undefined reference to 'mq_open'
验证 UNPv2里的一个例子时，连接时出现 undefined reference to 'mq_open' 错误。
man mq_open ，发现里面有这么一句话 ：link with -lrt 。加上 -lrt 后，连接成功。
运行，出现 Permission deny。继续查，man mq_overview 发现： Each message queue is identified by a name of the form /somename. 也就是说在指定 mqueue 的名字时，必须以 ”/" 开头，并且名字中只能有一个 “/"。UNPv2的 2.2 IPC names有详细的解释。
终于运行成功了，却发现找不到创建的 mqueue ， 继续 man mq_overview。发现下面一段：
Mounting the message queue file system
On Linux, message queues are created in a virtual file system. (Other implementations may also provide such a feature, but the details are likely to differ.) This file system can be mounted using the following commands:
$ mkdir /dev/mqueue
$ mount -t mqueue none /dev/mqueue
The sticky bit is automatically enabled on the mount directory.
After the file system has been mounted, the message queues on the system can be viewed and manipulated using the commands usually used for files (e.g., ls(1) and rm(1)).
The contents of each file in the directory consist of a single line containing information about the queue:
$ ls /dev/mqueue/mymq
QSIZE:129 NOTIFY:2 SIGNO:0 NOTIFY_PID:8260
$ mount -t mqueue none /dev/mqueue
These fields are as follows:
QSIZE Number of bytes of data in all messages in the queue.
NOTIFY_PID
If this is non-zero, then the process with this PID has used mq_notify(3) to register for asynchronous message notification, and the remaining fields describe how notification occurs.
NOTIFY Notification method: 0 is SIGEV_SIGNAL; 1 is SIGEV_NONE; and 2 is SIGEV_THREAD.
SIGNO Signal number to be used for SIGEV_SIGNAL.
运行，出现 Permission deny。继续查，man mq_overview 发现： Each message queue is identified by a name of the form /somename. 也就是说在指定 mqueue 的名字时，必须以 ”/" 开头，并且名字中只能有一个 “/"。UNPv2的 2.2 IPC names有详细的解释。
终于运行成功了，却发现找不到创建的 mqueue ， 继续 man mq_overview。发现下面一段：
Mounting the message queue file system
On Linux, message queues are created in a virtual file system. (Other implementations may also provide such a feature, but the details are likely to differ.) This file system can be mounted using the following commands:
$ mkdir /dev/mqueue
$ mount -t mqueue none /dev/mqueue
The sticky bit is automatically enabled on the mount directory.
After the file system has been mounted, the message queues on the system can be viewed and manipulated using the commands usually used for files (e.g., ls(1) and rm(1)).
The contents of each file in the directory consist of a single line containing information about the queue:
$ ls /dev/mqueue/mymq
QSIZE:129 NOTIFY:2 SIGNO:0 NOTIFY_PID:8260
$ mount -t mqueue none /dev/mqueue
These fields are as follows:
QSIZE Number of bytes of data in all messages in the queue.
NOTIFY_PID
If this is non-zero, then the process with this PID has used mq_notify(3) to register for asynchronous message notification, and the remaining fields describe how notification occurs.
NOTIFY Notification method: 0 is SIGEV_SIGNAL; 1 is SIGEV_NONE; and 2 is SIGEV_THREAD.
SIGNO Signal number to be used for SIGEV_SIGNAL.
