如何使用C程序中的函数在已有的结构中添加一个新值？

#include <stdio.h>
#include <stdlib.h>

// 已有的结构
struct MyStruct {
    int num;
    char ch;
};

int main() {
    // 创建已有结构的变量并赋值
    struct MyStruct existingStruct;
    existingStruct.num = 10;
    existingStruct.ch = 'A';

    // 创建新的结构体变量并赋值
    struct MyStruct* newStruct = (struct MyStruct*)malloc(sizeof(struct MyStruct));
    newStruct->num = 20;
    newStruct->ch = 'B';

    // 连接两个结构体
    existingStruct.next = newStruct;

    // 打印已有结构和新添加的值
    printf("Existing struct: num = %d, ch = %c\n", existingStruct.num, existingStruct.ch);
    printf("New struct: num = %d, ch = %c\n", existingStruct.next->num, existingStruct.next->ch);

    // 释放动态分配的内存
    free(newStruct);

    return 0;
}