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MSGCTL(2)                     System Calls Manual                    MSGCTL(2)



NAME
       msgctl - message control operations

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

       int msgctl (msqid, cmd, buf)
       int msqid, cmd;
       struct msqid_ds *buf;

DESCRIPTION
       msgctl()  provides a variety of message control operations as specified
       by cmd.  The following cmds are available:

       IPC_STAT       Place the current value  of  each  member  of  the  data
                      structure  associated  with  msqid  into  the  structure
                      pointed to by buf.  The contents of this  structure  are
                      defined in intro(2).

       IPC_SET        Set  the  value  of  the  following  members of the data
                      structure associated with  msqid  to  the  corresponding
                      value found in the structure pointed to by buf:
                             msg_perm.uid
                             msg_perm.gid
                             msg_perm.mode /* only low 9 bits */
                             msg_qbytes

                      This  cmd  can only be executed by a process that has an
                      effective user ID equal to either that of super-user, or
                      to  the  value  of  msg_perm.cuid or msg_perm.uid in the
                      data structure associated with msqid.   Only  super-user
                      can raise the value of msg_qbytes.

       IPC_RMID       Remove  the  message queue identifier specified by msqid
                      from the system and destroy the message queue  and  data
                      structure associated with it.  This cmd can only be exe-
                      cuted by a process that has an effective user  ID  equal
                      to  either  that  of  super-user,  or  to  the  value of
                      msg_perm.cuid or  msg_perm.uid  in  the  data  structure
                      associated with msqid.

       In  the msgop(2) and msgctl(2) system call descriptions, the permission
       required for an operation is given as "[token]", where "token"  is  the
       type of permission needed interpreted as follows:

              00400                       Read by user
              00200                       Write by user
              00060                       Read, Write by group
              00006                       Read, Write by others

       Read  and  Write permissions on a msqid are granted to a process if one
       or more of the following are true:

              The effective user ID of the process is super-user.

              The effective user ID of the process matches msg_perm.[c]uid  in
              the data structure associated with msqid and the appropriate bit
              of the "user" portion (0600) of msg_perm.mode is set.

              The  effective  user  ID  of  the   process   does   not   match
              msg_perm.[c]uid  and  the  effective  group  ID  of  the process
              matches msg_perm.[c]gid and the appropriate bit of  the  "group"
              portion (060) of msg_perm.mode is set.

              The   effective   user   ID   of  the  process  does  not  match
              msg_perm.[c]uid and the effective group ID of the  process  does
              not match msg_perm.[c]gid and the appropriate bit of the "other"
              portion (06) of msg_perm.mode is set.

       Otherwise, the corresponding permissions are denied.

RETURN VALUES
       msgctl() returns:

       0      on success.

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

ERRORS
       EACCES         cmd is equal to IPC_STAT and [READ] operation permission
                      is denied to the calling process (see intro(2)).

       EFAULT         buf points to an illegal address.

       EINVAL         msqid is not a valid message queue identifier.

                      cmd is not a valid command.

       EPERM          cmd is equal to IPC_RMID or IPC_SET.  The effective user
                      ID of the calling process is neither super-user, nor the
                      value  of  msg_perm.cuid  or  msg_perm.uid  in  the data
                      structure associated with msqid.

                      cmd is equal to IPC_SET, an attempt  is  being  made  to
                      increase  to  the value of msg_qbytes, and the effective
                      user ID of the calling process is not equal to  that  of
                      super-user.

SEE ALSO
       intro(2), msgget(2), msgop(2)



                                21 January 1990                      MSGCTL(2)