用C语言编写一个程序，满足有10个学生，每个学生的数据包括学号、姓名、3门功课的成绩。从键盘输入10个学生的数据，要求按平均成绩降序显示出所有学生的数据(包括学号、姓名、3门课成绩、平均成绩)，并将其写入文本文件resultxt中。

以下是用C语言编写的程序，可以实现上述要求：

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

#define MAX_STUDENTS 10
#define MAX_NAME_LENGTH 20

typedef struct {
    int id;
    char name[MAX_NAME_LENGTH];
    int scores[3];
    double average;
} Student;

void input_students(Student students[]);
void calculate_averages(Student students[]);
void sort_students(Student students[]);
void output_to_file(Student students[]);

int main() {
    Student students[MAX_STUDENTS];
    input_students(students);
    calculate_averages(students);
    sort_students(students);
    output_to_file(students);
    return 0;
}

void input_students(Student students[]) {
    printf("Please enter data for %d students:\n", MAX_STUDENTS);
    for (int i = 0; i < MAX_STUDENTS; i++) {
        printf("Student %d:\n", i+1);
        printf("  ID: ");
        scanf("%d", &students[i].id);
        printf("  Name: ");
        scanf("%s", students[i].name);
        for (int j = 0; j < 3; j++) {
            printf("  Score %d: ", j+1);
            scanf("%d", &students[i].scores[j]);
        }
    }
}

void calculate_averages(Student students[]) {
    for (int i = 0; i < MAX_STUDENTS; i++) {
        double sum = 0.0;
        for (int j = 0; j < 3; j++) {
            sum += students[i].scores[j];
        }
        students[i].average = sum / 3.0;
    }
}

void sort_students(Student students[]) {
    for (int i = 0; i < MAX_STUDENTS-1; i++) {
        for (int j = i+1; j < MAX_STUDENTS; j++) {
            if (students[j].average > students[i].average) {
                Student temp = students[i];
                students[i] = students[j];
                students[j] = temp;
            }
        }
    }
}

void output_to_file(Student students[]) {
    FILE *fp = fopen("result.txt", "w");
    if (fp == NULL) {
        printf("Error: cannot open file\n");
        exit(1);
    }
    fprintf(fp, "ID\tName\tScore 1\tScore 2\tScore 3\tAverage\n");
    for (int i = 0; i < MAX_STUDENTS; i++) {
        fprintf(fp, "%d\t%s\t%d\t%d\t%d\t%.2f\n", students[i].id, students[i].name,
                students[i].scores[0], students[i].scores[1], students[i].scores[2], students[i].average);
    }
    fclose(fp);
    printf("Data saved to result.txt\n");
}

该程序使用了一个 `Student` 结构体来存储每个学生的数据，其中包括学号、姓名、3门功课的成绩和平均成绩。程序首先调用 `input_students` 函数从键盘输入每个学生的数据，然后调用 `calculate_averages` 函数计算每个学生的平均成绩，最后调用 `sort_students` 函数按平均成绩降序排序所有学生，并将结果输出到文件中。