消息队列标识符已更改

我试图在两个进程之间实现简单的服务器 - 客户端通信 . 我在服务器进程中遇到有2个消息队列的问题 .

问题是第二个队列的消息队列ID在第一个队列的 msgrcv 调用之后立即以某种方式更改 .

请考虑下面我的服务器进程的简化版本（用于演示目的）：

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

#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/msg.h>

#define SERVER_KEY_PATHNAME "/student/m18512/m1851201/mqueue_server_key"
#define USERS_KEY_PATHNAME "/student/m18512/m1851201/labs"
#define PROJECT_ID 'M'
#define PROJECT_ID_U 'U'
#define QUEUE_PERMISSIONS 0660


struct buffer {
    int qid_a;
    char message_text [200];
    char dest_user[30];
};

struct message {
    long mtype;
    struct buffer buffer;
};

struct buffer_user {
    char source_user [30];
    long qid_source_user;
};

struct message_user {
    long mtype;
    struct buffer_user buffer_user;
};

int main (int argc, char **argv)
{
    setvbuf(stdout, NULL, _IONBF, 0);
    key_t queue_a_key, queue_b_queue;
    int qid_a, qid_b;
    struct message message;
    struct msqid_ds buf;

    //Create queue A
    if ((queue_a_key = ftok (SERVER_KEY_PATHNAME, PROJECT_ID)) == -1) {
        perror ("ftok");
        exit (1);
    }
    if ((qid_a = msgget (queue_a_key, IPC_CREAT | QUEUE_PERMISSIONS)) == -1) {
        perror ("msgget");
        exit (1);
    }

    //Create queue B
    if ((queue_b_queue = ftok (USERS_KEY_PATHNAME, PROJECT_ID_U)) == -1) {
        perror ("ftok");
        exit (1);
    } 
    if ((qid_b = msgget (queue_b_queue, IPC_CREAT | QUEUE_PERMISSIONS)) == -1) {
        perror ("msgget");
        exit (1);
    }


    while (1) { 
        printf("Step 1: qid A: %d\n", qid_a);
        printf("Step 1: qid B: %d\n", qid_b);

        // read an incoming message
        if (msgrcv (qid_a, &message, 300, 0, 0) == -1) {
            perror ("msgrcv");
            exit (1);
        }

        printf("Step 2: qid A: %d\n", qid_a);
        printf("Step 2: qid B: %d\n", qid_b);

    }
}

从输出中可以看出，一旦客户端进程向队列A发送消息，队列B的id就会改变：

Step 1: qid A: 1674706969
Step 1: qid B: 1679229087
Step 2: qid A: 1674706969
Step 2: qid B: 1679229013

队列B的原始id是 1679229087 （步骤1），但是在步骤2，该队列的id变为 1679229013 .

此外，看着ipcs，我没有看到一个带有新id 1679229013 的队列，但是 1679229087 仍然存在 .

请参阅下面的客户端流程的简化代码：

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

#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/msg.h>

#define SERVER_KEY_PATHNAME "/student/m18512/m1851201/mqueue_server_key"
#define USERS_KEY_PATHNAME "/student/m18512/m1851201/labs"
#define PROJECT_ID 'M'
#define PROJECT_ID_U 'U'

struct buffer {
    int qid;
    char message_text [200];
    char dest_user[30];
};

struct message {
    long mtype;
    struct buffer buffer;
};

struct buffer_user {
    char source_user [30];
    long qid_source_user;
};

struct message_user {
    long mtype;
    struct buffer_user buffer_user;
};


int main (int argc, char **argv)
{
    setvbuf(stdout, NULL, _IONBF, 0);
    struct message my_message, return_message;
    my_message.mtype = 1;   
    key_t server_queue_key, users_queue_key;
    int server_qid, myqid, users_qid;

    // create my client queue for receiving messages from server
    if ((myqid = msgget (IPC_PRIVATE, 0660)) == -1) {
        perror ("msgget: myqid");
        exit (1);
    }
    if ((server_queue_key = ftok (SERVER_KEY_PATHNAME, PROJECT_ID)) == -1) {
        perror ("ftok");
        exit (1);
    }
    if ((server_qid = msgget (server_queue_key, 0)) == -1) {
        perror ("msgget: server_qid");
        exit (1);
    }

    my_message.buffer.qid = myqid; 

    printf("Server qid %d, my qid %d\n", server_qid, myqid);

    printf ("Please type a message: ");

    while (fgets (my_message.buffer.message_text, 198, stdin)) {
        // remove newline from string
        int length = strlen (my_message.buffer.message_text);
        if (my_message.buffer.message_text [length - 1] == '\n')
           my_message.buffer.message_text [length - 1] = '\0';

        // send message to server
        if (msgsnd (server_qid, &my_message, sizeof (my_message) + 1, 0) == -1) {
            perror ("client: msgsnd");
            exit (1);
        }

    }
    exit (0);
}

EDIT ：当我像这样声明队列ID时，似乎存在问题：

int qid_a, qid_b;

但是，当我这样声明它们时：

int qid_a = 0, qid_b = 0

问题消失了 . 为什么会这样？