📄 linux环境进程间通信（三）.htm
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</PRE></TD></TR></TBODY></TABLE><BR>程序输出结果见 <A
href="http://www.ibm.com/developerworks/cn/linux/l-ipc/part3/index.html#listing3">附录
3</A>。 <BR><BR>
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<P><A name=N10252><SPAN class=atitle>小结：</SPAN></A></P>
<P>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。</P>
<P><B><A name=listing1>附录 1</A>：
</B>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： </P>
<OL>
<LI>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；
<LI>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；
<LI>随文件系统持续：IPC一直持续到显示删除该对象为止。 </LI></OL><BR>
<P><B><A name=listing2>附录 2</A>：
</B><BR>结构msg_queue用来描述消息队列头，存在于系统空间： </P>
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<TD class=code-outline><PRE class=displaycode>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;
};
</PRE></TD></TR></TBODY></TABLE><BR><BR>
<P>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；</P>
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<TD class=code-outline><PRE class=displaycode>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 */
};
</PRE></TD></TR></TBODY></TABLE><BR>//可以看出上述两个结构很相似。 <BR>
<P><B><A name=listing3>附录 3</A>： </B>消息队列实例输出结果： </P>
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<TD class=code-outline><PRE class=displaycode>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
</PRE></TD></TR></TBODY></TABLE><BR><BR><BR>
<P><A name=resources><SPAN class=atitle>参考资料 </SPAN></A></P>
<UL>
<LI>UNIX网络编程第二卷：进程间通信，作者：W.Richard
Stevens，译者：杨继张，清华大学出版社。对POSIX以及系统V消息队列都有阐述，对Linux环境下的程序开发有极大的启发意义。<BR><BR>
<LI>linux内核源代码情景分析（上），毛德操、胡希明著，浙江大学出版社，给出了系统V消息队列相关的源代码分析。<BR><BR>
<LI><A href="http://www.fanqiang.com/a4/b2/20010508/113315.html"
target=_blank>http://www.fanqiang.com/a4/b2/20010508/113315.html</A>，主要阐述linux下对文件的操作，详细介绍了对文件的存取权限位，对IPC对象的存取权限同样具有很好的借鉴意义。
<BR><BR>
<LI>msgget、msgsnd、msgrcv、msgctl手册<BR></LI></UL><BR><BR>
<P><A name=author><SPAN class=atitle>关于作者</SPAN></A></P>
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<P>郑彦兴，男，现攻读国防科大计算机学院网络方向博士学位。您可以通过电子邮件 <A
href="mailto:mlinux@163.com?cc=">mailto:mlinux@163.com?cc=</A>和他联系。
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