Why Segmentation Faults occur?-CodePudding

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.

CodePudding user response：

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.