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MSGOP(2)                      System Calls Manual                     MSGOP(2)



NAME
       msgop, msgsnd, msgrcv - message operations

SYNOPSIS
       #include <&lt;sys/types.h>&gt;
       #include <&lt;sys/ipc.h>&gt;
       #include <&lt;sys/msg.h>&gt;

       int msgsnd(msqid, msgp, msgsz, msgflg)
       int msqid;
       struct msgbuf *msgp;
       int msgsz, msgflg;

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

DESCRIPTION
       msgsnd()  is  used  to  send a message to the queue associated with the
       message queue identifier specified by msqid.  [WRITE]  (see  msgctl(2))
       msgp  points  to a structure containing the message.  This structure is
       composed of the following members:

              long    mtype;      /* message type */
              char    mtext[1];   /* message text */

       mtype is a positive integer that can be used by the  receiving  process
       for  message  selection  (see  msgrcv()  below).   mtext is any text of
       length msgsz bytes.  msgsz can range from 0 to a  system-imposed  maxi-
       mum.

       msgflg specifies the action to be taken if one or more of the following
       are true:

       o  The number of bytes already on the queue is equal to msg_qbytes (see
          intro(2)).

       o  The  total  number of messages on all queues system-wide is equal to
          the system-imposed limit.

       These actions are as follows:

       o  If (msgflg & IPC_NOWAIT) is "true", the message will not be sent and
          the calling process will return immediately.

       o  If  (msgflg  & IPC_NOWAIT) is "false", the calling process will sus-
          pend execution until one of the following occurs:

          o  The condition responsible for the suspension no longer exists, in
             which case the message is sent.

          o  msqid  is  removed  from  the  system (see msgctl(2)).  When this
             occurs, errno is set equal  to  EIDRM,  and  a  value  of  -1  is
             returned.

          o  The  calling  process receives a signal that is to be caught.  In
             this case the message is not sent and the calling process resumes
             execution in the manner prescribed in signal(3V).

       Upon  successful  completion,  the  following  actions  are  taken with
       respect to the data structure associated with msqid (see intro(2)).

       o  msg_qnum is incremented by 1.

       o  msg_lspid is set equal to the process ID of the calling process.

       o  msg_stime is set equal to the current time.

       msgrcv() reads a message from the queue  associated  with  the  message
       queue  identifier  specified  by  msqid  and places it in the structure
       pointed to by msgp.  [READ] This structure is composed of the following
       members:

              long    mtype;      /* message type */
              char    mtext[1];   /* message text */

       mtype  is  the  received  message's  type  as  specified by the sending
       process.  mtext is the text of the message.  msgsz specifies  the  size
       in bytes of mtext.  The received message is truncated to msgsz bytes if
       it is larger than msgsz and (msgflg  &  MSG_NOERROR)  is  "true".   The
       truncated  part of the message is lost and no indication of the trunca-
       tion is given to the calling process.

       msgtyp specifies the type of message requested as follows:

       o  If msgtyp is equal to 0, the first message on the queue is received.

       o  If msgtyp is greater than 0, the first message  of  type  msgtyp  is
          received.

       o  If  msgtyp is less than 0, the first message of the lowest type that
          is less than or equal to the absolute value of msgtyp is received.

       msgflg specifies the action to be taken if a  message  of  the  desired
       type is not on the queue.  These are as follows:

       o  If  (msgflg & IPC_NOWAIT) is "true", the calling process will return
          immediately with a return value of -1 and errno set to ENOMSG.

       o  If (msgflg & IPC_NOWAIT) is "false", the calling process  will  sus-
          pend execution until one of the following occurs:

               o  A message of the desired type is placed on the queue.

               o  msqid  is  removed from the system.  When this occurs, errno
                  is set equal to EIDRM, and a value of -1 is returned.

               o  The calling process receives a signal that is to be  caught.
                  In  this  case  a  message  is  not received and the calling
                  process resumes execution in the manner prescribed  in  sig-
                  nal(3V).

       Upon  successful  completion,  the  following  actions  are  taken with
       respect to the data structure associated with msqid (see intro(2)).

       o  msg_qnum is decremented by 1.

       o  msg_lrpid is set equal to the process ID of the calling process.

       o  msg_rtime is set equal to the current time.

RETURN VALUES
       msgsnd() returns:

       0      on success.

       -1     on failure and sets errno to indicate the error.

       msgrcv() returns the number of bytes actually placed into mtext on suc-
       cess.  On failure, it returns -1 and sets errno to indicate the error.

ERRORS
       msgsnd()  will  fail  and no message will be sent if one or more of the
       following are true:

       EACCES         Operation permission is denied to  the  calling  process
                      (see intro(2)).

       EAGAIN         The  message cannot be sent for one of the reasons cited
                      above and (msgflg & IPC_NOWAIT) is "true".

       EFAULT         msgp points to an illegal address.

       EIDRM          The message queue referred to by msqid was removed  from
                      the system.

       EINTR          The call was interrupted by the delivery of a signal.

       EINVAL         msqid is not a valid message queue identifier.

                      mtype is less than 1.

                      msgsz  is  less  than  zero  or greater than the system-
                      imposed limit.

       msgrcv() will fail and no message will be received if one  or  more  of
       the following are true:

       E2BIG          mtext  is  greater than msgsz and (msgflg & MSG_NOERROR)
                      is "false".

       EACCES         Operation permission is denied to the calling process.

       EFAULT         msgp points to an illegal address.

       EIDRM          The message queue referred to by msqid was removed  from
                      the system.

       EINTR          The call was interrupted by the delivery of a signal.

       EINVAL         msqid is not a valid message queue identifier.

                      msgsz is less than 0.

       ENOMSG         The queue does not contain a message of the desired type
                      and (msgtyp & IPC_NOWAIT) is "true".

SEE ALSO
       intro(2), msgctl(2), msgget(2), signal(3V)



                                21 January 1990                       MSGOP(2)