c语言订票系统 任务：通过此系统可以实现如下功能：录入（可以录入航班情况）、查询（可以查询某个航线的情况；可以输入起飞抵达城市，查询飞机航班情况）、订票（可以订票，如果该航班已经无票，可以提供相关可选择航班）、退票（可退票，退票后修改相关数据文件）、修改航班信息（当航班信息改变可以修改航班数据文件）

以下是一个简单的C语言订票系统，包含录入、查询、订票、退票、修改航班信息等功能。

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

struct flight {
    char flight_num[20]; // 航班号
    char departure[20]; // 起飞城市
    char destination[20]; // 目的地城市
    char date[20]; // 起飞日期
    int total_seats; // 总座位数
    int booked_seats; // 已预订的座位数
    float price; // 票价
};

void print_flight(struct flight f) {
    printf("航班号：%s\n", f.flight_num);
    printf("起飞城市：%s\n", f.departure);
    printf("目的地城市：%s\n", f.destination);
    printf("起飞日期：%s\n", f.date);
    printf("总座位数：%d\n", f.total_seats);
    printf("已预订座位数：%d\n", f.booked_seats);
    printf("票价：%f\n", f.price);
    printf("\n");
}

void input_flight(struct flight *f) {
    printf("请输入航班号：");
    scanf("%s", f->flight_num);
    printf("请输入起飞城市：");
    scanf("%s", f->departure);
    printf("请输入目的地城市：");
    scanf("%s", f->destination);
    printf("请输入起飞日期：");
    scanf("%s", f->date);
    printf("请输入总座位数：");
    scanf("%d", &f->total_seats);
    printf("请输入票价：");
    scanf("%f", &f->price);
    f->booked_seats = 0;
}

void save_flight(struct flight f) {
    FILE *fp = fopen("flights.dat", "ab");
    fwrite(&f, sizeof(struct flight), 1, fp);
    fclose(fp);
}

void list_flights() {
    FILE *fp = fopen("flights.dat", "rb");
    struct flight f;
    while (fread(&f, sizeof(struct flight), 1, fp) == 1) {
        print_flight(f);
    }
    fclose(fp);
}

void search_flight(char departure[20], char destination[20], char date[20]) {
    FILE *fp = fopen("flights.dat", "rb");
    struct flight f;
    int found = 0;
    while (fread(&f, sizeof(struct flight), 1, fp) == 1) {
        if (strcmp(f.departure, departure) == 0 &&
            strcmp(f.destination, destination) == 0 &&
            strcmp(f.date, date) == 0) {
            print_flight(f);
            found = 1;
        }
    }
    fclose(fp);
    if (!found) {
        printf("没有找到匹配的航班。\n");
    }
}

void book_flight(char flight_num[20]) {
    FILE *fp = fopen("flights.dat", "r+b");
    struct flight f;
    int found = 0;
    while (fread(&f, sizeof(struct flight), 1, fp) == 1) {
        if (strcmp(f.flight_num, flight_num) == 0) {
            if (f.booked_seats < f.total_seats) {
                f.booked_seats++;
                fseek(fp, -sizeof(struct flight), SEEK_CUR);
                fwrite(&f, sizeof(struct flight), 1, fp);
                printf("订票成功！\n");
                found = 1;
            } else {
                printf("该航班已经无票，可选择以下航班：\n");
                list_flights();
            }
            break;
        }
    }
    fclose(fp);
    if (!found) {
        printf("没有找到匹配的航班。\n");
    }
}

void cancel_flight(char flight_num[20]) {
    FILE *fp = fopen("flights.dat", "r+b");
    struct flight f;
    int found = 0;
    while (fread(&f, sizeof(struct flight), 1, fp) == 1) {
        if (strcmp(f.flight_num, flight_num) == 0) {
            if (f.booked_seats > 0) {
                f.booked_seats--;
                fseek(fp, -sizeof(struct flight), SEEK_CUR);
                fwrite(&f, sizeof(struct flight), 1, fp);
                printf("退票成功！\n");
                found = 1;
            } else {
                printf("该航班当前没有已订票，无法退票。\n");
            }
            break;
        }
    }
    fclose(fp);
    if (!found) {
        printf("没有找到匹配的航班。\n");
    }
}

void modify_flight(char flight_num[20]) {
    printf("请输入要修改的航班信息：\n");
    struct flight f;
    input_flight(&f);
    FILE *fp = fopen("flights.dat", "r+b");
    struct flight f_old;
    int found = 0;
    while (fread(&f_old, sizeof(struct flight), 1, fp) == 1) {
        if (strcmp(f_old.flight_num, flight_num) == 0) {
            fseek(fp, -sizeof(struct flight), SEEK_CUR);
            fwrite(&f, sizeof(struct flight), 1, fp);
            printf("修改成功！\n");
            found = 1;
            break;
        }
    }
    fclose(fp);
    if (!found) {
        printf("没有找到匹配的航班。\n");
    }
}

int main() {
    while (1) {
        printf("请输入操作编号：\n");
        printf("1. 录入航班信息\n");
        printf("2. 查询航班信息\n");
        printf("3. 订票\n");
        printf("4. 退票\n");
        printf("5. 修改航班信息\n");
        printf("6. 退出系统\n");
        int op;
        scanf("%d", &op);
        switch (op) {
            case 1: {
                struct flight f;
                input_flight(&f);
                save_flight(f);
                printf("保存成功！\n");
                break;
            }
            case 2: {
                printf("请输入起飞城市、目的地城市和起飞日期：\n");
                char departure[20], destination[20], date[20];
                scanf("%s", departure);
                scanf("%s", destination);
                scanf("%s", date);
                search_flight(departure, destination, date);
                break;
            }
            case 3: {
                printf("请输入要订票的航班号：\n");
                char flight_num[20];
                scanf("%s", flight_num);
                book_flight(flight_num);
                break;
            }
            case 4: {
                printf("请输入要退票的航班号：\n");
                char flight_num[20];
                scanf("%s", flight_num);
                cancel_flight(flight_num);
                break;
            }
            case 5: {
                printf("请输入要修改的航班号：\n");
                char flight_num[20];
                scanf("%s", flight_num);
                modify_flight(flight_num);
                break;
            }
            case 6: {
                exit(0);
            }
        }
    }
    return 0;
}

该程序中使用了结构体来表示航班信息，同时通过文件读写来实现航班信息的录入、保存和查询等功能。在订票和退票功能中，还需要修改航班文件中的已预订座位数。另外，为了使程序更加灵活，每次操作完成后都会回到菜单界面让用户选择下一步操作。