POSIX Message Queues

This chapter describes POSIX message queues, which allow processes to exchange data in the form of messages

POSIX message queues are similar to their System V counterparts, in that data is exchanged in units of whole messages

However, there are also some notable differences:

  1. POSIX message queues are reference counted. A queue that is marked for deletion is removed only after it is closed by all processes that are currently using it.
  2. Each System V message has an integer type, and messages can be selected in a variety of ways using msgrcv(). By contrast, POSIX messages have an associated priority, and messages are always strictly queued (and thus received) in priority order.
  3. POSIX message queues provide a feature that allows a process to be asynchronously notified when a message is available on a queue.

52.1 Overview

The main functions in the POSIX message queue API are the following:

  1. The mq_open() function creates a new message queue or opens an existing queue, returning a message queue descriptor for use in later calls.
  2. The mq_send() function writes a message to a queue.
  3. The mq_receive() function reads a message from a queue.
  4. The mq_close() function closes a message queue that the process previously opened.
  5. The mq_unlink() function removes a message queue name and marks the queue for deletion when all processes have closed it

The above functions all serve fairly obvious purposes. In addition, a couple of features are peculiar to the POSIX message queue API:

  1. Each message queue has an associated set of attributes. Some of these attributes can be set when the queue is created or opened using mq_open(). Two functions are provided to retrieve and change queue attributes: mq_getattr() and mq_setattr().
  2. The mq_notify() function allows a process to register for message notification from a queue. After registering, the process is notified of the availability of a message by delivery of a signal or by the invocation of a function in a separate thread.

52.2 Opening, Closing, and Unlinking a Message Queue

However, if O_CREAT is specified in flags, two further arguments are required: mode and attr.

mq_queue 两种模式，打开和创建

Upon successful completion, mq_open() returns a message queue descriptor, a value of type mqd_t, which is used in subsequent calls to refer to this open message queue.

所以 message queue 不像其他的 IPC 机制那样，使用 key_t 创建，而是使用 char * name

52.3 Relationship Between Descriptors and Message Queues

The relationship between a message queue descriptor and an open message queue is analogous to the relationship between a file descriptor and an open file

On Linux, POSIX message queues are implemented as i-nodes in a virtual file system, and message queue descriptors and open message queue descriptions are implemented as file descriptors and open file descriptions, respectively

  1. Two processes can hold message queue descriptors (descriptor x in the diagram) that refer to the same open message queue description. This can occur because a process opens a message queue and then calls fork(). These descriptors share the state of the O_NONBLOCK flag.
  2. Two processes can hold open message queue descriptors that refer to different message queue descriptions that refer to the same message queue (e.g., descriptor z in process A and descriptor y in process B both refer to /mq-r). This occurs because the two processes each used mq_open() to open the same queue.

52.4 Message Queue Attributes

struct mq_attr {
  long mq_flags;  /* Message queue description flags: 0 or
                    O_NONBLOCK [mq_getattr(), mq_setattr()] */
  long mq_maxmsg; /* Maximum number of messages on queue
                    [mq_open(), mq_getattr()] */
  long mq_msgsize; /* Maximum message size (in bytes)
                    [mq_open(), mq_getattr()] */
  long mq_curmsgs; /* Number of messages currently in queue
                    [mq_getattr()] */
}

接下来分析了几个例子

52.5 Exchanging Message

介绍 mq_send 和 mq_receive 的使用，并且演示了一个例子，说明 block 的含义

If the message queue is already full (i.e., the mq_maxmsg limit for the queue has been reached), then a further mq_send() either blocks until space becomes available in the queue, or, if the O_NONBLOCK flag is in effect, fails immediately with the error EAGAIN.

The mq_timedsend() and mq_timedreceive() functions are exactly like mq_send() and mq_receive(), except that if the operation can’t be performed immediately, and the O_NONBLOCK flag is not in effect for the message queue description, then the abs_timeout argument specifies a limit on the time for which the call will block.

52.6 Message Notification

The sigev_notify field of this structure is set to one of the following values:

下面分别讲解三个取值的含义 SIGEV_NONE SIGEV_SIGNAL SIGEV_THREAD 但是没有看

52.6.1 Receiving Notification via a Signal

skip

52.6.2 Receiving Notification via a Thread

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问题

If oflag includes O_CREAT, a new, empty queue is created if one with the given name doesn’t already exist. If oflag specifies both O_CREAT and O_EXCL, and a queue with the given name already exists, then mq_open() fails.

似乎这两个标志反复的说明出现，但是有一种情况没有说明: 当 queue 已经出现, 没有 O_EXCL 会出现什么情况。 创建两个 queue ? 报错 ?(如何报错，那么 O_EXCL 就没有意义了

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