I get "Segmentation fault"

[vincegata]

Hello,

I am sporadically getting "Segmentation fault" error. googling says that it happens when the program is trying to write to a memory segment that it does not have access to.

I use try-catch block on the out most program function (as below) but still my program shuts down abruptly without even going into catch block.

int main()
{
try
{
MyClass app = MyClass();
app.Run();
}
catch(exception& e)
{
cerr << "my app exception: " << e.What() << endl;
}
}

So, why try-catch is not catching it, and what is the way to determine what causes this error. I run Ubuntu, g++, it is sockets application.

TIA!

[Peter87]

Reading and writing to invalid memory locations doesn't throw exceptions. It is undefined behaviour so the C++ standard gives no guarantees on what will happen.

If you want to find out where the problem occur you can use a memory debugger such as valgrind.

[vincegata]

I see, I need to start reading on valgrind - thank you!

[vincegata]

I found the source of my Segmentation fault with Valgrind however I would have expected to get an Index out of range exception.

TickClass tick[5];
for(int i=0;i<5;i++) tick[i] = TickClass();

int j = -1;
tick[j].get_symbol(); //  Segmentation fault is here.

Could someone comment why it was not an exception? I am new to programming in Linux. Thanks!

[Peter87]

Arrays are not bound checked in C++. Accessing an array out of bounds gives undefined behaviour which means anything could happen.

[vincegata]

Thanks again.

[Luc Lieber]

If you want bounds checking, you can use a std::vector.

Keep in mind however, that std::vector::operator[] does not provide bounds checking, whereas std::vector::at does.

std::vector<TickClass> ticks;
for(int i=0;i<5;i++)
{
  ticks.push_back(TickClass());
}

int j = -1;
ticks[j].get_symbol(); //  Segmentation fault is here.
ticks.at(j).get_symbol(); // out_of_range thrown here. 

[vincegata]

Thanks for the suggestion, I use STL vector in some other places but here I need my code to be as fast as possible hence the C-style array. vector is probably adding some performance overhead for all those niceties.

It's just I remember getting more meaningful messages for array out of bounds error when I was coding with Visual C++ & Windows - not that I regret abandoning Windows :)

[Peter87]

std::vector::operator[] shouldn't have any overhead with optimizations turned on. You can enable bounds checking for std::vector::operator[] (and other things like iterators) by defining the macro _GLIBCXX_DEBUG. That way you can have bounds checking while testing and when you later want speed you just remove it.

[vincegata]

THX

[LauraArnold]

Getting "Segmentation Fault" tells us that it doesn't work, but it's not enough for debugging. Can you try running ns2 with gdb and post the output you get?

http://www.versionfull.com/search/accounting-software/1

[vincegata]

I marked it as Solved :)