在程序中想要获取消息队列长度可使用消息队列的属性这个数据结构：
需要#include <sys/msg.h>
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简单程序示例：
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数据结构各个参数详解如下：
msg_perm
An instance of the ipc_perm structure, which is defined for us in linux/ipc.h. This holds the permission information for the message queue, including the access permissions, and information about the creator of the queue (uid, etc).
msg_first
Link to the first message in the queue (the head of the list).
msg_last
Link to the last message in the queue (the tail of the list).
msg_stime
Timestamp (time_t) of the last message that was sent to the queue.
msg_rtime
Timestamp of the last message retrieved from the queue.
msg_ctime
Timestamp of the last ``change‘‘ made to the queue (more on this later).
wwait
and
rwait
Pointers into the kernel‘s wait queue. They are used when an operation on a message queue deems the process go into a sleep state (i.e. queue is full and the process is waiting for an opening).
msg_cbytes
Total number of bytes residing on the queue (sum of the sizes of all messages).
msg_qnum
Number of messages currently in the queue.
msg_qbytes
Maximum number of bytes on the queue.
msg_lspid
The PID of the process who sent the last message.
msg_lrpid
The PID of the process who retrieved the last message.