Qwt with CMake

I have downloaded and built qwt-6.2.0 and installed it in /usr/local/qwt-6.2.0/
I have verified that the Qwt installation is sound by compiling and running the exampled provided, using Qt Creator 9.02.
qmake --version reports Qt version 5.15.3, which I believe is the version Kubuntu 22.04 is built on.
I am trying to use Qwt as part of a program built with Qt6, using KDevelop / cmake, but I have not been able to configure CMakeLists.txt correctly.
I have found several discussions of this issue, but suspect that they might be deprecated. Anybody that knows about an up to date CMakeLists.txt for Qwt?

You can build and install Qwt for Qt6 exactly the same way like for Qt5 - there is no difference beside that you use a qmake from a Qt6 installation.
I recommend to do it from the command line - simply: /your/path/to/Qt6/bin/qmake qwt.pro.

The way how Qwt has been installed does not prevent you from using your preferred build system of choice ( f.e. cmake ) in your project.

Out of my head I would expect, that you have to:

1. set the QWT_DLL define for the preprocessor
2. add the path to the headers
3. add the lib, when linking

More or less what you have to do for almost any other 3rd party library.

QWT_DLL? I am on Ubuntu 22.04.
I can find the qwt .h-files using
include_directories(/usr/local/qwt-6.2.0/include)
But cmake does not find the libs in
/usr/local/qwt-6.2.0/lib
(the libs are there) I have tried
set(CUSTOM_LIBRARY_PATH /usr/local/qwt-6.2.0/lib )
set(CMAKE_PREFIX_PATH /usr/local/qwt-6.2.0/lib)
but all I get is:
/usr/bin/ld: cannot find -lqwt: No such file or directory
collect2: error: ld returned 1 exit status

Running
ld -lqwt --verbose
in a shell returns failure, but that is perhaps a different environment?

It seems I am misunderstanding how cmake works, but what am I doing wrong?

Can you post your CMakelists.txt file for your project? If it is too big or you can't, then create a CMake project from one of the Qwt example programs (simpleplot maybe) and post your complete CMakelists.txt file for that.

It looks like this:

Code:

---

cmake_minimum_required(VERSION 3.22)
 
project(plottapp)
SET(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++17 -pthread")
 
# Find includes in corresponding build directories
set(CMAKE_INCLUDE_CURRENT_DIR ON)
 
# Instruct CMake to run moc automatically when needed
set(CMAKE_AUTOMOC ON)
 
# Find the QtWidgets libraries
find_package(Qt6Widgets CONFIG REQUIRED)
find_package(Qt6Charts)
 
include_directories(/usr/local/qwt-6.2.0/include)  # works
set(CUSTOM_LIBRARY_PATH /usr/local/qwt-6.2.0/lib)  # does not work
set(CMAKE_PREFIX_PATH /usr/local/qwt-6.2.0/lib)    # does not work
 
set(plottapp_SRC
  main.cpp
  plottapp.cpp
  mslider.cpp
  etc.
  etc.
)
 
qt6_wrap_ui(plottapp_SRC plottapp.ui)
add_executable(plottapp ${plottapp_SRC})
target_link_libraries(plottapp Qt6::Widgets Qt6::Charts qwt ) # <-- Chokes on qwt here
 
# Install the executable
install(TARGETS plottapp DESTINATION bin)

---

Instead of using the CUSTOM_LIBRARY_PATH or CMAKE_PREFIX_PATH, you should probably use the find_library() command, something like this:

Code:

---

find_library( QWT NAMES qwt qwt6 PATHS /usr/local/qwt-6.2.0/lib REQUIRED )
 
target_link_libraries( plottapp Qt6::Widgets Qt6::Charts ${QWT} )

---

However, your problem all along could be that the library might not be named "qwt" but "qwt6" :p

In any case, find_library() is a better solution than changing CMake variables, which could have unintended consequences.

With Microsoft compilers, the order of libraries does not matter. In linux and gcc, the libraries might have to be listed in dependency order, i.e. (plottapp ${QWT} Qt6::Charts Qt6::Widgets)

That got me a bit further, as the libries are now found. However, I get a page full of linking errors, that I cannot make sense of. The order I list the libraries in target_link_libraries doen't seem to make a difference, and the errors are the same whether I compile for Release or Debug:

Code:

---

[100%] Linking CXX executable plottapp
/usr/bin/ld: /usr/local/qwt-6.2.0/lib/libqwt.so: undefined reference to `QtPrivate::QMetaTypeInterfaceWrapper<QVariant>::metaType@Qt_6'
/usr/bin/ld: /usr/local/qwt-6.2.0/lib/libqwt.so: undefined reference to `QtPrivate::QMetaTypeInterfaceWrapper<bool>::metaType@Qt_6'
/usr/bin/ld: /usr/local/qwt-6.2.0/lib/libqwt.so: undefined reference to `QtPrivate::QMetaTypeInterfaceWrapper<QSize>::metaType@Qt_6'
  .
  .
  .
/usr/bin/ld: /usr/local/qwt-6.2.0/lib/libqwt.so: undefined reference to `QtPrivate::QMetaTypeInterfaceWrapper<QTime>::metaType@Qt_6'
collect2: error: ld returned 1 exit status
make[2]: *** [CMakeFiles/plottapp.dir/build.make:591: plottapp] Error 1
make[1]: *** [CMakeFiles/Makefile2:84: CMakeFiles/plottapp.dir/all] Error 2
make: *** [Makefile:136: all] Error 2
*** Failure: Exit code 2 ***

---

Finding Qt6Widgets is supposed to also bring in the dependencies (Qt6Gui and Qt6Core) but maybe you need to add those. Those errors look like there is a low level Qt library that is not being linked in. Maybe try putting ${QWT} at the beginning of the link libraries list, followed by Charts, Widgets, Gui, and Core. I think that is the correct dependency order.

I tried adding Qt6Gui and Qt6Core, and all kinds of reordering, but to no avail. I have no problem using Qt Charts, so the issue must be related to Qwt.

Are you sure you are building qwt against your Qt6 installation? Can you successfully use qwt in a project built with QtCreator? (I know these are "Have you checked to see if it is plugged in?" questions, but I've done things like this in the past, where I have had mismatched builds and nothing seems to work).

It is MOC that results in all of the QMetaType references so something could be wrong with your qwt build.

I am not absolutely sure I am building Qwt corectly. I built Qwt thus:
/usr/bin/qmake qwt.pro
make
sudo make install

The strange thing is that all the examples supplied with Qwt works perfectly when build by QtCreator.

The file qwtconfig.pri starts of with:
QWT_VER_MAJ = 6
QWT_VER_MIN = 2
QWT_VER_PAT = 0
QWT_VERSION = $${QWT_VER_MAJ}.$${QWT_VER_MIN}.$${QWT_VER_PAT}
which looks correct to me.

I do not know if it is relevant, but qwt.pro has this information at the start:
lessThan(QT_MAJOR_VERSION, 5) {

lessThan(QT_MINOR_VERSION, 8) {
error(Qt >= 4.8 required.)
}
}

include( qwtconfig.pri )

The version 5 here probably refers to Qt 5.15.3 used by Kubuntu 22.04?
qmake --version
returns
QMake version 3.1
Using Qt version 5.15.3 in /usr/lib/x86_64-linux-gnu

I noticed that there is also qmake6, which returns:
qmake6 --version
QMake version 3.1
Using Qt version 6.2.4 in /usr/lib/x86_64-linux-gnu

So I tried:
make clean
/usr/bin/qmake6 qwt.pro
make
sudo make install
but the problem persists.

What do you see when you run this?

Code:

---

ldd -d -r libqwt.so

---

Does it show dependencies on any of the Qt6 libraries?

How about opening the qwt.pro file in QtCreator and ensuring that you are building using a Qt6 kit?

I am afraid I am not really a linux developer, so this is getting to the limits of my knowledge and experience.

I see:

Code:

---

ldd: ./libqwt.so: No such file or directory

---

Q
I then created a link:

Code:

---

ln -s /usr/local/qwt-6.2.0/lib/libqwt.so.6.2.0 libqwt.so

---

but it made no difference. But output like

Code:

---

/usr/bin/ld: /usr/local/qwt-6.2.0/lib/libqwt.so: undefined reference to `QtPrivate::QMetaTypeInterfaceWrapper<QVariant>::metaType@Qt_6'

---

indicated that the lib is found. It also indicates that qwt v. 6.2.0 is used, as well as Qt6, so it appears that there isn't a version mismatch. Also, I can build the qwt examples with Qt Creator, so libqwt.so seems to be OK.

Well. sorry, but I am at the end of my ideas. I think qmake creates a Makefile, right? (I use Visual Studio on Windows for all my development, which does not use Makefiles). Maybe take a look at one of those for your qwt examples and see if there is something in the linking that is missing from your CMakelists file. Otherwise, I don't know what else to suggest.

Yes, cmake creates a makefile, but I don't find any clues there. There is a file called CMakeCache.txt, which includes:

Code:

---

Name  Value
QWT  /usr/local/qwt-6.2.0/lib/libqwt.so

---

which is the correct path. Anyway, don't spend more time on this problem. I will keep scratching my head though. One thing I haven't thought about before: does qmake use hard-coded paths?

Quote:

---

cmake creates a makefile

---

No, I meant qmake. Check the Makefile from that to see if there are any ideas about what could be wrong with the cmake configuration.