Building Cross-plattform Applications (for real!)

I have been writing many posts about the Qt library, without making a proper introduction.
Qt is a cross-plattform C++ framework, that provides support for a lot of things such as UI, multi-threading, Graphics, etc. Since C++ itself does not have so many libraries built in as you would find on other libraries, it is a good idea to use something like this. There are other frameworks specific things such as graphics (for instance GTK+), but I don’t know of any as complete as Qt. The .NET framework is quite complete, but unlike Qt it is not cross-platform, neither it has a permissive GPL/LGPL license, so if you care about these two things it is clearly *not* an option.

Having said this, I am quite happy with the Qt libraries in the long run, even things are not as easy as they would seem.

The Windows and Linux environment are different enough, even if you stick to C++ and Qt.
In my Linux environment, I use g++ make (the default compiler), and let the system (aka package manager) to decide what is the most appropriated version. It actually takes care of all the environmental variables and I do not have to worry too much about setting a build environment, whether I use Qt creator (the “native” Qt IDE) or just the command line.
On the other hand, Windows has got a couple of options regarding compilers:

  • there is the mingGW compiler, which people use for portability (but I don’t particularly like it since you need to setup a whole set of tools, that are not native on Windows)
  • there is the native Microsoft Visual Studio toolchain, which is by excellence, a “Windows compiler”
  • there is the Qt creator “jom” compiler, which allows using multi core, but somehow I am a bit reluctant in using it, because I don’t see anybody using it outside Qt creator.

The thoughts and “suspicions” above, are nothing else but that: thoughts and suspicions. Since I have been programming in Windows for quite a while using Microsoft Visual Studio and I am quite happy about it, I just decided to use it in my Windows projects.

If you want to stick to basic configurations, the differences between Windows and Linux projects may be small, but as soon as you start to complicate them a bit it is not so simple anymore. Recently I decided to link my application statically, in order to ease deployment in a complete “user-proof” scenario. These required rebuilding Qt statically, in both OS.
After successfully compiling Qt, I tried to build my project on Linux using Qt creator and it “worked as a charm”. I did not have to change any of the environment variables, but only have to point to which version of Qt I wanted to use inside Qt creator. There were only a few things that I had to keep in mind:

  • My application uses a plugin, so I had to also link it statically (creating a .la)
  • There were some minor changes in the qmake project files, *both* of the plugin and the application, in order to compile them statically.
  • It is better to clean/rebuild the projects, so that we don’t run into the risk of having files *left* from a previous dynamic linking. From my experience, “make clean” is not so tidy, so I would recommend going inside the directories and remove the .obj, or any other intermediate files by hand…

This is the only change that I had to make in the project file of my designer plugin

dynamic linking:

CONFIG += debug_and_release

static linking:

CONFIG += release staticlib

(with static linking, there is generally no point on debugging, so I am generating only a release build)

Then I could compile my application, and link against the static versions of Qt and of this plugin, by slightly modifying the project file:

dynamic linking:

CONFIG += debug_and_release

static linking:

CONFIG += release static

And that was about it: I got a binary file with 26.9 MB, that I can take with me to any Ubuntu system 🙂

Now the Windows part was a bit more painful. First I could not get the Qt Creator to work, since it did not correctly pick up the VC+ settings, and complained about the static version of Qt not having been built with the same compiler I was trying to use. To speed up things, I decided to compile the application on the command line, more specifically inside the Visual studio shell, which is the same place where I compiled Qt.

I had some persistent linking errors regarding a DLL linkage with my plugin. I read somewhere that by default in Windows, qmake will attempt a dll linkage unless its explictly told otherwise; thus I added this flag to the DEFINES of my project files:


In Linux I did not run into such a problem. In any case, it did not solve the linking errors. After researching a bit more, I found out that to call a plugin statically, you have to invoke a specific macro on the “main” of the application, *and* include QtPlugin. Another thing that was not necessary in Linux…

#include <QtPlugin>;


And, finally the plugin has to be *explicitly* added to the project file, in this way:

QTPLUGIN += catchinputctrl

I still had linking errors, this time regarding *not* finding the plugin library; the directive that I had in Linux did not worked, and I messed around a bit with the “-L” and “-l” options in LIBS but without success, so I ended up copying the .lib file to my project directory (a quick fix). After that, I could generate the binary, but not without some linking warnings:

Creating library ..\release\faocas.lib and object ..\release\faocas.exp
frmcatch.obj : warning LNK4217: locally defined symbol ??0CatchInputCtrl@@QAE@PA
VQWidget@@@Z (public: __thiscall CatchInputCtrl::CatchInputCtrl(class QWidget *)
) imported in function "public: void __thiscall Ui_FrmCatch::setupUi(class QWidg
et *)" (?setupUi@Ui_FrmCatch@@QAEXPAVQWidget@@@Z)
frmoperation.obj : warning LNK4049: locally defined symbol ??0CatchInputCtrl@@QA
E@PAVQWidget@@@Z (public: __thiscall CatchInputCtrl::CatchInputCtrl(class QWidge
t *)) imported
frmtrip.obj : warning LNK4049: locally defined symbol ??0CatchInputCtrl@@QAE@PAV
QWidget@@@Z (public: __thiscall CatchInputCtrl::CatchInputCtrl(class QWidget *))
frmcatch.obj : warning LNK4217: locally defined symbol ??1CatchInputCtrl@@UAE@XZ
(public: virtual __thiscall CatchInputCtrl::~CatchInputCtrl(void)) imported in
function "public: virtual void * __thiscall CatchInputCtrl::`scalar deleting des
tructor'(unsigned int)" (??_GCatchInputCtrl@@UAEPAXI@Z)
frmoperation.obj : warning LNK4049: locally defined symbol ??1CatchInputCtrl@@UA
E@XZ (public: virtual __thiscall CatchInputCtrl::~CatchInputCtrl(void)) imported

frmtrip.obj : warning LNK4049: locally defined symbol ??1CatchInputCtrl@@UAE@XZ
(public: virtual __thiscall CatchInputCtrl::~CatchInputCtrl(void)) imported
mt.exe -nologo -manifest "release\faocas.intermediate.manifest" -outputr

The output dir contained my executable, as well as a exports library file and a copy of the lib. These are unnecessary, and I can happily copy my 12.2MB binary around, without having to ship anything else.

Some questions that remain in my mind:

  • Why are the binaries generated by Windows and Linux so different in size? (one almost *doubles* the size of the other)
  • Why is the static and dynamic linking in Windows so different?
  • Why is the static linking in Windows so different from the one in Linux, and why is this so undocumented? (does anybody use it at all??)
Static linked app in Windows

Static linked app in Windows

Static linked app in Linux

Static linked app in Linux


Cross-compiling, using Qt

On a previous post I described a simple example, of how-to cross compile an Windows application in a Linux environment.
Although this worked fine and raised my motivation, it is not a very useful example since the real world is much more complex. One of the complexities that I want to introduce is the use of Qt libraries, that are used thoroughly in my projects. I will now go in detail, through a slightly more complicated “Hello World”, where I make use of these libraries.

A key thing to understand, when cross compiling with Qt, is that you need to setup the development environment, both in the host and in the target OS (those are in this case: Ubuntu and Windows).

I started by downloading the Qt distribution, compiled for MinGW (of course, you may also decide to download the source codes and compile it yourself, which will add another complexity layer to this task…):


Then I ran this with Wine, and followed the installation instructions:

wine qt-win-opensource-4.8.5-mingw.exe

Qt got installed under my “wine drive”, in the $HOME directory:


As I mentioned before, you also need a working Qt environment for Linux. I had Qt 4.8.7 installed in my machine, and I stupidly thought I could use that; it turns out I can not. The Qt versions in Windows and Linux need to match! Since MinGW did not released a 4.8.7 version yet, I decided to download the Linux version of Qt 4.8.5, and installed it along with my current version (let us see what problems this may bring me in the future, if I keep developing in 4.8.7!).
If you did not understand my grump in the last paragraph, the only thing you need to know is that I downloaded Qt 4.8.5 for Linux, and installed it in:


The next step is to configure the mkspecs for cross-compiling; for that you need to navigate to /usr/local/Trolltech/Qt-4.8.5/mkspecs and create a new directory there. I called it “win32-x-g++”.


Inside this directory, you need to place a version of qmake.conf, with all the necessary information for cross compiling; mainly you need to pay attention to the paths of the Qt installed in Wine, and the minGW paths in Linux. Here is a working version for me; please amend it to reflect your paths.

Then I wrote a very simple Qt “Hello World”:

// Hello World in C++ for the Qt framework
int main(int argc, char *argv[])
	QApplication a(argc, argv); 
	QLabel l("Hello World!", 0); 
	l.resize(300, 200);;

This is just a Window with “Hello World” written, but it is calling the Qt Libraries.
My Qt project file, looks like this (I generated it with qmake, and then added the relevant libraries to the Qt variable):

# Automatically generated by qmake (2.01a) Fri Dec 20 13:18:06 2013

TARGET = hello
QT += core gui
CONFIG += qt

# Input
SOURCES += hello.cpp

I make a small parenthesis here, to note that the qmake version should be the one on:


If you do have more than one Qt version (like I do!), it is worth to set the $QTDIR variable to this directory, while cross compiling. You can check if this working as expected, by checking the path of qmake:

which qmake

If this points to /usr/local/Trolltech/Qt-4.8.5, then everything is ok; otherwise, please take a moment to export your $QTDIR.

To generate the makefiles, using the created mkspecs file above, you can use the following syntax:

qmake -spec win32-x-g++

Then you can compile the application, using make. If the compilation was successful, you should be able to run the executable “hello.exe”, using wine. When I attempted to do this, I had some errors that related to not finding some libraries. As in my previous example, I copied “mingwm10.dll” to the current directory, but I still had errors related to Wine not finding the Qt libraries.

I edited the QTDIR and PATH variables on Wine(Windows), by calling regedit.


Unfortunately that still did not do it for me. I thought a quick fix, for forcing the application to pick the libraries would be to copy the required DLL’s to the app path. Unfortunately that also did not worked 😦

err:module:import_dll Library libgcc_s_dw2-1.dll (which is needed by L"Z:\\home\\joana\\projects\\cross-compiling\\hello\\release\\QtCore4.dll") not found
err:module:import_dll Library QtCore4.dll (which is needed by L"Z:\\home\\joana\\projects\\cross-compiling\\hello\\release\\hello.exe") not found
err:module:import_dll Library libgcc_s_dw2-1.dll (which is needed by L"Z:\\home\\joana\\projects\\cross-compiling\\hello\\release\\QtCore4.dll") not found
err:module:import_dll Library QtCore4.dll (which is needed by L"Z:\\home\\joana\\projects\\cross-compiling\\hello\\release\\QtGui4.dll") not found
err:module:import_dll Library libgcc_s_dw2-1.dll (which is needed by L"Z:\\home\\joana\\projects\\cross-compiling\\hello\\release\\QtGui4.dll") not found
err:module:import_dll Library QtGui4.dll (which is needed by L"Z:\\home\\joana\\projects\\cross-compiling\\hello\\release\\hello.exe") not found
err:module:LdrInitializeThunk Main exe initialization for L"Z:\\home\\joana\\projects\\cross-compiling\\hello\\release\\hello.exe" failed, status c0000135

Then I thought, that even if I cannot run the executable with Wine, it does not necessarily mean that I did not generate it correctly (which is ultimately my purpose, with all of this!). So I went to a Windows guest in VirtualBox, and I managed to successfully run the application! 🙂


This was a nice outcome, however it is still necessary to do this for the entire project, tracking all required Qt dll’s, Boost, and any other needed libraries. It would probably be a good idea to consider static linking, since I would not have to ship the DLL’s, and would also avoid digging into the problem of “why Wine is not finding the shared libraries”. However, from my experience, static linking of the Qt framework is quite an onerous task, and in this case I would have to do it twice (for Linux and for Windows).