c - 消息队列 - 没有正确的响应

Question

*我正在使用消息队列来发送和接收消息，但是结构 msqid_ds 中的一些参数没有给出正确的值为什么会这样？ *上次消息发送时间=1525240214- 为什么显示垃圾值？

        struct mesg_q
    {
        char msg_txt[100];
        long msg_typ;
    };

    int main()
    {
        int msgid;
       key_t key;
        char buffer[100]; 

        struct mesg_q msgq;
         struc answerst msqid_ds info;
     //  key = ftok("/home/yash72/krishna/thread_test/msgq.text", 65);
        msgid=msgget((key_t)1234, 0666 | IPC_CREAT);

        if(msgid== -1)
        {    
            printf("msgget failed\n");
            return -1;
        }

        while(1)
        { 
            printf("Text Message\n");
            fgets(msgq.msg_txt,100,stdin);

            if(msgsnd(msgid,&msgq,100,0)==-1)
            {
                printf("Send failed\n");
                return -1;
            }  
            else
            {
                printf("Message send\n");
            }    
         msgctl(msgid, IPC_STAT, &info);
         printf("Time of last message send=%d\n",info.msg_stime);
         printf("uid=%d\n",info.msg_perm.uid);
        }   
    }    

OUTPUT:

 Text Message
qwerty
Message send
Time of last message send=1525240214 // Why this is showing junk?
uid=0
Text Message



Receiver code:

• 此代码中出现垃圾值的原因是什么？

  结构 mesg_q { char msg_txt[100]; 长消息类型； };

  int main()
  {
      int msgid;
      char buffer[100];
      long int rec_buff=0;
      key_t  key;
      struct mesg_q msgq;
       struct msqid_ds info;
  
     // key = ftok("/home/yash72/krishna/thread_test/msgq.text", 65);
      msgid=msgget((key_t)1234, 0666 | IPC_CREAT);
  
      if(msgid == -1)
      {
          printf("Msgget failed\n");
          return -1;
      }
  
      while(1)
  
      {
          if(msgrcv(msgid,&msgq,100,rec_buff,0)==-1)
          {
              printf("Mesg recv failed\n");
              return -1;
          }
          else
          {
              printf("Mesg Recvd\n");
          }
          printf("Recvd mesg=%s\n",msgq.msg_txt);
  
         msgctl(msgid, IPC_STAT, &info);
  
          printf("Num:of bytes on queue= %d\n", info.msg_cbytes);
          printf("num:of messages on queue=%d\n", info.msg_qnum);
          printf("Max bytes on queue=%d\n", info.msg_qbytes);
          printf("Time of last message recvd=%d\n",info.msg_rtime);
      }     
  }
  

  输出；

  队列中的字节数= 0 队列中的消息数=0 队列中的最大字节数=16384 上次收到消息的时间=1525240214

  从 struct msqid_ds 中得到这个错误值的原因是什么？

Answer 1

首先你应该知道为什么要使用消息队列 IPC，我假设你在使用消息队列之前，已经经历了FIFO。在 FIFO 中，进程相互通信的条件是什么？这两个进程在通信时都应该活着。

为避免上述FIFO问题，您正在使用消息队列，因为在MQ中您可以一次放入数据并可以随时读取，为此您必须指定mtype 即 mtype process_1 正在发送数据，mtype process_2 正在接收数据。

来自msgsnd的手册页

struct msgbuf {
      long mtype;       /* message type, must be > 0, I'm talking baout this */
      char mtext[1];    /* message data */
};

所以在两个进程(读者和发送者)中指定mtype

msgq.msg_typ = 1; /*specify this in both process */

其次，上次消息发送时间=1525240214-为什么显示垃圾值？ => 它不是垃圾数据，它是从 EPOCH 时间开始的秒数，使用 ctime() 以人类可读的格式打印。

sender.c

#include<stdio.h>
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/msg.h>
#include<stdint.h>
struct mesg_q {
        char msg_txt[100];
        long msg_typ;
};
int main(void) {
        int msgid;
        struct mesg_q msgq;
        struct msqid_ds info;
        msgid=msgget((key_t)1234, 0666 | IPC_CREAT);
        if(msgid== -1) {
                printf("msgget failed\n");
                return -1;
        }
        while(1) {
                printf("Text Message\n");
                fgets(msgq.msg_txt,sizeof(msgq.msg_txt),stdin);
                /* with what msg_type you are sending, you didn't mention ? mention it */
                msgq.msg_typ = 1; /* I am sending with msg_type 1 */
                if(msgsnd(msgid,&msgq,sizeof(msgq),msgq.msg_typ)==-1) {
                        printf("Send failed\n");
                        return -1;
                }
                else {
                        printf("Message send\n");
                }
                msgctl(msgid, MSG_STAT, &info);
                printf("Time of last message send = %jd\n",(intmax_t)info.msg_stime);/* use the correct format specifier */
                /* what ever time info.msg_stime printing, its not junk, its seconds from EPOCH, use ctime() to print in readable format */
                printf("uid = %d\n",info.msg_perm.uid);
        }
        return 0;
}

receiver.c

struct mesg_q {
        char msg_txt[100];
        long msg_typ;
};
int main(void ){
        int msgid;
        long int rec_buff=0;
        struct mesg_q msgq;
        struct msqid_ds info;
        msgid=msgget((key_t)1234, 0666 | IPC_CREAT);
        if(msgid == -1) {
                printf("Msgget failed\n");
                return -1;
        }
        while(1) {
                /* with what msg_typ you are reciving, you have to mention ? u didn't */
                msgq.msg_typ = 1;
                if(msgrcv(msgid,&msgq,sizeof(msgq),rec_buff, msgq.msg_typ)==-1) {
                        printf("Mesg recv failed\n");
                        return -1;
                }
                else{
                        printf("Mesg Recvd\n");
                }
                printf("Recvd mesg=%s\n",msgq.msg_txt);

                msgctl(msgid, MSG_STAT, &info);
                /* enable compiler warning, and use correct format specifier */
                printf("Num:of bytes on queue= %u\n", (int)info.msg_cbytes);
                printf("num:of messages on queue=%d\n", (int)info.msg_qnum);
                printf("Max bytes on queue=%d\n", (int)info.msg_qbytes);
                printf("Time of last message recvd=%jd\n",(intmax_t)info.msg_rtime);
        }
        return 0;
}