# I. Introduction
- Environment Setting:
- Ubuntu Linux: 24.04.1 LTS
- gcc: 13.3.0
- What is segmentation fault?
- How can segmentation occurs?
# II. Experiment
## Case 1: Invalid write to a read-only section

int main()
{
    char *s = "Hello World";
    *s = "H";
    return 0;
}

- Compile the `.c` file into an object file using the following command.

    $ gcc segFault.c -g -o segFault

- The error message occurs after executing the`./segFault` command.
::: warning
Segmentation fault (core dumped)
:::
- This error is due to an invalid write to read-only memory. A string literal is stored in a read-only memory section.
- Solution:
- This code can be corrected by using an array instead of a character pointer, and this allocates memory on stack and initializes it to the value of the string literal.

    int main()
    {
        char s[] = "Hello World";
        s[0] = "H";
        return 0;
    }

## Case 2: Null pointer derefence

#include <stdlib.h>
#include <stdio.h>

int main()
{
    int *ptr = NULL;
    printf("%d", *ptr);
    return 0;
}

- There is no any warning or error message after compiling this `.c` file into an object file.
- The same error message occurs aftering executing it.
::: warning
Segmentation fault (core dumped
:::
- Deferecing a null pointer and assigning value to it also causes a segmentation fault as the code follows:

#include <stdlib.h>
#include <stdio.h>

int main()
{
    int *ptr = NULL;
    *ptr = 1;
    printf("%d",*ptr);
    return 0;
}

## Case 3: Buffer overflow

#include <stdlib.h>
#include <stdio.h>

int main()
{
    char s[] = "Hello World";
    printf("%s", s[20]);
    return 0;
}

- The code above accesses the character array `s` beyond its upper boundary
- No error message is prompted after compiling it into an object file.
- However, the same error message is prompted after executing it.
::: warning
Segmentation fault (core dumped)
:::
## Case 4: Stack overflow

int main()
{
    return main();
}

- The infinite recursive call leads to a stack overflow error because the called function is allocated in stack section.
- This case is similar to the other cases presented above. No error message is prompted after compiling it. However, a *segmentation fault* occurs after executing it.
# III. Reference
- [Segmentation fault](https://en.wikipedia.org/wiki/Segmentation_fault)