C++ allocation segfault

Question

I have this code:

size_t count = new_data.size();
std::cout << "got the count: " << count << std::endl;
double g_x[count];
std::cout << "First array done\n";
double g_y[count];
std::cout << "Allocated array of size" << count << std::endl;

which gives me the output:

got the count: 1506538
Segmentation fault: 11

I honestly don't understand why. It work on another data set, but not on this one.

@David wrong. You can. But you shouldnt – MPogoda Feb 23 '13 at 18:29 — MPogoda, Feb 23 '13 at 18:29

Sergey Vyacheslavovich Brunov · Accepted Answer · 2013-02-23T18:36:48.057

3

You're probably just getting a stack overflow here. Try to dynamically allocate the memory, i.e. use the heap.

double* g_x = new double[count];
...
delete[] g_x;

Even better solution would be to use std::vector<>:

#include <vector>

...

std::vector<double> g_x(count); // Creates vector with the specified size.

edited Feb 23 '13 at 18:36

answered Feb 23 '13 at 18:28

Sergey Vyacheslavovich Brunov

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But you should remember: when you are using `new`, you are around a memory leak if some exception will be thrown in place of `...`. Simply use STL's containers in such situation – MPogoda Feb 23 '13 at 18:31
allocating 1506538 double 8byte storage is about 11.5MB! – Arcturus Feb 23 '13 at 18:33
@Arcturus, that's okay when it is allocated on **heap**. – Sergey Vyacheslavovich Brunov Feb 23 '13 at 18:35
well I usually would use a container, but I have to use a strange framework which doesn't work with those, not even if I pass it a pointer or such – Benedikt Bergenthal Feb 23 '13 at 18:38
@BenediktWutzi, interesting. Could you please add the signature of some framework function to the question? – Sergey Vyacheslavovich Brunov Feb 23 '13 at 18:39
It's cern's ROOT framework. I'm using the TGraph class to plot some data http://root.cern.ch/root/html/TGraph.html – Benedikt Bergenthal Feb 23 '13 at 18:52
@BenediktWutzi, the framework accepts **arrays**: **pointer** to the first element and **number** of elements. For example, `void DrawGraph(Int_t n, const Double_t* x = 0, const Double_t* y = 0, Option_t* option = "")`. To get pointer to the first element for `std::vector<>` just use `&g_x[0]`. – Sergey Vyacheslavovich Brunov Feb 23 '13 at 18:57
@BenediktWutzi, please show us the code which can't be not compiled. – Sergey Vyacheslavovich Brunov Feb 23 '13 at 19:03
I mean when I try to use &g_x[0] – Benedikt Bergenthal Feb 23 '13 at 19:05
@BenediktWutzi, which function of the framework you're trying to call with `&g_x[0]`? – Sergey Vyacheslavovich Brunov Feb 23 '13 at 19:06
The constructor of TGraph so TGraph *g = new TGraph(count, &g_x[0], &g_y[0]); – Benedikt Bergenthal Feb 23 '13 at 19:07
let us [continue this discussion in chat](http://chat.stackoverflow.com/rooms/25023/discussion-between-sergey-brunov-and-benedikt-wutzi) – Sergey Vyacheslavovich Brunov Feb 23 '13 at 19:08

score 1 · Answer 2 · edited May 23 '17 at 11:43

1

If you want a dynamically sized array, you either need to create it using new, or use one of the STL containers.

Take a look at some of the answers at Static array vs. dynamic array in C++

edited May 23 '17 at 11:43

Community

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answered Feb 23 '13 at 18:28

David Kiger

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score 1 · Answer 3 · answered Feb 23 '13 at 18:41

Your problem is that in C and C++ arrays are by definition defined at compile time so what you do is wrong and it's very strange that this code even compile (compiler should scream at you)

However if you need a runtime defined array you should either use std::vector or manually allocate memory (not reccomended if you don't have very specific needs)

P.S. you should check your compiler warning level and the verbosity because this code has serious flaws so it shouldn't even compile (it's also bad for you if you have a very low warning level etc because you could pick up some bad coding habits because you are using not knowingly compiler specific extensions to the language and this is going to drive you mad when changing environement)

C++ allocation segfault

3 Answers3