为了在 qsort 中按指定字段排序，我们需要编写一个比较函数。下面是一个示例代码，它包含一个结构体和两个比较函数，它们按照结构体的不同字段排序：

#include 
#include 
#include 

typedef struct {
    int id;
    char name[50];
    float score;
} Student;

int compare_by_id(const void *a, const void *b) {
    Student *s1 = (Student *) a;
    Student *s2 = (Student *) b;
    if (s1->id < s2->id) return -1;
    else if (s1->id > s2->id) return 1;
    else return 0;
}

int compare_by_name(const void *a, const void *b) {
    Student *s1 = (Student *) a;
    Student *s2 = (Student *) b;
    return strcmp(s1->name, s2->name);
}

int main() {
    Student students[] = {
        {2, "Alice", 63.2},
        {1, "Bob", 71.5},
        {3, "Charlie", 84.7}
    };
    int n = sizeof(students) / sizeof(Student);

    // sort by id
    qsort(students, n, sizeof(Student), compare_by_id);
    for (int i = 0; i < n; i++) {
        printf("%d %s %.1f\n", students[i].id, students[i].name, students[i].score);
    }
    printf("\n");

    // sort by name
    qsort(students, n, sizeof(Student), compare_by_name);
    for (int i = 0; i < n; i++) {
        printf("%d %s %.1f\n", students[i].id, students[i].name, students[i].score);
    }
    printf("\n");

    return 0;
}

在上述代码中，我们定义了一个结构体 Student，其中包含三个字段：id, name 和 score。我们还定义了两