6.3.1/gcc_64/lib/libQt6Gui.so /home/chris/Qt/6.3.1/gcc_64/lib/libQt6Core.so -lpthread -lGL   
/usr/bin/ld: bbsmonitor.o: in function `bbsMonitor::bbsMonitor(QWidget*)':
/home/chris/sourceCode/qtCode/falkenbbs/build-bbsMonitor-Desktop_Qt_6_3_1_GCC_64bit-Debug/../bbsMonitor/bbsmonitor.cpp:10: undefined reference to `util_init()'
collect2: error: ld returned 1 exit status
make: *** [Makefile:342: bbsMonitor] Error 1
09:40:58: The process "/usr/bin/make" exited with code 2.
Error while building/deploying project bbsMonitor (kit: Desktop Qt 6.3.1 GCC 64bit)
When executing step "Make"
09:40:58: Elapsed time: 00:03.

The util_init function resides within the library I have linked to the binary. Actually calling any function within the library generates the same error...

Chrisw01

@Christian-Ehrlicher said in Linking to external shared library.:

Then you maybe don't use any function from this library so the linker doesn't need to add it. qmake pass the correct flags to g++/the linker so it's not a qmake (or cmake) problem.

Hey, thanks, you made me rethink things... And you were 100% correct. I needed to declare the function as an extern "C" function. Once that was done it compiles and works great!

Thanks for the insight..