Why do I keep getting segmentation fault(core dumped)

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So I am doing exercises off of a book and for some reason when I ran this code that I made, it didn't work and I kept getting segmentation fault(core dumped).
Can you please explain to me why an tips to prevent this in the future here is the code.

C++

<pre>class listNode{
public:
    char letter;
    listNode * next;
};
typedef listNode * str;

void append(str& s, char c){
    listNode* loopPtr = s;
    loopPtr = s;
    while(loopPtr->next != NULL){
        loopPtr = loopPtr->next;
    }
    if(loopPtr->next == NULL){
        listNode* newNode = new listNode();
        newNode->letter = c;
        newNode->next = NULL;
        loopPtr->next = newNode;
    }
}


char characterAt(str s, int nodeNum){
    listNode* loopPtr = s;
    for(int i = 0; i < nodeNum; i++){
        loopPtr = loopPtr->next;
        if(nodeNum <= 0 && nodeNum >= 100000000){
            break;
        }
    }
    if(nodeNum <= 0 && nodeNum >= 100000000){
        return ' ';
    }
    else{
        return loopPtr->letter;
    }
}


int main(){
    str s;
    append(s, 'T');
    append(s, 'e');
    append(s, 's');
    append(s, 't');
 

    return 0;
}

What I have tried:

I have tried revisiting chapters in the book that might help and going to tutorial websites.

Posted 28-Jul-21 10:53am

MARLON LESUEUR

Updated 28-Jul-21 13:38pm

v2

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Comments

merano99 28-Jul-21 17:12pm

Essential parts of the source code are missing. It cannot be compiled without knowing the exact declaration for the self-defined data types.

error C2065: "str": undeclared identifier
error C2065: "listNode": undeclared identifier
error C2440: "Initialization": "str" ??cannot be converted to "listNode *"

MARLON LESUEUR 28-Jul-21 17:36pm

Sorry not all of the code was put down I will edit it

4 solutions

Solution 2

A segmentation fault means that you are using a bad pointer (that is, trying to read or write a memory address that is off limits to your program). No one here will manually execute your code to find the bug, although they will point out something obvious if they take the time to read your code carefully.

Although this bug might be found by manually executing the code with pencil and paper, that is simply not a viable approach for a program of even moderate complexity. Learning to use a debugger will make your life much easier and make it possible for you to write more complex programs.

Posted 28-Jul-21 12:19pm

Greg Utas

Solution 3

The pointer s is already not initialized and therefore undefined.
Since the class does not have a constructor either, the member variables are not automatically created or initialized.
What could possibly get out of here?

C++

str loopPtr = s;
while (loopPtr->next) ...

Posted 28-Jul-21 12:37pm

merano99

Updated 28-Jul-21 12:49pm

v2

Solution 4

Quote:
Can you please explain to me why an tips to prevent this in the future here is the code.

C/C++ rule of thumb: You are responsible of everything, every detail. C/C++ does not do a single check for you. If you own a memory space, you can do any stupid thing you want. Only the OS check that you are in your own memory, sometimes (MS-DOS do not do this check and you can mess everywhere in a DOS machine), windows does this check.

Quote:
I kept getting segmentation fault(core dumped).

Something is gone so bad that you try to access (read/write) a memory address that you do not own, it can be an uninitialized pointer, or a buffer/array/structure overrun.
Not your problem, but a little mistake:

C++

void append(str& s, char c){
    listNode* loopPtr = s;
    loopPtr = s; // Here you do exactly the same thing as in previous line, this line can be removed.

General solution: Use the debugger, it will help you to locate where your code crash and what are the values of variables when crash occurs. When you understand why you get a crash, you can backtrack the problem to when the variable go wrong if a variable is corrupted.

Your code do not behave the way you expect, or you don't understand why !

There is an almost universal solution: Run your code on debugger step by step, inspect variables.
The debugger is here to show you what your code is doing and your task is to compare with what it should do.
There is no magic in the debugger, it don't know what your code is supposed to do, it don't find bugs, it just help you to by showing you what is going on. When the code don't do what is expected, you are close to a bug.
To see what your code is doing: Just set a breakpoint and see your code performing, the debugger allow you to execute lines 1 by 1 and to inspect variables as it execute.

Debugger - Wikipedia, the free encyclopedia[^]

Mastering Debugging in Visual Studio 2010 - A Beginner's Guide[^]
Basic Debugging with Visual Studio 2010 - YouTube[^]

1.11 — Debugging your program (stepping and breakpoints) | Learn C++[^]

The debugger is here to only show you what your code is doing and your task is to compare with what it should do.

Posted 28-Jul-21 13:38pm

Patrice T

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OriginalGriff · Accepted Answer · 2021-07-28T11:11:00

Compiling does not mean your code is right! :laugh:
Think of the development process as writing an email: compiling successfully means that you wrote the email in the right language - English, rather than German for example - not that the email contained the message you wanted to send.

So now you enter the second stage of development (in reality it's the fourth or fifth, but you'll come to the earlier stages later): Testing and Debugging.

Start by looking at what it does do, and how that differs from what you wanted. This is important, because it give you information as to why it's doing it. For example, if a program is intended to let the user enter a number and it doubles it and prints the answer, then if the input / output was like this:

Input   Expected output    Actual output
  1            2                 1
  2            4                 4
  3            6                 9
  4            8                16

Then it's fairly obvious that the problem is with the bit which doubles it - it's not adding itself to itself, or multiplying it by 2, it's multiplying it by itself and returning the square of the input.
So with that, you can look at the code and it's obvious that it's somewhere here:

C#

int Double(int value)
   {
   return value * value;
   }

Once you have an idea what might be going wrong, start using the debugger to find out why. Put a breakpoint on the first line of the method, and run your app. When it reaches the breakpoint, the debugger will stop, and hand control over to you. You can now run your code line-by-line (called "single stepping") and look at (or even change) variable contents as necessary (heck, you can even change the code and try again if you need to).
Think about what each line in the code should do before you execute it, and compare that to what it actually did when you use the "Step over" button to execute each line in turn. Did it do what you expect? If so, move on to the next line.
If not, why not? How does it differ?
Hopefully, that should help you locate which part of that code has a problem, and what the problem is. We can't do any of that for you as we can't run that code in isolation - there is just too much stuff missing.

This is a skill, and it's one which is well worth developing as it helps you in the real world as well as in development. And like all skills, it only improves by use!