This PR updates the mqueue documentation example and adds some helpful documentation to explain the limits of the MqAttr input values.

Author: hicklin

src/mqueue.rs

@@ -9,17 +9,18 @@
 //! use nix::sys::stat::Mode;
 //!
 //! const MSG_SIZE: mq_attr_member_t = 32;
-//! let mq_name= "/a_nix_test_queue";
+//! let attr = MqAttr::new(0, 10, MSG_SIZE, 0);
+//! let mq_name= "/a_nix_doc_test_queue";
 //!
 //! let oflag0 = MQ_OFlag::O_CREAT | MQ_OFlag::O_WRONLY;
 //! let mode = Mode::S_IWUSR | Mode::S_IRUSR | Mode::S_IRGRP | Mode::S_IROTH;
-//! let mqd0 = mq_open(mq_name, oflag0, mode, None).unwrap();
+//! let mqd0 = mq_open(mq_name, oflag0, mode, Some(&attr)).unwrap();
 //! let msg_to_send = b"msg_1";
 //! mq_send(&mqd0, msg_to_send, 1).unwrap();
 //!
 //! let oflag1 = MQ_OFlag::O_CREAT | MQ_OFlag::O_RDONLY;
-//! let mqd1 = mq_open(mq_name, oflag1, mode, None).unwrap();
-//! let mut buf = [0u8; 32];
+//! let mqd1 = mq_open(mq_name, oflag1, mode, Some(&attr)).unwrap();
+//! let mut buf = [0u8; MSG_SIZE as usize];
 //! let mut prio = 0u32;
 //! let len = mq_receive(&mqd1, &mut buf, &mut prio).unwrap();
 //! assert_eq!(prio, 1);
@@ -100,7 +101,9 @@ impl MqAttr {
     ///
     /// - `mq_flags`:   Either `0` or `O_NONBLOCK`.
     /// - `mq_maxmsg`:  Maximum number of messages on the queue.
+    ///   The maximum value allowed by the system can be obtained from `/proc/sys/fs/mqueue/msg_max`.
     /// - `mq_msgsize`: Maximum message size in bytes.
+    ///   The maximum value allowed by the system can be obtained from `/proc/sys/fs/mqueue/msgsize_max`.
     /// - `mq_curmsgs`: Number of messages currently in the queue.
     pub fn new(
         mq_flags: mq_attr_member_t,