c - 追加到字符串后使其为空，C

Question

我有一个向字符串添加字符的函数，下面代码中的注释解释了问题。我认为我错误地使用了指针。

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

void crack_password(char * hash) {
     printf("%d\n", 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("%d\n", hash) // prints correctly.
     char first_two_letters[2] = "";
     append(first_two_letters, hash[0]);

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

void append(char * s, char c) {
    int len = strlen(s);
    s[len] = c;
    s[len + 1] = '\0';
}

Answer 1

您的first_two_letters声明太小。它始终是您要保存的字符数+1(对于附加的“\0”)。
hash 存储在堆栈中紧接first_two_letters 之后，这意味着您将first_two_letters 的“\0”写入哈希变量，这就是它显示为空的原因。

char first_two_letters[3] = "";

应该可以解决问题。