CODEWITHSUNDEEP
c++segmentation-fault
Christine Logan
Christine Logan

Reputation: 9

Mysterious segmentation fault in C++?

I have been searching all over and cannot find anything like this. Now, I won't bore you with my whole program. It's incredibly long. But, here's your basic overview:

int main()
{
  int i=0;
  int h=5;

  cout << "h(IS) = " << h << endl;

  cout << "testing comment.";

  while(i < 10)
  {
  cout << "I'm in the loop!";
  i++;
  }

  return 0;
}

Looks great, right? Okay, so here's the problem. I run it, and I get a segmentation fault. The weirdest part is where I'm getting it. That testing comment doesn't even print. Oh, and if I comment out all the lines before the loop, I still get the fault.

So, here's my output, so you understand:

h(IS) = 5
Segmentation fault

I am completely, and utterly, perplexed. In my program, h calls a function - but commenting out both the line that prints h and the function call have no effect, in fact, all it does is give the segmentation fault where the line ABOVE the printing h line used to be.

What is causing this fault? Anything I can do to test where it's coming from?

Keep your answers simple please, I'm only a beginner compared to most people here :)

Note: I can provide my full code upon request, but it's 600 lines long.

EDIT: I have pasted the real code here: http://pastebin.com/FGNbQ2Ka

Forgive the weird comments all over the place - and the arrays. It's a school assignment and we have to use them, not pointers. The goal is to print out solutions to the 15-Puzzle. And it's 1 AM, so I'm not going to fix my annoyed comments throughout the thing.

I most recently got irritated and commented out the whole first printing just because I thought it was something in there...but no...it's not. I still get the fault. Just with nothing printed.

For those interested, my input information is 0 6 2 4 1 10 3 7 5 9 14 8 13 15 11 12

THANK YOU SO MUCH, EVERYONE WHO'S HELPING! :)

Upvotes: 0

Views: 1206

Answers (3)

diverscuba23
diverscuba23

Reputation: 2185

Lines 57 - 67:

for (i=0; i<=4; i++)
{
  for (j=0; j<=4; j++)
  {
    if (cur[i][j] == 0)
        {
          row = i;
          col = j;
        }
  }
}

at least one of your errors is in this code, cur is declared int cur[4][4]; this means then when j==4 (and when i==4) you are not within the bounds of your array (well you are within the memory for some of them, but not all) valid values will be 0 - 3.

Upvotes: 0

pts
pts

Reputation: 87271

valgrind is a great tool for debugging memory access problems. It's very easy to use on Linux. Just install G++ and valgrind, and then run (without the $ signs):

$ g++ -g -o prog prog.cpp
$ valgrind ./prog

It will print very detailed error messages about memory access problems, with source code line numbers. If those still don't make sense to you, please post the full source code (prog.cpp) and the full output of valgrind.

I've run valgrind for you, its output is here: http://pastebin.com/J13dSCjw

It seems that you use some values which you don't initialize:

==21408== Conditional jump or move depends on uninitialised value(s)
==21408==    at 0x8048E9E: main (prog.cpp:61)
...
==21408== Conditional jump or move depends on uninitialised value(s)
==21408==    at 0x804A809: zero(int (*) [4], int (*) [4], int*, int, int, int, int, int, int) (prog.cpp:410)
==21408==    by 0x804A609: lowest(int (*) [4], int (*) [4], int, int, int, int, int, int) (prog.cpp:354)
==21408==    by 0x804932C: main (prog.cpp:125)
...

To fix these problems, add code which initializes the variables depicted in the error lines above (e.g. line 61, 410), then recompile, and rerun with valgrind again, until all errors disappear.

If your program behaves weirdly even after fixing all problems reported by valgrind, please let us know.

Upvotes: 0

SomeWittyUsername
SomeWittyUsername

Reputation: 18358

You slip over array boundaries, causing the corruption:

for (i=0; i<=4; i++)
{
  for (j=0; j<=4; j++)
  {
        if (cur[i][j] == 0)
        {
          row = i;
          col = j;
        }
  }
}

Your i and j indices must not reach 4.

Upvotes: 5

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