Why Segmentation Faults occur?

Question

I just started to learn C and need some helps. I already compiled my code and fixed all warnings that occur. However, when I run my program it says 'Segmentation Fault' and this is my code.

#include <stdio.h>
#include <string.h>


char *sorting(char word[51], int n) 
{
    for (int i = 0; i < n; i++) {
        for (int j = i+1; j < n; j++) {
            if (word[i] > word[j]) {
                char temp = word[i];
                word[i] = word[j];
                word[j] = temp;
            }
        }
    }
    return word;
}

From above, I just sort the word in order to use in strcmp.

First I will read file that contains jumbled words. Next read dictionary's file. Then I will check whether two words are the same.

int main(int argc, char **dict, char **jambles)
{
    const char *j = jambles[1];
    FILE *jambles_file = fopen(j, "r");
    char jambles_words[51];

    while (fgets(jambles_words, sizeof(jambles_words), jambles_file)) {

        int count = 0;
        const char *d = dict[1];
        FILE *dict_file = fopen(d ,"r");
        char dict_words[51];

        printf("%s", jambles_words);

        while (fgets(dict_words, sizeof(dict_words), dict_file)) {

            int length_jambles = strlen(jambles_words);
            int length_dict = strlen(dict_words);
            char *j_ = jambles_words;
            char *d_ = dict_words;
            const char *sort_jambles = sorting(j_, length_jambles);
            const char *sort_dict = sorting(d_, length_dict);

            if (length_jambles == length_dict) {
                int compare = strcmp(sort_jambles, sort_dict);
                if (compare == 0) {
                    printf("%s", dict_words);
                    count++;
                }
            }
            else if (count == 0) {
                printf("NO MACTHES");
            }
        }
        printf("\n");
    }
    return 0;
}

I still don't know what are the mistakes of my code even I already search on the internet for the causes of Segmentation Fault.

Short answer: You messed something up and accessed memory you don't have access to. One way to do this is to blow past the limits of your buffer. Tip: %50s and always check important return values. — tadman
– tadman, Commented Oct 23, 2022 at 17:34
That's an invalid main function declaration. Depending on system it might work, but not as you expect. — Some programmer dude
– Some programmer dude, Commented Oct 23, 2022 at 17:38
@JosephSible-ReinstateMonica I searched on the internet about how to create arguments for int main() — user19159910
– user19159910, Commented Oct 23, 2022 at 17:38
@linggS. That is a valid way to define a main function on POSIX systems. I'm just worried that based on the names you chose for the arguments, that you don't know what they actually contain. — Joseph Sible-Reinstate Monica
– Joseph Sible-Reinstate Monica, Commented Oct 23, 2022 at 17:39

Joseph Sible-Reinstate Monica · Accepted Answer · 2022-10-23 17:53:21Z

1

I suspect the root cause is that you misunderstand how command-line arguments are passed to C programs. When you run ./a.out foo bar and it calls into int main(int argc, char **argv, char **envp), "foo" will be in argv[1] and "bar" will be in argv[2]. envp will just contain environment variables like "TERM=xterm" and "PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin". You appear to think that "bar" will instead end up in envp[1], which is incorrect.

The more immediate cause is likely that because of the above, your call to fopen is failing due to a file called "TERM=xterm" or something not existing, and so returning NULL, which you then pass to fgets blindly. In general, it's Undefined Behavior to pass null pointers to any standard library function that doesn't specifically say what doing so will do.