2．编写程序：用指针变量编写下列字符串处理函数： 1）字符串拼接函数 2）字符串比较函数 3）取字符串长度函数

1）字符串拼接函数：

#include <stdio.h>

void string_concat(char *s1, char *s2) {
    while (*s1 != '\0') {
        s1++;
    }
    while (*s2 != '\0') {
        *s1 = *s2;
        s1++;
        s2++;
    }
    *s1 = '\0';
}

int main() {
    char s1[20] = "hello";
    char s2[20] = "world";
    string_concat(s1, s2);
    printf("%s\n", s1);
    return 0;
}

2）字符串比较函数：

#include <stdio.h>

int string_compare(char *s1, char *s2) {
    while (*s1 == *s2) {
        if (*s1 == '\0') {
            return 0;
        }
        s1++;
        s2++;
    }
    return (*s1 - *s2);
}

int main() {
    char s1[20] = "hello";
    char s2[20] = "world";
    int result = string_compare(s1, s2);
    if (result < 0) {
        printf("%s is less than %s\n", s1, s2);
    } else if (result > 0) {
        printf("%s is greater than %s\n", s1, s2);
    } else {
        printf("%s is equal to %s\n", s1, s2);
    }
    return 0;
}

3）取字符串长度函数：

#include <stdio.h>

int string_length(char *s) {
    int length = 0;
    while (*s != '\0') {
        length++;
        s++;
    }
    return length;
}

int main() {
    char s[20] = "hello";
    int length = string_length(s);
    printf("The length of %s is %d\n", s, length);
    return 0;
}