Error linking static boost_filesystem library with Qt Creator - qt

Question

I am getting an error when trying to link a static boost_filesystem library with QMake.
I know that my boost install is working, because I can link to it no problem outside of QMake like this:
g++ -o my_file my_file.cpp -IC:\boost_1_55_0\boost_1_55_0 -LC:\boost_1_55_0\boost_1_55_0\stage\lib -lboost_system -lboost_filesystem
I tried linking Qt with boost_filesystem like this at first, but it seems that it could not find the libraries:
LIBS += -LC:\boost_1_55_0\boost_1_55_0\stage\lib -lboost_system -lboost_filesystem
"-lboost_system not found -lboost_filesystem not found"
I read something about how you need to specify absolute paths to static libs when using QMake, so I've changes my LIBS line in my .pro to the following:
LIBS += "C:\boost_1_55_0\boost_1_55_0\stage\lib\libboost_system-mgw48-mt-1_55.a" \
"C:\boost_1_55_0\boost_1_55_0\stage\lib\libboost_filesystem-mgw48-mt-1_55.a"
The libraries are found with this method, however I get the following error message:
undefined reference to boost::system::generic_category()
I was under the impression that this error only happen when boost_system isn't linked! I seems like it is linked though, because it finds the proper library for it, and I know my install of boost is good because the same library links fine (with the same compiler) outside of Qt Creator.
Here's the include in mainwindow.h:
include <boost/filesystem.hpp>
Here the full .pro:
QT += core gui
QT += network
greaterThan(QT_MAJOR_VERSION, 4): QT += widgets
QMAKE_CXXFLAGS += -std=c++11
TARGET = Downloader
TEMPLATE = app
INCLUDEPATH += C:\boost_1_55_0\boost_1_55_0
LIBS += "C:\boost_1_55_0\boost_1_55_0\stage\lib\libboost_system-mgw48-mt-1_55.a" \
"C:\boost_1_55_0\boost_1_55_0\stage\lib\libboost_filesystem-mgw48-mt-1_55.a"
SOURCES += main.cpp\
mainwindow.cpp\
rar_process.cpp\
HEADERS += mainwindow.h\
rar_process.h\
FORMS += mainwindow.ui
RESOURCES += \
graphics.qrc
And here's the full compiler output:
C:\Qt\Qt5.1.1\5.1.1\mingw48_32\bin\qmake.exe -spec win32-g++ -o Makefile ..\qt- downloader\Downloader.pro
C:/Qt/Qt5.1.1/Tools/mingw48_32/bin/mingw32-make -f Makefile.Release
mingw32-make[1]: Entering directory 'C:/Users/Patrick/Desktop/cpp/build-Downloader- Desktop_Qt_5_1_1_MinGW_32bit-Release'
g++ -Wl,-s -Wl,-subsystem,windows -mthreads -o release\Downloader.exe release/main.o release/mainwindow.o release/rar_process.o release/qrc_graphics.o release/moc_mainwindow.o release/moc_rar_process.o -lglu32 -lopengl32 -lgdi32 -luser32 -lmingw32 -lqtmain C:\boost_1_55_0\boost_1_55_0\stage\lib\libboost_system-mgw48-mt-1_55.a C:\boost_1_55_0\boost_1_55_0\stage\lib\libboost_filesystem-mgw48-mt-1_55.a - LC:\Qt\Qt5.1.1\5.1.1\mingw48_32\lib -lQt5Widgets -lQt5Network -lQt5Gui -lQt5Core
release/main.o:main.cpp:(.text.startup+0x1a): undefined reference to `boost::system::generic_category()'
release/main.o:main.cpp:(.text.startup+0x24): undefined reference to `boost::system::generic_category()'
Makefile.Release:86: recipe for target 'release\Downloader.exe' failed
c:/qt/qt5.1.1/tools/mingw48_32/bin/../lib/gcc/i686-w64-mingw32/4.8.0/../../../../i686- w64-mingw32/bin/ld.exe: release/main.o: bad reloc address 0x24 in section `.text.startup'
collect2.exe: error: ld returned 1 exit status
mingw32-make[1]: *** [release\Downloader.exe] Error 1
mingw32-make[1]: Leaving directory 'C:/Users/Patrick/Desktop/cpp/build-Downloader- Desktop_Qt_5_1_1_MinGW_32bit-Release'
makefile:34: recipe for target 'release' failed

Answer 1

GCC is very sensitive to the order in which you specify libraries during the linking stage. For example, if libA.a depends on libB.a and an executable App depends on both, then you'd have to invoke linker in the following way:
gcc main.o object1.o ... object2.o -lA -lB -o App
This implies that you have to change it to:
LIBS += "C:\boost_1_55_0\boost_1_55_0\stage\lib\libboost_filesystem-mgw48-mt-1_55.a" \
"C:\boost_1_55_0\boost_1_55_0\stage\lib\libboost_system-mgw48-mt-1_55.a"
To be cross-platform, it's recommended to make your build more robust:
Boost_VERSION = 1_55
COMPILER = mgw48
win32-g++* {
LIBS += $$(Boost_DIR)/stage/lib/libboost_filesystem-$${COMPILER}-mt-$${Boost_VERSION}.a \
$$(Boost_DIR)/stage/lib/libboost_system-$${COMPILER}-mt-$${Boost_VERSION}.a
PRE_TARGETDEPS += $$(Boost_DIR)/stage/lib/libboost_filesystem-$${COMPILER}-mt-$${Boost_VERSION}.a \
$$(Boost_DIR)/stage/lib/libboost_system-$${COMPILER}-mt-$${Boost_VERSION}.a
}
linux-g++* {
LIBS += -L$$(Boost_DIR)/stage/lib -lboost_filesystem-$${COMPILER}-mt-$${Boost_VERSION} \
-lboost_system-$${COMPILER}-mt-$${Boost_VERSION}
PRE_TARGETDEPS += $$(Boost_DIR)/stage/lib/libboost_filesystem-$${COMPILER}-mt-$${Boost_VERSION}.a \
$$(Boost_DIR)/stage/lib/libboost_system-$${COMPILER}-mt-$${Boost_VERSION}.a
}
NOTE: Boost_DIR would be treated as an environment variable here. So that you can flexibly build it on any system without hardcoding the system-dependent path to Boost. This is general practice in professional software development: try to hardcode as little as possible in your build system, whatever it is. Furthermore, it's reasonable to even make Boost_VERSION environment variable too. All you'd have to change then, are brackets to parentheses, i.e. $${Boost_VERSION} to $$(Boost_VERSION).
Finally, notice that I've used forward slash / as a path separator for both Windows and Linux. Backward slash \ is not only deprecated in QMake, but it is also discouraged in general. Once again, be cross-platform: simply let QMake handle the conversion.

Answer 2

Apparently boost-system will not link against mingw gcc 4.8.0 if it was built with mingw gcc 4.8.1.
I re-built boost with mingw gcc 4.8.0 and linked against those libs, and now it works!