Thread: Can't find the cause of a segmentation fault

Our lab for class today was this simple program, but I got a segmentation fault somewhere. After trying everything listed below, neither I nor my TA could figure it out! Any ideas?

Code:

//Lab Exercise 3
//lab3.cpp

#include <iostream>
#include <fstream>
#include <cstdlib>

using namespace std;

void selection_sort(int[], int);

int main()
{
   ifstream infile;

   int numbers[100];
   char in_file_name[16];

   cout << "Integer Sort\n"
        << "------------\n" //seg fault 1

        << "Please enter the filename to sort: ";
   
   cin >> in_file_name;
//seg fault 2
cout << in_file_name;

cout << "opening file..."<<endl;
   infile.open(in_file_name);

cout << "opening file...";

   if (infile.fail())
   {
      cout <<  "Error opening file.  Please try again.\n";
      exit (1);
   }

cout<<"Adios";

   int count = 0;

cout << "Inputting...";

   while ((infile >> numbers[count])  && (count < 50))
      ++count;

cout << "Sorting...";
   selection_sort(numbers, count);
   
   ofstream outfile;
   outfile.open("sorted.txt");

   for (int i = 0; i < count; ++i)
      outfile << numbers[[i];

   cout << "The sorted numbers have been written to sorted.txt." 
          << "Have a nice day!\n"<<endl;

   infile.close();
   outfile.close();
   return 0;
}

Please forgive the odd indenting. The unindented cout statements are for debugging only. I don't indent them so they are easy to remove later.

//seg fault 1 happened when I hardcoded the input filename, like:

Code:

istream infile;
infile.open("int.txt");

instead of asking the user for input.

The file as is seg faults at //seg fault 2.

Now comes the really weird part. If I input a file that does not exist, it acts properly. That is, it outputs the error message as described and exits the program. It does not give me a segmentation fault at all.

Weird #2 - When the filename is hardcoded, and I combine the three separate string literals when asking for the filename to look like:

Code:

cout << "Integer Sort\n------------\nPlease enter blah blah blah";

it still seg faults at //seg fault 1, right in the middle of outputting a string literal! The output is:

Code:

Integer Sort
------------
<segmentation fault> core dumped  <--this isn't the exact message, but you get the idea

Weird #3 - If I add endl's to a cout statement, I can make it to the next cout before it crashes. So if I add endls to the request for input, outputting in_file_name, and the lines

Code:

 cout << "Opening file...";

and

Code:

cout << "Adios";

it will run that far, but crash somewhere before

Code:

cout << "Inputting...";

and if I add and endl to that line, I make it to

Code:

cout << "Sorting...";

before I crash.

In my search of the board I found references to permission, which shouldn't be the case (I think), since a) no one else in the class experienced this, and b) I can view the file with "more int.txt" from my directory. I am working in UNIX, of which I have almost no knowledge. I don't think I'm accessing parts of my arrays I shouldn't be, and even if I were I don't imagine it'd give me this kind of behavior. FYI, this is not for credit anymore since the lab is over, so I don't think it falls under the homework clause. I removed the selection sort function to shorten this already-too-long post. I can post that, too, if need be.

>The actual segmentation fault is most likely in your selection_sort code.
You're probably right, I looked it over (can't test it at work) and in one for loop I'm incrementing the wrong variable, which can only be trouble.

I seem to recall hearing that segmentation faults are a colossal pain because they are hard to pin down sometimes, but how in the [insert your favorite expletive] can it cause a problem if I'm not overstepping an array or accessing forbidden memory at the time? I'm in the middle of outputting a string for [insert favorite saint or holy figure]'s sake! In the case of //seg fault 1, all I've done is declare the array in question - I haven't accessed or referenced it in any way!

I'm not disputing that this is the trouble. I'm sure that for loop is it. I just don't get how it crashed from an error in that function if I haven't really dealt with any of the variables used in the function, let alone the function itself at the time it creates a segmentation fault.

Also, why don't I need to use endl when outputting normally? (or do I?)

Code:

#include<iostream>
int main()
{
   std::cout << "Hello, world!";
   return 0;
}

works without endl...or is it printed to the screen by flushing the output buffer in some way unknown to me?

oooooooooohhhhhh.

I get it.

Daved, I don't mean to hump your leg or anything, but you. are. awesome. Thanks.