Random segmentation faults? - c++

Question

Note, problem has been identified, refer to second section for current detail and the third section for the most likely reason and a question on how to remedy it
I'm writing a program in Qt and I stopped a couple days ago at a segmentation fault problem. It seemed to complain about memory not being allocated. I searched around and couldn't figure out why there was an allocation problem and I took a break. Now, looking at it again, the segmentation fault is still there, but it's in a completely different, mostly unrelated function. What could be causing these random segmentation faults?
For some concreteness, in this case, I am currently getting a segmentation fault here:
Q_DECL_CONSTEXPR inline int QSize::height() const
{ return ht; }
I was getting it here before I commented out the lines (the last three lines)
qDebug() << QString("---SETTINGS DEBUG---")+QString("\r\n")<<
"netProcPage: "+netProcPage.url()+"\r\n" <<
"mapSize.width(): "+QString::number(mapSize.width())+"\r\n"<<
"mapSize.height(): "+QString::number(mapSize.height())+"\r\n"<<
"mapZoom: "+QString::number(mapZoom)+"\r\n"<<
"mappxSize.width(): "+QString::number(mappxSize.width())+"\r\n"<<
"mappxSize.height(): "+QString::number(mappxSize.height())+"\r\n"<<
"UserCoords[0]: "+QString::number(UserCoords[0])+"\r\n"<<
"UserCoords[1]: "+QString::number(UserCoords[1])+"\r\n"<<
"mapCoordOffsets[0]: "+QString::number(mapCoordOffsets[0])+"\r\n"<<
"mapCoordOffsets[1]: "+QString::number(mapCoordOffsets[1])+"\r\n"<<
"getWindowSize.width(): "+QString::number(getWindowSize.width())+"\r\n"<<
"getWindowSize.height(): "+QString::number(getWindowSize.height())+"\r\n"<<
"mappxOffsets[0]: "+QString::number(mappxOffsets[0])+"\r\n"<<
"mappxOffsets[1]: "+QString::number(mappxOffsets[1])+"\r\n"<<
QString("---END SETTINGS DEBUG---")+QString("\r\n");
Before then, without changing anything (just waiting a couple days and a couple restarts later), it was here:
mkeMap.genMap(QString("Map1"), tempmapSize, tempmapZoom, mapDisp->ui);
In the MainWindow class constructor complaining about tempmapSize, which is defined by:
QSize tempmapSize;
tempmapSize = settings->mapSize; //<--- The error might be coming from here, is there an alternative?
which is where the segmentation faults have been associated with. This is the settings class:
#include "settings.h"
Settings::Settings(QWidget *parent)
{
netProcPage = "http://localhost:81";
// Max image size is 32767x32767 pixels (divided by 4*mapZoom since 4 panes are used at mapZoom zoom)
// If max mapZoom is 20, max size of map is 409x409, or 408x408 to keep it even
mapSize.setWidth(250);
mapSize.setHeight(250);
mapZoom = 10;
mappxSize.setWidth(mapSize.width()*mapZoom);
mappxSize.setHeight(mapSize.height()*mapZoom);
//downloadMap(netProcPage,"getMap","Username","Password");
//makeMap("bingbong",mapSize,mapZoom);
UserCoords[0] = 0;
UserCoords[1] = 0;
mapCoordOffsets[0] = UserCoords[0] + .5 * mapSize.width();
mapCoordOffsets[1] = UserCoords[1] + .5 * mapSize.height();
//getWindowSize.setWidth(parent->width());
//getWindowSize.setHeight(parent->height());
getWindowSize.setWidth(500);
getWindowSize.setHeight(500);
mappxOffsets[0] = UserCoords[0]*mapZoom + .5 * getWindowSize.width() - .5 * mappxSize.width();
mappxOffsets[1] = UserCoords[1]*mapZoom + .5 * getWindowSize.height() - .5 * mappxSize.height();
}
void Settings::debug()
{
qDebug() << QString("---SETTINGS DEBUG---")+QString("\r\n")<<
"netProcPage: "+netProcPage.url()+"\r\n" <<
"mapSize.width(): "+QString::number(mapSize.width())+"\r\n"<<
"mapSize.height(): "+QString::number(mapSize.height())+"\r\n"<<
"mapZoom: "+QString::number(mapZoom)+"\r\n"<<
"mappxSize.width(): "+QString::number(mappxSize.width())+"\r\n"<<
"mappxSize.height(): "+QString::number(mappxSize.height())+"\r\n"<<
"UserCoords[0]: "+QString::number(UserCoords[0])+"\r\n"<<
"UserCoords[1]: "+QString::number(UserCoords[1])+"\r\n"<<
"mapCoordOffsets[0]: "+QString::number(mapCoordOffsets[0])+"\r\n"<<
"mapCoordOffsets[1]: "+QString::number(mapCoordOffsets[1])+"\r\n"<<
"getWindowSize.width(): "+QString::number(getWindowSize.width())+"\r\n"<<
"getWindowSize.height(): "+QString::number(getWindowSize.height())+"\r\n";//<<
//"mappxOffsets[0]: "+QString::number(mappxOffsets[0])+"\r\n"<<
//"mappxOffsets[1]: "+QString::number(mappxOffsets[1])+"\r\n"<<
//QString("---END SETTINGS DEBUG---")+QString("\r\n");
}
QSize* Settings::getmapSize()
{
return &mapSize;
}
int Settings::getmapZoom()
{
return mapZoom;
}
---
Here's the problem (identified)
I've refactored the code as suggested and I have pinpointed the exact problem, but I don't know how to fix it.
void makeMap::genMap(QString name, QPointF* inSize, int* zoom, Ui::MapDisp* ui)
{
QVector<QString> mapvector;
QPointF mapSize = *inSize; // <--- The problem is right here
...
}
The problem occurs when dereferencing the QPointF object found in an earlier instance of Settings, which was sent to genMap(...).
The call was done like this:
QPointF* tempmapSize;
tempmapSize = settings->getmapSize();
int* tempmapZoom = settings->getmapZoom();
mkeMap.genMap(QString("Map1"), tempmapSize, tempmapZoom, mapDisp->ui);
Wherever I moved the dereference (*Settings::inSize*, or *inSize), that's where the error occurred (in the debugger). Everything has been compiling fine.
When running the program, it crashes with this error:
Starting C:\program-debug.exe...
ASSERT failure in QVector<T>::operator[]: "index out of range", file ../../../../../Qt/5.2.0/mingw48_32/include/QtCore/qvector.h, line 369
Invalid parameter passed to C runtime function.
Invalid parameter passed to C runtime function.
C:\program-debug.exe exited with code 3
This reference to QVector<T> is referring to later in genMap,
void makeMap::genMap(QString name, QPointF* inSize, int* zoom, Ui::MapDisp* ui)
{
QVector<QString> mapvector;
QPointF mapSize = *inSize; //<---Here's the segmentation fault
/* Using this instead of the above works, as well as replacing zoom which causes another segmentation fault when dereferenced
QPointF mapSize;
mapSize.setX(250);
mapSize.setY(250);
int zoom0 = 10;
*/
QFile file(name+"_"+QString::number(mapSize.x())+"x"+QString::number(mapSize.y())+".rtsmap");
file.open(QIODevice::WriteOnly | QIODevice::Text);
QTextStream out(&file);
mapvector.resize(mapSize.x() * mapSize.y());
for(int x = 0; x < mapSize.x(); x++){
for(int y = 0; y < mapSize.y(); y++){
uint decimalcolor = (((qSin(x)+1) + (qSin(y)+1))/4)>1?16777215:(((qSin(x)+1) + (qSin(y)+1))/4)*16777214;
QString hexadecimalcolor;
hexadecimalcolor.setNum(decimalcolor,16);
mapvector[index(x, y, mapSize)] = "#" + hexadecimalcolor;
//drawRect(x*10,y*10,10,10,"#"+hexadecimalcolor,zoom);
out << "#" << hexadecimalcolor+'\n';
}
}
file.close();
drawMap(mapvector,zoom0,ui,mapSize);
}
In Short
I think the problem is a dangling pointer. More specifically, when I pass the settings pointer to the class constructor, here:
MapCtrl::MapCtrl(QWidget *parent, Settings *settingsIn) :
QWidget(parent),
ui(new Ui::MapCtrl)
{
ui->setupUi(this);
mapDisp = new MapDisp(parent, settingsIn);
addMap();
settings = settingsIn;
}
The pointer settingsIn probably gets deleted at the end of the constructor with settings still pointing there, so later when I dereference a value from settings, it doesn't exist, causing a segmentation fault. So, the question is, how do I prevent the pointer called settingsIn from getting deleted at the end of the constructor?
requested code
Here's where the MapCtrl construtor MapCtrl::MapCtrl gets invoked and Settings is instantiated:
MainWindow::MainWindow(QWidget *parent) :
QMainWindow(parent),
ui(new Ui::MainWindow)
{
ui->setupUi(this);
wScr = new WelcomeScreen(this);
Settings *settings = new Settings(this);
mapCtrl = new MapCtrl(parent,settings);
...
}
solution (thanks to Kuba Ober for the guidance and the plethora of useful C++ knowledge)
In addition to checking for dangling pointers and fixing some possible causes, the final step was to change this:
MapCtrl::MapCtrl(QWidget *parent, Settings *settingsIn) :
QWidget(parent),
ui(new Ui::MapCtrl)
{
ui->setupUi(this);
mapDisp = new MapDisp(parent, settingsIn);
addMap();
qDebug() << "bingbong!!" << settingsIn->mapSize.x();
settings = settingsIn;
}
to this:
MapCtrl::MapCtrl(QWidget *parent, Settings *settingsIn) :
QWidget(parent),
ui(new Ui::MapCtrl)
{
ui->setupUi(this);
mapDisp = new MapDisp(parent, settingsIn);
qDebug() << "bingbong!!" << settingsIn->mapSize.x();
settings = settingsIn;
addMap();
}
settings was being set after a function that needed settings to be set.

Answer 1

In your entire codebase:
Get rid of places where you return Qt containers (QSize, QList, etc.) by pointer, and replace them with returning them by value.
Declare accessor methods const.
E.g. change Settings::getmapSize() to the below, idiomatic, code:
QSize Settings::getmapSize() const
{
return mapSize;
}
Use QPointF for coordinates, instead of naked arrays. Pass those into methods/functions by either value or const reference; the former may actually work a tad faster on modern hardware and compilers.
E.g. change the code in the constructor to be like this:
mapCoordOffsets = UserCoords + (toPoint(mapSize) * .5);
Where
static QPointF toPoint(const QSize & size) {
return QPointF(size.width(), size.height());
}
Get rid of the C-style output parameters, like in
void makeMap::genMap(..., QPointF* inSize, int* zoom, ...)
If you're returning something, simply return a structure that's called, say MapParams or somesuch:
MapParams makeMap::genMap(...);
If you're taking a structure and potentially modifying it, just modify it and return the modified one:
MapParams makeMap::genMap(..., MapParams params) {
...
params.size = ...;
params.zoom = ...;
...
return params;
}
My worry is that somewhere you're passing a bogus pointer or otherwise a pointer is obfuscating a lifetime issue. Naked pointers in modern C++ are highly suspect, and your code seems to use a lot of them for no good reason at all. Output parameters implemented with pointers are C-ism that has no place in modern C++.
Another common bug could be this: you have a Settings * settings member in the MainWindow class. Then you happily create a Settings * local variable in MainWindow constructor, but the MainWindow::settings member remains uninitialized. That's why you should always prepend member names with m_, so that they are harder to confuse with local variables. Ideally, you should use initializer lists to initialize members in a constructor.
When doing debugging, when you see an assert about out-of-bounds access, you need to debug its cause. A debugger should stop on such an assert, if not then you've got something misconfigured (what's your exact platform and toolset?). Whatever happens afterwards is meaningless if it was a write access, as memory has been overwritten, and you're chasing squirrels at this point.
Doing those things cuts down a whole bunch of errors that you've possibly made in your code. It may not fix your problem, but will get you on a good trajectory towards the solution.