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(ipc.info)msgrcv

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msgrcv
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     int msgrcv (int msqid, struct msgbuf *msgp, int msgsz, long msgtyp,
     			int msgflg);

   * msqid  : id obtained by a call to msgget.

   * msgsz  : maximum size of message to receive.

   * msgp   : allocated by user to store the message in.

   * msgtyp :
          0 => get first message on queue.

          > 0 => get first message of matching type.

          < 0 => get message with least type  which is <= abs(msgtyp).

   * msgflg :
          IPC_NOWAIT : Return immediately if message not found.

          MSG_NOERROR : The message is truncated if it is larger than
          msgsz.

          MSG_EXCEPT : Used with msgtyp > 0 to receive any msg except
          of specified type.

   * returns : size of message if found. -1 on error.

   The first message that meets the `msgtyp' specification is
identified. For msgtyp < 0, the entire queue is searched for the
message with the smallest type.

   If its length is smaller than msgsz or if the user specified the
MSG_NOERROR flag, its text and type are copied to msgp->mtext and
msgp->mtype, and it is taken off the queue.

   The `msg_cbytes', `msg_qnum', `msg_lrpid', and `msg_rtime' fields
are updated. Writers waiting on the queue are awakened.

Errors:
E2BIG  : msg bigger than msgsz and MSG_NOERROR not specified.
EACCES : Do not have permission for reading the queue.
EFAULT : msgp not accessible.
EIDRM  : msg queue was removed.
EINTR  : msg not found ... would have slept but ... was interrupted.
EINVAL : msgsz > msgmax or msgsz < 0, msqid < 0 or unused.
ENOMSG : msg of requested type not found and IPC_NOWAIT specified.