IPC 队列：- 接收错误数据

Question

My program is receiving wrong data on msgrcv function if flag is set to 0, however, if i set flag to 1 for specific data then msgrcv function is keep waiting. I defined send and receive function in common file and calling it from sender and receiver file. I checked debug data and it was sent properly but on receiving end if flag set to 0 means receive first packet then i am getting wrong data. If set flag to 1 which means specific message type data msgrcv function ended in waiting state. Please help. / CommonMsgQueue.c /

    int InitializeMsgQueue(char *keypath)
    {
       key_t key;
       int msgid;
       key = ftok(keypath,'B');
       if (key == -1 )
       {
          perror("Failed to generate key ");
          exit(0);
       }
       msgid = msgget(key, 0666 | IPC_CREAT );
       if ( msgid == -1 )
       {
          perror("Failed to get msgid");
          exit(0);
       }
       return msgid;
    }
    int SendMsg(key_t msgqid ,StMesgQ *MsgData , int msglength)
    {
        int iRetVal =0;
        iRetVal = msgsnd(msgqid,&MsgData,sizeof(StMesgQ),IPC_NOWAIT);
        if ( iRetVal == -1 )
        {
           perror("Failed to send message");
        }
        return iRetVal;
    }
    int RcvMsg(key_t msgqid, StMesgQ *MsgData , int msglength)
    {
       int iRetVal =0;
       iRetVal = msgrcv(msgqid,&MsgData,msglength,0,0);
       if (iRetVal == -1 )
       {
          perror("Failed to receive message ");
          exit(0);
       }
       
       return iRetVal;
    
    }
   /*Sender.c*/
   msgqid = InitializeMsgQueue("/home/nprabhat/Nitesh/Nitesh_FrameWork/Key/ClientQueue");
   CommonSHMData = (struct CommonSHM*)ShmData;
   StMesgQ obj;
   while (1)
   {
       
       printf("Sender:-- Enter the value For iIndex\n");
       scanf("%d",&value);
       CommonSHMData->iIndex = value;
       CommonSHMData->iIndex2 = value;
       DataPktToSend = (char*)CommonSHMData;
     memcpy(&obj.buf,&DataPktToSend,strlen(DataPktToSend));
       obj.iMsgType = 1;
       SendMsg(msgqid,&obj,sizeof(StMesgQ));
    }
  /*Reciever.c*/
  RcvMsg(msgqid,&MsgData,sizeof(StMesgQ));
  CommonSHMData_Recv = (struct CommonSHM*)MsgData.buf;
  printf("Data Received from queue is %d",CommonSHMData_Recv->iIndex);

Answer 1

regarding:

   iRetVal = msgrcv( msgqid, &MsgData, msglength, 0, 0 );
   
   if (iRetVal == -1 )
   {
      perror("Failed to receive message ");
      exit(0);
   }
   
   memcpy(&MsgData,&RcvMsgData,sizeof(StMesgQ));

the function: memcpy() copies from right to left.

Here is the syntax:

void *memcpy(void *destination, const void *source, size_t n);

So the call to memcpy() is overlaying the data placed in MsgData by the call to msgrcv() with whatever trash happens to be in RcvMsgData