消息队列.txt

这个资料非常不错

//创建一个消息队列后，输出消息队列缺省属性
msg_stat(msgid,msg_ginfo);
sflags=IPC_NOWAIT;
msg_sbuf.mtype=10;
msg_sbuf.mtext[0]='a';
reval=msgsnd(msgid,&msg_sbuf,sizeof(msg_sbuf.mtext),sflags);
if(reval==-1)
{
    printf("message send error\n");
}
//发送一个消息后，输出消息队列属性
msg_stat(msgid,msg_ginfo);
rflags=IPC_NOWAIT|MSG_NOERROR;
reval=msgrcv(msgid,&msg_rbuf,4,10,rflags);
if(reval==-1)
    printf("read msg error\n");
else
    printf("read from msg queue %d bytes\n",reval);
//从消息队列中读出消息后，输出消息队列属性
msg_stat(msgid,msg_ginfo);
msg_sinfo.msg_perm.uid=8;//just a try
msg_sinfo.msg_perm.gid=8;//
msg_sinfo.msg_qbytes=16388;
//此处验证超级用户可以更改消息队列的缺省msg_qbytes
//注意这里设置的值大于缺省值
reval=msgctl(msgid,IPC_SET,&msg_sinfo);
if(reval==-1)
{
    printf("msg set info error\n");
    return;
}
msg_stat(msgid,msg_ginfo);
//验证设置消息队列属性
reval=msgctl(msgid,IPC_RMID,NULL);//删除消息队列
if(reval==-1)
{
    printf("unlink msg queue error\n");
    return;
}
}
void msg_stat(int msgid,struct msqid_ds msg_info)
{
int reval;
sleep(1);//只是为了后面输出时间的方便
reval=msgctl(msgid,IPC_STAT,&msg_info);
if(reval==-1)
{
    printf("get msg info error\n");
    return;
}
printf("\n");
printf("current number of bytes on queue is %d\n",msg_info.msg_cbytes);
printf("number of messages in queue is %d\n",msg_info.msg_qnum);
printf("max number of bytes on queue is %d\n",msg_info.msg_qbytes);
//每个消息队列的容量（字节数）都有限制MSGMNB，值的大小因系统而异。在创建新的消息队列时，//msg_qbytes的缺省值就是MSGMNB
printf("pid of last msgsnd is %d\n",msg_info.msg_lspid);
printf("pid of last msgrcv is %d\n",msg_info.msg_lrpid);
printf("last msgsnd time is %s", ctime(&(msg_info.msg_stime)));
printf("last msgrcv time is %s", ctime(&(msg_info.msg_rtime)));
printf("last change time is %s", ctime(&(msg_info.msg_ctime)));
printf("msg uid is %d\n",msg_info.msg_perm.uid);
printf("msg gid is %d\n",msg_info.msg_perm.gid);
}

程序输出结果见附录 3。
小结：
消息队列与管道以及有名管道相比，具有更大的灵活性，首先，它提供有格式字节流，有利于减少开发人员的工作量；其次，消息具有类型，在实际应用中，可作为优先级使用。这两点是管道以及有名管道所不能比的。同样，消息队列可以在几个进程间复用，而不管这几个进程是否具有亲缘关系，这一点与有名管道很相似；但消息队列是随内核持续的，与有名管道（随进程持续）相比，生命力更强，应用空间更大。
附录 1：在参考文献[1]中，给出了IPC随进程持续、随内核持续以及随文件系统持续的定义：
1. 随进程持续：IPC一直存在到打开IPC对象的最后一个进程关闭该对象为止。如管道和有名管道； 
2. 随内核持续：IPC一直持续到内核重新自举或者显示删除该对象为止。如消息队列、信号灯以及共享内存等； 
3. 随文件系统持续：IPC一直持续到显示删除该对象为止。 

附录 2：
结构msg_queue用来描述消息队列头，存在于系统空间：
struct msg_queue {
    struct kern_ipc_perm q_perm;
    time_t q_stime;         /* last msgsnd time */
    time_t q_rtime;         /* last msgrcv time */
    time_t q_ctime;         /* last change time */
    unsigned long q_cbytes;     /* current number of bytes on queue */
    unsigned long q_qnum;       /* number of messages in queue */
    unsigned long q_qbytes;     /* max number of bytes on queue */
    pid_t q_lspid;          /* pid of last msgsnd */
    pid_t q_lrpid;          /* last receive pid */
    struct list_head q_messages;
    struct list_head q_receivers;
    struct list_head q_senders;
};

结构msqid_ds用来设置或返回消息队列的信息，存在于用户空间；
struct msqid_ds {
    struct ipc_perm msg_perm;
    struct msg *msg_first;      /* first message on queue,unused  */
    struct msg *msg_last;       /* last message in queue,unused */
    __kernel_time_t msg_stime;  /* last msgsnd time */
    __kernel_time_t msg_rtime;  /* last msgrcv time */
    __kernel_time_t msg_ctime;  /* last change time */
    unsigned long  msg_lcbytes; /* Reuse junk fields for 32 bit */
    unsigned long  msg_lqbytes; /* ditto */
    unsigned short msg_cbytes;  /* current number of bytes on queue */
    unsigned short msg_qnum;    /* number of messages in queue */
    unsigned short msg_qbytes;  /* max number of bytes on queue */
    __kernel_ipc_pid_t msg_lspid;   /* pid of last msgsnd */
    __kernel_ipc_pid_t msg_lrpid;   /* last receive pid */
};

//可以看出上述两个结构很相似。
附录 3：消息队列实例输出结果：
current number of bytes on queue is 0
number of messages in queue is 0
max number of bytes on queue is 16384
pid of last msgsnd is 0
pid of last msgrcv is 0
last msgsnd time is Thu Jan  1 08:00:00 1970
last msgrcv time is Thu Jan  1 08:00:00 1970
last change time is Sun Dec 29 18:28:20 2002
msg uid is 0
msg gid is 0
//上面刚刚创建一个新消息队列时的输出
current number of bytes on queue is 1
number of messages in queue is 1
max number of bytes on queue is 16384
pid of last msgsnd is 2510
pid of last msgrcv is 0
last msgsnd time is Sun Dec 29 18:28:21 2002
last msgrcv time is Thu Jan  1 08:00:00 1970
last change time is Sun Dec 29 18:28:20 2002
msg uid is 0
msg gid is 0
read from msg queue 1 bytes
//实际读出的字节数
current number of bytes on queue is 0
number of messages in queue is 0
max number of bytes on queue is 16384   //每个消息队列最大容量（字节数）
pid of last msgsnd is 2510
pid of last msgrcv is 2510
last msgsnd time is Sun Dec 29 18:28:21 2002
last msgrcv time is Sun Dec 29 18:28:22 2002
last change time is Sun Dec 29 18:28:20 2002
msg uid is 0
msg gid is 0
current number of bytes on queue is 0
number of messages in queue is 0
max number of bytes on queue is 16388   //可看出超级用户可修改消息队列最大容量
pid of last msgsnd is 2510
pid of last msgrcv is 2510  //对操作消息队列进程的跟踪
last msgsnd time is Sun Dec 29 18:28:21 2002
last msgrcv time is Sun Dec 29 18:28:22 2002
last change time is Sun Dec 29 18:28:23 2002    //msgctl()调用对msg_ctime有影响
msg uid is 8
msg gid is 8