QFontDatabase causes Segfault [Solved]

[Pippin]

Hello,

I was getting a segmentation fault out of the line

auto s = QFontDatabase::addApplicationFont("construct/Type.ttf");

and I wasn't sure why that was. After some testing, I tried the line

QFontDatabase k;

and I got a segmentation fault out of it, too. (Valgrind tells me there's something in the constructor that causes the segfault.) I don't understand. I'm including <QFontDatabase> and all. Any ideas?

Thanks in advance.

[SGaist]

Hi,

What does the debugger tell you ?

[Pippin]

@SGaist What do you mean?

Valgrind says

==5551==    at 0x672E3FA: ??? (in /usr/lib/x86_64-linux-gnu/libQt5Gui.so.5.2.1)
==5551==    by 0x672E460: QFontDatabase::QFontDatabase() (in /usr/lib/x86_64-linux-gnu/libQt5Gui.so.5.2.1)

[SGaist]

I mean: what is the stack trace when you run your application through the debugger ? In your case, gdb

[Pippin]

I get this:

./Menu[0x4611fd]
/lib/x86_64-linux-gnu/libpthread.so.0(+0x10340)[0x7fe46bebc340]
/usr/lib/x86_64-linux-gnu/libQt5Gui.so.5(+0x1743fa)[0x7fe46c8e43fa]
/usr/lib/x86_64-linux-gnu/libQt5Gui.so.5(_ZN13QFontDatabaseC2Ev+0x41)[0x7fe46c8e4461]
./Menu[0x429172]
./Menu[0x412b3d]
/lib/x86_64-linux-gnu/libc.so.6(__libc_start_main+0xf5)[0x7fe46b26fec5]
./Menu[0x412f10]

[SGaist]

You should run a debug build of your application, it will give more precise information.

[Pippin]

@SGaist I'm already compiling with QMAKE_CXXFLAGS += -std=c++14 -Wall -Wextra -pedantic -g -ggdb -rdynamic, what exactly would you like me to do?

[SGaist]

Did you set these flags your self or use Qt Creator Run in debug mode ?

[Pippin]

I set these flags myself, I never use Qt Creator for anything. I compile and run everything through the terminal with qmake and make

There was also this line though: QMutex: destroying locked mutex but I'm not using any mutex.

[SGaist]

Try again with CONFIG+=debug. You currently build a debug version of your app running against a release build of Qt.

You might not be using mutex directly but depending on what part of Qt you use, Qt is.

[Pippin]

@SGaist I've added CONFIG+=debug in the .pro file but the outcome is the same.:S

[SGaist]

Do you have the Qt debug package for your distribution installed ?

[Pippin]

@SGaist I've just installed qtbase5-dbg to make sure, but the result remains the same.

Anyway, do we really need to get more information about what exactly is causing the segmentation fault in QFontDatabase::QFontDatabase ? It seems that almost anything in <QFontDatabase> causes a segmentation fault. Well, at least QFontDatabase::QFontDatabase and QFontDatabase::addApplicationFont do.

[Pippin]

Wait, Valgrind seems to be more accurate than the stack trace for some reason.

==2786==    at 0x672E3FA: initializeDb() (in /usr/lib/x86_64-linux-gnu/libQt5Gui.so.5.2.1)
==2786==    by 0x672E460: QFontDatabase::QFontDatabase() (in /usr/lib/x86_64-linux-gnu/libQt5Gui.so.5.2.1)

I don't know how to get more info.

[SGaist]

When are you using it in your application ?

[Pippin]

@SGaist

int main(int argc, char *argv[])
{
	DS::Initialize();
	\\ ...
}

where DS::Initialize is a static function of the class DS:

void DS::Initialize(void)
{
	struct sigaction act;
	sigemptyset(&act.sa_mask);
	act.sa_handler = ::Handler;
	act.sa_flags = 0;
	sigaction(SIGSEGV, &act, 0);
	
	QFontDatabase k;
	std::cout << "foo" << std::endl;
	auto s = QFontDatabase::addApplicationFont("construct/Type.ttf");
	std::cout << "foo" << std::endl;
	\\ ...
}

[SGaist]

You're missing a QGuiApplication in order to use QFontDatabase

[Pippin]

@SGaist So it was only a matter of order! It works now, thank you so much! How do I add <Solved> ?

[SGaist]

You're welcome !

Just edit the thread title a prepend it or rather [solved] so that the forum can pick it up and modify the view accordingly :)