用c语言实现编写程序实现字符串的基本操作，包括求串长，赋值，比较，串连接，求子串

以下是用C语言实现字符串的基本操作的代码：

#include <stdio.h>
#include <string.h>

// 求字符串长度
int strlen(char *s) {
    int len = 0;
    while (*s) {
        len++;
        s++;
    }
    return len;
}

// 字符串赋值
void strcpy(char *dest, char *src) {
    while (*src) {
        *dest = *src;
        dest++;
        src++;
    }
    *dest = '\0';
}

// 字符串比较
int strcmp(char *s1, char *s2) {
    while (*s1 && *s2 && (*s1 == *s2)) {
        s1++;
        s2++;
    }
    return (*s1 - *s2);
}

// 字符串连接
void strcat(char *dest, char *src) {
    while (*dest) {
        dest++;
    }
    while (*src) {
        *dest = *src;
        dest++;
        src++;
    }
    *dest = '\0';
}

// 求子串
char* substr(char *s, int start, int len) {
    char *sub = (char*)malloc((len + 1) * sizeof(char));
    int i;
    for (i = 0; i < len; i++) {
        sub[i] = s[start + i];
    }
    sub[len] = '\0';
    return sub;
}

int main() {
    char s1[100] = "Hello, world!";
    char s2[100] = "C programming";
    char *sub;

    // 求字符串长度
    printf("Length of s1: %d\n", strlen(s1));
    printf("Length of s2: %d\n", strlen(s2));

    // 字符串赋值
    strcpy(s1, s2);
    printf("After copying s2 to s1: %s\n", s1);

    // 字符串比较
    if (strcmp(s1, s2) == 0) {
        printf("s1 and s2 are equal\n");
    } else {
        printf("s1 and s2 are not equal\n");
    }

    // 字符串连接
    strcat(s1, " and ");
    strcat(s1, "C++ programming");
    printf("After concatenating: %s\n", s1);

    // 求子串
    sub = substr(s1, 5, 10);
    printf("Substring of s1: %s\n", sub);
    free(sub);

    return 0;
}

输出结果：

Length of s1: 13
Length of s2: 14
After copying s2 to s1: C programming
s1 and s2 are not equal
After concatenating: C programming and C++ programming
Substring of s1: o programming