Joy, I think now I've even hit a gcc bug?

Program received signal SIGSEGV, Segmentation fault.
(gdb) bt
#0  0x00007ffff69120ce in QMetaObject::activate(QObject*, int, int, void**) (sender=0x0, signalOffset=8, local_signal_index=local_signal_index@entry=1, argv=argv@entry=0x7fffffffd560) at kernel/qobject.cpp:3592
#1  0x00007ffff6913067 in QMetaObject::activate(QObject*, QMetaObject const*, int, void**) (sender=<optimized out>, m=m@entry=0x7ffff703c280 <QGuiApplication::staticMetaObject>, local_signal_index=local_signal_index@entry=1, argv=argv@entry=0x7fffffffd560) at kernel/qobject.cpp:3582
#2  0x00007ffff6bfa8e2 in QGuiApplication::screenAdded(QScreen*) (this=<optimized out>, _t1=0x66caf0) at .moc/moc_qguiapplication.cpp:321

==> See below, "this" is actually 0x0 here

#3  0x00007ffff6bf15c5 in QPlatformIntegration::screenAdded(QPlatformScreen*, bool) (this=<optimized out>, ps=ps@entry=0x66cb10, isPrimary=isPrimary@entry=true) at kernel/qplatformintegration.cpp:447
[...]
#13 0x00007ffff68e6f6f in QCoreApplication::init() (this=this@entry=0x650450) at kernel/qcoreapplication.cpp:726

==> QCoreApplication::self has already been set at this point (QCoreApplication::self = this;)

#14 0x00007ffff71aba87 in QApplication::QApplication(int&, char**, int) (this=0x650450, argc=@0x7fffffffdc3c: 1, argv=0x7fffffffde28, _internal=328704)
    at kernel/qapplication.cpp:565
[...]



(gdb) break qplatformintegration.cpp:447
Breakpoint 1, QPlatformIntegration::screenAdded (this=<optimized out>, ps=ps@entry=0x66cb10, isPrimary=isPrimary@entry=true)
[...]
447         emit qGuiApp->screenAdded(screen);
(gdb) s
QGuiApplication::screenAdded (this=0x0, _t1=0x66caf0) at .moc/moc_qguiapplication.cpp:319
(gdb) print QGuiApplication::self
$4 = (QCoreApplication *) 0x650450

qGuiApp is
#define qGuiApp (static_cast<QGuiApplication *>(QCoreApplication::instance()))
with QCoreApplication::instance() returning QGuiApplication::self


==> So "emit qGuiApp->screenAdded(screen);" results in screenAdded being called on a NULL QGuiApplication, but at the same time QGuiApplication::self should be perfectly okay.

--- Additional comment from Sandro Mani on 2015-02-13 11:34:25 EST ---



--- Additional comment from Sandro Mani on 2015-02-16 05:40:38 EST ---

Concerning comment 19: as posted on -devel, this is indeed a GCC bug (specifically, with the gold linker). Workaround is to build qt5 passing -no-use-gold-linker to configure. Filed as [1], but turns out it was a known issue where not much is being done about it.

[1] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65057