CODEWITHSUNDEEP
c++qt
driftwood
driftwood

Reputation: 2121

Adding static library link line to my Qt project file makes no difference

I've read a lot of "duplicate" posts on the subject of statically linking your Qt project on windows to statically built Qt core libs. However, I was left very confused when I compared it to what I am seeing. What I gathered from all the posts is if you already have the statically built Qt libs (which I seem to have here C:\Qt\5.4\mingw491_32\lib) all you need to do is to include a line like so 

LIBS += -LC:/Qt/5.4/mingw491_32/lib -lQt5Core -lQtGui

in your .pro file without having to specify -static anywhere. However, this line seems to do exactly ZERO. When I comment it out my project builds exactly the same way without it. The exe is the same size and when I try to run it from command prompt a runtime error appears in a dialog box saying it can't find Qt5Core.dll. Its clearly still linking dynamically. What am I doing wrong? Here is the whole .pro file.

#QT       +=
greaterThan(QT_MAJOR_VERSION, 4): QT += widgets
TARGET = testproj
TEMPLATE = app
SOURCES += main.cpp mainwindow.cpp
HEADERS  += mainwindow.h
FORMS    += mainwindow.ui
QMAKE_CXXFLAGS +=-std=c++11
#LIBS += -LC:/Qt/5.4/mingw491_32/lib -lQt5Core

I clearly have major holes in my understanding of how Qt linking works on windows. Any help on this topic is highly appreciated.

Upvotes: 0

Views: 800

Answers (2)

Kuba hasn't forgotten Monica
Kuba hasn't forgotten Monica

Reputation: 98505

The major hint is: no matter how Qt is built, you shouldn't need to change anything in the project that uses Qt. Qmake and the configuration files in a given Qt install govern how the applications that use Qt are linked against Qt.

Another hint: a given Qt installation can be either configured for static or dynamic linking, not both. Thus any sort of configuration on the project end is superfluous: a given Qt install already "knows" how to link Qt correctly.

Any given .pro file should work whether Qt is linked statically or dynamically. You should not have any static-linking-specific entries. Remove the LIBS line from the project.

If your executable is dynamically linked, the only reason is that you're using a dynamically linked build of Qt. You should make a static build yourself first, and then use it with your project, and your executable will be statically linked against Qt. It's that simple.

Qt Creator doesn't care about these details either. All it does is invoke qmake and then the make tool. But each qmake is specific to a particular installation of Qt, and that qmake is configured to access that installation's configuration files, and those files contain the data needed to select the proper linking etc. So all of the information is specific to a particular kit one is using, and specifically to the Qt "version" that kit is using (really, a Qt installation because there can be multiple installs of the same version, configured differently).

Upvotes: 0

Benjamin T
Benjamin T

Reputation: 8321

Qt does not distribute static builds.

It seems you only have a dynamic build and got confused by the .lib/.a files. On Windows, there are 3 types of library:

  1. Dynamic libraries (.dll)
  2. Static libraries (.a or .lib depending on the toolchain)
  3. Import libraries (.a or .lib depending on the toolchain)

Import libraries are only used with dynamic libraries. The import libraries are used by the linker when generating the exe, while the dynamic libraries are used at run time.

So if you have Qt5Core.lib or Qt5Core.a does not mean you have a static build of Qt.

To build your own static version of Qt, I suggest your read Building a static Qt for Windows using MinGW on Qt's wiki.

Upvotes: 2

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