写一个飞机订票系统c语言代码，要求包括以下内容5.用户改签功能：可进行机票改签。 6.用户自我查询购票信息功能。

好的，下面是一个简单的飞机订票系统的C语言代码，包括以上要求的功能：

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

struct flight {
    char flight_no[10];   // 航班号
    char from[20];        // 出发地
    char to[20];          // 目的地
    char departure_time[20];  // 起飞时间
    char arrival_time[20];    // 到达时间
    float price;          // 票价
    char passenger_name[20];  // 乘客姓名
    char passenger_id[20];    // 乘客身份证号
    char payment_method[20];  // 支付方式
};

int num_flights = 0;
struct flight flights[100];

void register_user();
void login_user();
void search_flights();
void book_flight();
void change_flight();
void query_booked_flights();

int main() {
    int choice;
    do {
        printf("\nWelcome to Airline Reservation System\n");
        printf("1. Register user\n");
        printf("2. Login user\n");
        printf("3. Search flights\n");
        printf("4. Book flight\n");
        printf("5. Change flight\n");
        printf("6. Query booked flights\n");
        printf("0. Exit\n");
        printf("Enter your choice: ");
        scanf("%d", &choice);
        switch (choice) {
            case 1:
                register_user();
                break;
            case 2:
                login_user();
                break;
            case 3:
                search_flights();
                break;
            case 4:
                book_flight();
                break;
            case 5:
                change_flight();
                break;
            case 6:
                query_booked_flights();
                break;
            case 0:
                printf("Thank you for using Airline Reservation System\n");
                break;
            default:
                printf("Invalid choice\n");
        }
    } while (choice != 0);
    return 0;
}

void register_user() {
    char username[20], password[20];
    printf("\nEnter username: ");
    scanf("%s", username);
    printf("Enter password: ");
    scanf("%s", password);
    // 在这里可以将用户名和密码存储到文件或数据库中
    printf("User registered successfully\n");
}

void login_user() {
    char username[20], password[20];
    printf("\nEnter username: ");
    scanf("%s", username);
    printf("Enter password: ");
    scanf("%s", password);
    // 在这里可以从文件或数据库中验证用户名和密码是否正确
    printf("User logged in successfully\n");
}

void search_flights() {
    char from[20], to[20];
    printf("\nEnter source: ");
    scanf("%s", from);
    printf("Enter destination: ");
    scanf("%s", to);
    int i;
    for (i = 0; i < num_flights; i++) {
        if (strcmp(flights[i].from, from) == 0 && strcmp(flights[i].to, to) == 0) {
            printf("%s %s %s %s %s %.2f\n", flights[i].flight_no, flights[i].from, flights[i].to, flights[i].departure_time, flights[i].arrival_time, flights[i].price);
        }
    }
}

void book_flight() {
    char flight_no[10], passenger_name[20], passenger_id[20], payment_method[20];
    printf("\nEnter flight no: ");
    scanf("%s", flight_no);
    int i;
    for (i = 0; i < num_flights; i++) {
        if (strcmp(flights[i].flight_no, flight_no) == 0) {
            printf("Enter passenger name: ");
            scanf("%s", passenger_name);
            printf("Enter passenger id: ");
            scanf("%s", passenger_id);
            printf("Enter payment method: ");
            scanf("%s", payment_method);
            strcpy(flights[i].passenger_name, passenger_name);
            strcpy(flights[i].passenger_id, passenger_id);
            strcpy(flights[i].payment_method, payment_method);
            printf("Flight booked successfully\n");
            return;
        }
    }
    printf("Invalid flight no\n");
}

void change_flight() {
    char flight_no[10], passenger_name[20], passenger_id[20], payment_method[20];
    printf("\nEnter flight no: ");
    scanf("%s", flight_no);
    int i;
    for (i = 0; i < num_flights; i++) {
        if (strcmp(flights[i].flight_no, flight_no) == 0) {
            printf("Enter passenger name: ");
            scanf("%s", passenger_name);
            printf("Enter passenger id: ");
            scanf("%s", passenger_id);
            printf("Enter payment method: ");
            scanf("%s", payment_method);
            strcpy(flights[i].passenger_name, passenger_name);
            strcpy(flights[i].passenger_id, passenger_id);
            strcpy(flights[i].payment_method, payment_method);
            printf("Flight changed successfully\n");
            return;
        }
    }
    printf("Invalid flight no\n");
}

void query_booked_flights() {
    char passenger_name[20];
    printf("\nEnter passenger name: ");
    scanf("%s", passenger_name);
    int i;
    for (i = 0; i < num_flights; i++) {
        if (strcmp(flights[i].passenger_name, passenger_name) == 0) {
            printf("%s %s %s %s %s %.2f %s %s %s\n", flights[i].flight_no, flights[i].from, flights[i].to, flights[i].departure_time, flights[i].arrival_time, flights[i].price, flights[i].passenger_name, flights[i].passenger_id, flights[i].payment_method);
        }
    }
}

这个程序使用了一个全局数组`flights`来存储航班信息，使用结构体来表示每个航班。函数`register_user()`和`login_user()`用来注册用户和登录用户，可以将用户名和密码存储到文件或数据库中。函数`search_flights()`用来查询航班信息，函数`book_flight()`用来预订航班，函数`change_flight()`用来改签航班，函数`query_booked_flights()`用来查询已预订的航班信息。

这只是一个简单的例子，实际上一个完整的飞机订票系统要涉及到很多其他的问题，比如票价计算、座位选择、支付等等，需要仔细设计和实现。