Segmentation faults when initializing large arrays

Hi!

I have come across some weird behavior the other day which results in unexplained segmentation faults. Let's say I have a program which reads:

Code:

---

#include <iostream>
#include <vector>
using namespace std;

class SomeClass
{
};

void someFunc()
{
    vector<SomeClass*> HugeArray[1000][1000];
    HugeArray[0][0].clear();
}

int main()
{
    someFunc();
    return 0;
}

---

It compiles fine, but then sigsegvs on me upon entering someFunc (when control has not even yet reached the clear()-call). But if I make the function global, it runs perfectly fine:

Code:

---

#include <iostream>
#include <vector>
using namespace std;

class SomeClass
{
};

vector<SomeClass*> HugeArray[1000][1000];

void someFunc()
{
    HugeArray[0][0].clear();
}

int main()
{
    someFunc();
    return 0;
}

---

It also runs fine without making HugeArray global, but then I have to reduce its size to f.ex. 100x100.

Is this a compiler-related problem? I have tried this on GCC 4.x

Thanks in advance

This has been discussed numerous times in the last week or so. Stack size is much more limited than the heap. Massive memory declarations should be one of the following:

• static and local
• global
• allocated dynamically

I generally recommend the last option, however, there is nothing wrong with the first one, and in fact, it might even be preferred in some cases. Global variables in general, though, are a bad idea when you can avoid them.

Edit: For clarification, local variables are generally allocated on the stack while dynamically allocated memory comes from the heap. Static local variables and global variables are usually built right into the executable.

FYI, you are abusing vectors. You want a 2D array, not an array OF vectors.
So then, you want a vector of vectors:
std::vector< std::vector<SomeClass*> >

Quote:

---

Originally Posted by MacGyver

This has been discussed numerous times in the last week or so.

---

Oh, sorry, didn't know that :o And thanks for the detailed explanation!

Quote:

---

Originally Posted by Elysia

You want a 2D array, not an array OF vectors.

---

No, I actually want a 3d array with limited, predefined x and y-dimensions and an unlimited z-dimension. That is why I have a predefined 2d-array of vectors :)

Quote:

---

Originally Posted by MathFan

Oh, sorry, didn't know that :o And thanks for the detailed explanation!

---

I mentioned it more along the lines of a suggestion that you might glean more information from the recent discussions on the subject since people might not be willing to re-explain the same thing at the same level of detail each time. Wasn't meant to condemn you, although searching the forums before posting is a good practice, I do admit.

Hope this was helpful.

Quote:

---

Originally Posted by MacGyver

I mentioned it more along the lines of a suggestion that you might glean more information from the recent discussions on the subject since people might not be willing to re-explain the same thing at the same level of detail each time. Wasn't meant to condemn you, although searching the forums before posting is a good practice, I do admit.

Hope this was helpful.

---

Yes, I am sorry, my fault. I do tend to get carried away asking questions while forgetting to try finding the answer myself.

And again, thank you for taking the time to respond to my question despite that :)