(Hacker2) Why am I getting a seg fault here from fgets?

Question

I'm certain that the error is from fgets. The dictionary I'm using for this pset is preprocessed(?) so that all the words are ≤ 8.

I had a segmentation fault earlier with copying the first 2 characters from the user inputted password. After searching, I found I could solve it by using memset to fill all 3 chars with terminating characters.

I tried the same thing with this seg fault, but to no avail. I also tried changing the file pointer to one in which I have only one line, 7 characters in length→ this didn't work either.

#define _XOPEN_SOURCE   
#include <crypt.h>      
#include <cs50.h>       
#include <stdio.h>      
#include <stdlib.h>
#include <string.h>     
#include <unistd.h>     

int main (int argc, char* argv[])
{
    FILE* crackDict = fopen("/Dropbox/CS50/hacker2/CrackDictionary/Cracked_Dictionary.txt", "r");   
    string encryptedPass, crackWord, salt = NULL, engWord = NULL;

    if ((argc == 2) && (strlen(argv[1]) == 13))
    {
        encryptedPass = argv[1];
    }   
    else
    {
        printf("Usage: ./Crack [encrypted password...13 chars long]\n");
        return 0;
    }

    //clear the salt with terminating chars and copy first 2 chars of userinput
    memset(salt, '\0', 3);  
    strncpy(salt, encryptedPass, 2);


    memset(engWord, '\0', 9);
    for (int i = 0; fgets (engWord, 9, crackDict)!= NULL; i++)
    {
        crackWord = crypt(engWord, salt);
        if (strcmp(crackWord, encryptedPass) == 0)
        {
            break;
        }
        else
            memset(engWord, '\0', 9);   
    }
    printf("%s\n", engWord);

    return 0;    
}

Any help is greatly appreciated.

Answer 1

Not really. The cause of the error involves some topics that you will study later in the course. I'll try to simplify my answer as much as possible, but don't panic if you don't fully understand what I say because this is often a confusing topic even for students who already got to this stage in the course.

• C does not have a type named string. The cs50 library defines the type synonym string for another built-in type, namely char *.

• char * is what is referred to as a pointer type. A variable of a pointer type is used to store (point to) a memory address (i.e., an address of a location in memory).

• The value NULL represents a memory address here (address 0 or the 0th location in memory). The 0th location in memory is a special location that you can't read/write data from/to it.

Initially, you set salt and engWord to NULL, then applied them to memset which tries to write data into the locations that they point to and that is what's causing the error (because you can't read/write data from/to the 0th location in memory).

A quick fix to that would be declaring your string variables as char [] rather than string.

#define LENGTH 3
char salt[LENGTH];
memset(salt, '\0', LENGTH * sizeof (char)); // should work just fine