c - 在这种情况下，通过指针或通过值传递结构有什么区别？

Question

所以我正在阅读 C Primer Plus 这本书，并且遇到了关于结构和文件的编程作业。主要目标是为飞机制作座位预订程序。

我定义了以下结构，

typedef struct {
    unsigned short identification_number;
    unsigned short status;
    char last_name[MAX_NAM_LEN];
    char first_name[MAX_NAM_LEN];
} Seat;

typedef struct {
    unsigned short empty_seats;
    Seat seats[]; // Flexible Array Member
} Plane;

我有以下函数打印空座位的标识号，

/*
 * Function:  print_empty_seats_ID
 * -------------------------
 * Prints a list of empty seats identification numbers of the given plane.
 *
 */
void print_empty_seats_ID (Plane* plane, int num_seats) {

    // Clear the screen.
    system("clear");

    printf("Identification number of empty seats:");

    for (int i = 0; i < num_seats; i++) {
        if (plane->seats[i].status == EMPTY)
            printf(" %d", plane->seats[i].identification_number);
    }
    puts("");

    // Redirect the user to the main menu.
    puts("Redirecting in 4 seconds...");
    sleep(4);
}

以这种方式从另一个函数调用，print_empty_seats_ID (plane, num_seats); 这样 plane 是指向先前在堆上定义的平面结构的指针，并且 num_seats 是飞机上的座位总数。

此函数运行良好，但如果我将其更改为使用按值调用而不是指针，它会打印垃圾值，如下图所示。

/*
 * Function:  print_empty_seats_ID
 * -------------------------
 * Prints a list of empty seats identification numbers of the given plane.
 *
 */
void print_empty_seats_ID (Plane plane, int num_seats) {

    // Clear the screen.
    system("clear");

    printf("Identification number of empty seats:");

    for (int i = 0; i < num_seats; i++) {
        if (plane.seats[i].status == EMPTY)
            printf(" %d", plane.seats[i].identification_number);
    }
    puts("");

    // Redirect the user to the main menu.
    puts("Redirecting in 4 seconds...");
    sleep(4);
}

在这种情况下，函数以这种方式调用 print_empty_seats_ID (*plane, num_seats); 并输出以下废话 Identification number of empty seats: 64 0 0.

我的问题是为什么第一个功能有效而第二个功能无效？

Answer 1

问题是您使用的是所谓的灵活数组成员，规范对此有说明:

In particular, the size of the structure is as if the flexible array member were omitted except that it may have more trailing padding than the omission would imply.

因此，当您按值传递它时，不会复制座位本身。当你通过指针传递它时，指针指向原来有座位的平面，所以它有效..