C语言里的快排函数用法（qsort）

#include 
#include 
#include 
struct student
{
    int id;
    char name[12];
    char sex;
};
int compare(const void* a,const void* b)//基本数据类型排序
{
    return *(char*)a-*(char*)b;//从小到大
       //取值//强转为相应类型的指针！！
}
int compare_struct(const void* a,const void* b)
{
    return (*(struct student*)a).id-((struct student*)b)->id;
                  //注意优先级诶！//否则报错在非结构体中。。。
}
int compare_struct_duoji(const void* a,const void* b)//多级排序
{
    struct student student_a=*(struct student*)a;
    struct student student_b=*(struct student*)b;

    if(student_a.id==student_b.id)
    {
        return student_a.sex-student_b.sex;
    }
    else
    {
        return student_a.id-student_b.id;
    }
}
void main()
{
//*************char型*************
    char a[5]="hello";
    qsort(a,5,sizeof(a[0]),compare);
           //元素个数//元素大小//函数指针
    int i;
    for(i=0;i<5;i++)
            printf("%c ",a[i]);
    printf("\n");

//************struct型************
    struct student e[4]={{100,"chen",'m'},{100,"li",'f'}, \
                         {70,"wang",'f'},{100,"zhang",'m'}};
    qsort(e,4,sizeof(e[1]),compare_struct_duoji);

    for(i=0;i<4;i++)
            printf("%d %s %c\n",e[i].id,e[i].name,e[i].sex);
}