C语言有6个学生，每个学生的数据包括学号、姓名、性别、4门课的成绩、总成绩、平均成绩。从键盘输入每个学生信息及4门课成绩，总成绩及平均成绩要通过4门课算出。然后用选择排序法按照总成绩由高到低对6个学生数据进行排序并输出排序结果。

下面是实现这个功能的C语言代码：

#include <stdio.h>

struct Student {
    int id;
    char name[20];
    char gender;
    float scores[4];
    float total_score;
    float avg_score;
};

void calc_scores(struct Student *stu) {
    float total_score = 0;
    for (int i = 0; i < 4; i++) {
        total_score += stu->scores[i];
    }
    stu->total_score = total_score;
    stu->avg_score = total_score / 4;
}

void print_student(struct Student stu) {
    printf("%d\t%s\t%c\t", stu.id, stu.name, stu.gender);
    for (int i = 0; i < 4; i++) {
        printf("%.1f\t", stu.scores[i]);
    }
    printf("%.1f\t%.1f\n", stu.total_score, stu.avg_score);
}

void sort_students(struct Student *students, int n) {
    for (int i = 0; i < n - 1; i++) {
        int max_index = i;
        for (int j = i + 1; j < n; j++) {
            if (students[j].total_score > students[max_index].total_score) {
                max_index = j;
            }
        }
        if (max_index != i) {
            struct Student temp = students[i];
            students[i] = students[max_index];
            students[max_index] = temp;
        }
    }
}

int main() {
    struct Student students[6];
    for (int i = 0; i < 6; i++) {
        printf("Enter student %d info:\n", i + 1);
        printf("ID: ");
        scanf("%d", &students[i].id);
        printf("Name: ");
        scanf("%s", students[i].name);
        printf("Gender (M/F): ");
        scanf(" %c", &students[i].gender);
        printf("Scores (Math, English, Chinese, Programming): ");
        for (int j = 0; j < 4; j++) {
            scanf("%f", &students[i].scores[j]);
        }
        calc_scores(&students[i]);
    }

    printf("\nID\tName\tGender\tMath\tEnglish\tChinese\tProgramming\tTotal\tAvg\n");
    for (int i = 0; i < 6; i++) {
        print_student(students[i]);
    }

    printf("\nSorting by total score...\n");
    sort_students(students, 6);
    printf("\nID\tName\tGender\tMath\tEnglish\tChinese\tProgramming\tTotal\tAvg\n");
    for (int i = 0; i < 6; i++) {
        print_student(students[i]);
    }

    return 0;
}

这个程序定义了一个结构体 `Student`，表示一个学生的信息。在 `main` 函数中，通过循环输入6个学生的信息，并调用 `calc_scores` 函数计算总成绩和平均成绩。然后按照要求输出每个学生的信息，并调用 `sort_students` 函数排序。排序使用的是选择排序算法。最后再次输出排序后的结果。