int main(int argc, string argv[]) {
    string hash = argv[1];    // Read from argv
    crack_password(hash);
}

void crack_password(char * hash) {
     printf("%dn", hash) // prints correctly.
     string * password_guess = // some functionality, hash doesn't change
     match_password(password_guess, hash);
}

match_password(char * password_guess, char * hash) {
     printf("%dn", hash) // prints correctly.
     char first_two_letters[2] = "";
     append(first_two_letters, hash[0]);

     printf("Hash: %sn", hash);
     append(first_two_letters, hash[1]);
     printf("%dn", first_two_letters) // prints first two letters of the hash.
     printf("%dn", hash) // hash null here.
}

void append(char* s, char c) {
    int len = strlen(s);
    s[len] = c;
    s[len+1] = '';
}