Segmentation fault: 11

Question

I'm completing a pretty easy programming excersize with the following code:

    using System;

    namespace Factorial

{
    class MainClass
    {

        static int fives(int x) {

            int r = 0;
            while(x % 5 == 0) {
                r++;
                x /= 5;
            }
            return r;

        }

        static int z(int x) {

            if (x == 1)
                return 0;
            else
                return z (x-1) + fives (x);

        }

        public static void Main (string[] args)
        {
            int testCases = Convert.ToInt32 (Console.ReadLine ());
            int[] xs = new int[testCases];
            for (int i=0; i<testCases; i++)
                xs [i] = Convert.ToInt32 (Console.ReadLine ());
            foreach (int x in xs)
                Console.WriteLine (z (x));
        }
    }
}

It seems to work OK with small numbers, but with 8735373 from the example it prints "Segmentation fault: 11". Does it mean that I run out of memory because of recursion going in too deep? What causes it?

(I run C# in Mono 2.10.8 on a Mac.)

P.S.: If anyone's interested in the excersize itself, here's my final solution (much more optimized).

Have you tried to run it from debugger? Any exception details? — Carsten Schütte, May 29 '12 at 21:40
Oh, then maybe, although that would be weird, you'd probably get a `StackOverflowException`. Can you post more detail? — user703016, May 29 '12 at 21:41
Have you tried using a debugger? Sounds like stack-overflow to me. The `namespace Factorial` is just there to trick us, right? — Marc Gravell, May 29 '12 at 21:41
I don't see any exception details even in debug mode; it looks like a regular console output rather than exception. — Max Yankov, May 29 '12 at 21:42
Tried it in LINQPad, indeed it crashes with a StackOverflowException, caused by the `return z (x-1)` if `xs[i]` is huge. — Michael Stum, May 29 '12 at 21:42
On the spoj.pl it shows "runtime error (SIGABRT)" as a status. — Max Yankov, May 29 '12 at 21:45
Michael, thanks for this detail! Now I'll have at least something to try. — Max Yankov, May 29 '12 at 21:46
I would think that int is too small for the datatype for the z function and that's blowing up with large numbers. Try int64 or longint — MikeTWebb, May 29 '12 at 21:51
@MikeTWebb C# is `unchecked` by default - a valid concern, but if you run it in the debugger: not the actual error — Marc Gravell, May 29 '12 at 21:52
Still think that could be the issue and just not bubbling up correctly....do you know the lowest number where it crashes...i.e, 100,000 or 64,000? Try 32768....that's the first number larger than a small int — MikeTWebb, May 29 '12 at 21:56
@MikeTWebb nah, really - it's a stack-overflow; run it in a debugger... (feel free to add some `checked` blocks to ensure it isn't silent arithmetic overflow) — Marc Gravell, May 29 '12 at 22:05

score 4 · Answer 1 · answered May 29 '12 at 21:48

4

That looks like an unhandled StackOverflowException - common when overusing recursion. So... don't overuse recursion. It might be fine for maths and some very specific languages (F# would probably cope), but C#... not so much.

That looks like (not verified):

    static int z(int x)
    {
        int accumulator = 0;
        while(x!=1)
        {
            accumulator += fives(x);
            x--;
        }
        return accumulator;
    }

which doesn't error - it doesn't recurse (although it does call fives per iteration). Even better: do the algebra to find the direct formula.

answered May 29 '12 at 21:48

Marc Gravell

1,026,079
266
2,566
2,900

1

@Cicada meh... `StackOverflowException` is in that special category of crazy (along with `OutOfMemoryException`) where "random" is pretty much guaranteed. – Marc Gravell May 29 '12 at 21:51
That's exactly what I done, just a moment ago — even added my own answer before I saw yours ;) – Max Yankov May 29 '12 at 21:52
F# tail-recursion optimization FTW! – Jwosty Mar 01 '13 at 03:29

score 2 · Answer 2 · edited May 23 '17 at 12:13

When you use the larger number your recursion steps is causing StackOverflowException and if you look into the debugger you will see after how many recursive steps the exception occurred.

I think Mono code somehow consider stackoverflow exception to Segmentaiton fault which could be how it is handled.

enter image description here

You could debug it if you wish: How do I debug a segmentation fault in Mono on Ubuntu without any debugger?

score 1 · Answer 3 · answered May 29 '12 at 21:51

1

It seems that simple conversion from recursion to loop solved the problem:

    static int z (int x)
    {
        int result = 0;
        for (int i=1; i<=x; i++) {
            result += fives (i);
        }
        return result;
    }

answered May 29 '12 at 21:51

Max Yankov

12,551
12
67
135

sure you don't want to start with `i=2` ? – Marc Gravell May 29 '12 at 21:54
I'm mainly worried about the special case of `1` when translating `z`. If your `i=5` relates to further analysis of `fives(...)`, then that is separate: the first job is to translate `z` correctly, and *you haven't done that*. It is executing `fives(1)`, which the old version ***did not***. – Marc Gravell May 29 '12 at 21:57

score 1 · Accepted Answer · edited May 27 '15 at 17:36

If the issue was caused by the large amount of recursion, the error would probably be a StackOverflowException. As golergka, previously stated, I'm pretty sure it's a Mono fault. A segmentation fault occurs arises when memory is being mismanaged by trying to reach memory addresses that should not be reached. This type of error is a system error... not a C# exception. I'm almost sure that Mono isn't managing memory very well with large numbers. I hope that helps you in your research.

Segmentation fault: 11

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