Segmentation fault at output

Question

I get a segmentation fault in the following C++ program.

#include<iostream>

const int N = 3000;

int main() {
    bool coprimes[N][N];
    for (int i = 0; i < N; ++i) {
        for (int j = 0; j < N; ++j) {
            coprimes[i][j] = ((3 * i + j) % 17 == 0);
        }
    }

    for (int c = 1; c < N; ++c) {
        for (int a = 1; a < c / 2; ++a) {
            int b = c - a;
            if (!coprimes[a][b] || !coprimes[a][c] || !coprimes[b][c]) {
                continue;
            }

            std::cout << c << std::endl;
        }
    }
}

(The program as is does not make much sense. I just tried to create somewhat of a minimum working example.)

The weird thing is, it's at the line std::cout << c << std::endl;. If I remove that line, the program works fine.

Can anyone explain to me why this happens and whether there is a deeper logic behind it? I am still a C++ beginner, and since the compiler error only says "segmentation fault" without any additional information, I'm a bit at a loss here.

And also, how can I fix it? :-)

Any help highly appreciated!

PS: I know there are a gazillion threads about this topic. But that somehow makes it only harder to obtain useful information. I checked out the Wiki entry, but it does not seem to cover my problem.

Possible duplicate of [Segmentation fault on large array sizes](http://stackoverflow.com/questions/1847789/segmentation-fault-on-large-array-sizes?rq=1), and *hundreds* of similar questions. — WhozCraig, Feb 28 '15 at 21:17
Recognizing stack overflows can sometimes be difficult. Using posix on Linux, the stack size is available using "::pthread_attr_getstack(3 parameters)". My 12.04 system reports that the thread default is 8,388,608 bytes, quite a bit smaller than (3000*3000) booleans. — 2785528, Mar 01 '15 at 01:37

score 0 · Answer 1 · edited May 23 '17 at 12:06

0

Probably you're trying to reserve too much memory in your main() function (I mean coprimes array, 3000x3000 of bool it is more than 8MB). See here for more details.

edited May 23 '17 at 12:06

Community

1
1

answered Feb 28 '15 at 21:08

Marcin Kolny

633
7
14

score 0 · Accepted Answer · answered Feb 28 '15 at 21:14

0

There is a stack overflow issue because of too big coprimes matrix. You can allocate memory dynamically on heap (don't forget to release).

bool **coprimes = new bool*[N];
for (int i = 0; i < N; ++i)
    coprimes[i] = new bool[N];

answered Feb 28 '15 at 21:14

Andrey Derevyanko

550
5
12

Thank you. There is also the [auto keyword](http://stackoverflow.com/a/16239446/4500580) keyword that seems to do the same thing in C++11. But still ... why does the segmentation only occur when I include the line std::cout << c << std::endl;? – Sacha Mar 01 '15 at 07:49
Actually it shouldn't depend on `std::cout << c << std::endl;`. At least in my compiler program terminates without this line as well. Do you have any optimization on? – Andrey Derevyanko Mar 01 '15 at 08:07
Hrgh ... yes. That was it. Thanks a lot. I was jusing the -O3 compiler flag. Without it, the segmentation fault occurs with or without said line. Thanks a lot, I learned an important lesson: Remove optimization flags to debug. – Sacha Mar 01 '15 at 09:15

Segmentation fault at output

2 Answers2