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(ipc.info)Messages

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Messages
========

   A message resource is described by a struct `msqid_ds' which is
allocated and initialized when the resource is created. Some fields in
`msqid_ds' can then be altered (if desired) by invoking `msgctl'.  The
memory used by the resource is released when it is destroyed by a
`msgctl' call.

     struct msqid_ds
         struct ipc_perm msg_perm;
         struct msg *msg_first;  /* first message on queue (internal) */
         struct msg *msg_last;   /* last message in queue (internal) */
         time_t msg_stime;       /* last msgsnd time */
         time_t msg_rtime;       /* last msgrcv time */
         time_t msg_ctime;       /* last change time */
         struct wait_queue *wwait; /* writers waiting (internal) */
         struct wait_queue *rwait; /* readers waiting (internal) */
         ushort msg_cbytes;      /* number of bytes used on queue */
         ushort msg_qnum;        /* number of messages in queue */
         ushort msg_qbytes;      /* max number of bytes on queue */
         ushort msg_lspid;       /* pid of last msgsnd */
         ushort msg_lrpid;       /* pid of last msgrcv */

   To send or receive a message the user allocates a structure that
looks like a `msgbuf' but with an array `mtext' of the required size.
Messages have a type (positive integer) associated with them so that
(for example) a listener can choose to receive only messages of a given
type.

     struct msgbuf
         long mtype;      type of message (Note: msgrcv).
         char mtext[1];   message text .. why is this not a ptr?

   The user must have write permissions to send and read permissions to
receive messages on a queue.

   When `msgsnd' is invoked, the user's message is copied into an
internal struct `msg' and added to the queue. A `msgrcv' will then read
this message and free the associated struct `msg'.

msgget

msgsnd

msgrcv

msgctl

msglimits

Implementation defined limits.