c - 关于 strcat()、strncpy()、strncat() 等字符串函数的问题？

Question

代码1

int main()
{
    char str[]="abc";
    char str1[]="hello computer";
    strcat(str,str1);
    printf("the concatenated string is : %s\n",str);
    return 0;
}

输出- abchello computer

代码2

int main()
{
    char str[100];  //notice the change from code 1
    char str1[]="hello computer";
    strcat(str,str1);
    printf("the concatenated string is : %s\n",str);
    return 0;
}

输出- @#^hello computer

代码3

int main()
{
    char str[100];
    char str1[]="hello computer";
    strncpy(str,str1,5);
    str[5]='\0';   //external addition of NULL
    printf("the copied string is : %s\n",str);
    return 0;
}

输出- hello

代码4

int main()
{
    char str[100]="abc";
    char str1[]="hello computer";
    strncat(str,str1,5);
    printf("the concatenated string is : %s\n",str);
    return 0;
}

输出- abchello

问题

Q-1) 为什么 abchello computer显示在 code 1 中和 @#^hello computer在code 2 ？垃圾来自哪里@#^来了吗？

Q-2) 为什么外部添加NULL '\0'在 strncpy() 中是必需的但不在strncat()如图code 3和 code 4分别？

注意- 如果在 code 4 中我做char str[100];然后@#^hello已显示，但字符串仍未添加 NULL 结束

Answer 1

代码1

char str[]="abc";
char str1[]="hello computer";
strcat(str,str1);

这会溢出 str。该行为是未定义的，可以假设任何输出(实际上，str1 可能正在被自身覆盖，这就是为什么您看到 abshel​​lo computer 作为输出)。

代码2

char str[100];  //notice the change from code 1
char str1[]="hello computer";
strcat(str,str1);

您在 hello computer 之前看到垃圾，因为 str 未初始化。碰巧它在第一个 NUL 之前的测试时包含 "@#^"。

代码3

char str[100];
char str1[]="hello computer";
strncpy(str,str1,5);
str[5]='\0';   //external addition of NULL

这是正确的。 strncpy 复制 "hello computer" 的前五个字符并且不以 NUL 结尾。你应该自己做(就像你所做的那样)。如需进一步引用，请参阅 strcpy 的手册页:

The strcpy() function copies the string pointed to by src, including the terminating null byte ('\0'), to the buffer pointed to by dest. The strings may not overlap, and the destination string dest must be large enough to receive the copy. Beware of buffer overruns! (See BUGS.)

The strncpy() function is similar, except that at most n bytes of src are copied. Warning: If there is no null byte among the first n bytes of src, the string placed in dest will not be null-terminated.

代码4

char str[100]="abc";
char str1[]="hello computer";
strncat(str,str1,5);

strncpy 和strncat 之间的一个区别是strncat NUL 终止结果字符串。来自 strncat 的手册页:

As with strcat(), the resulting string in dest is always null-terminated.