msgctl(2)                        DG/UX 5.4.2                       msgctl(2)


NAME
       msgctl - get or set message queue attributes or destroy a message
       queue

SYNOPSIS
       #include <sys/types.h>
       #include <sys/ipc.h>
       #include <sys/msg.h>

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

   where:
       msqid     A message queue identifier
       cmd       The message control operation to be performed
       buf       The address of a message queue attribute record (used only
                 if cmd is IPC_STAT or IPC_SET)

DESCRIPTION
       Msgctl is used to get and set message queue attributes or to destroy
       a message queue.  The subject message queue is identified by msqid.
       The action performed by msgctl depends on the value of cmd as
       follows:

       IPC_STAT  The user-visible msqid_ds structure is returned in buf.  If
                 an error occurs, the contents of buf are undefined.

       IPC_SET   The following message queue attributes are set to the
                 values found in the structure pointed to by buf: user id
                 (msg_perm.uid), group id (msg_perm.gid), permission rights
                 (in msg_perm.mode), and the maximum size (msg_qbytes).

                 If an error occurs, the message queue remains unchanged.
                 Otherwise, the last change time (msg_ctime) is set to the
                 current time.

       IPC_RMID  The message queue is destroyed.  All resources consumed by
                 the message queue are freed and the message queue
                 identifier is invalidated.  All queued messages are lost.

                 If an error occurs, the message queue remains unchanged.

ACCESS CONTROL
       Operation permission depends on the value of cmd as follows:

       IPC_STAT  The calling process is required to have read access to the
                 message queue.

       IPC_SET   The effective user id of the calling process must be equal
                 to the message queue's user id, the message queue creator's
                 user id, or that of the superuser.  If the maximum size of
                 the message queue is increased, the effective user id of



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msgctl(2)                        DG/UX 5.4.2                       msgctl(2)


                 the calling process must be the superuser.

       IPC_RMID  The effective user id of the calling process must be equal
                 to the message queue's user id, the message queue creator's
                 user id, or that of the superuser.

RETURN VALUE
       0      Completed successfully.

       -1     An error occurred.  errno is set to indicate the error.

DIAGNOSTICS
       Errno may be set to one of the following error codes regardless of
       the value of cmd:

       EINVAL    msqid is not a valid message queue identifier.

       EINVAL    cmd is not a valid command.

       If cmd is IPC_STAT, errno may be set to one of these values:

       EACCES    Read permission is denied to the calling process.

       EFAULT    buf points to an illegal address.

       If cmd is IPC_SET, errno may be set to one of these values:

       EPERM     Permission to change the message queue attributes is denied
                 to the calling process.

       EPERM     Permission to increase to the maximum size of the message
                 queue is denied to the calling process.

       EFAULT    buf points to an illegal address.

       If cmd is IPC_RMID, errno may be set to this value:

       EPERM     Permission to remove the message queue is denied to the
                 calling process.

SEE ALSO
       intro(2), ipcrm(1), ipcs(1), msgget(2), msgrcv(2), msgsnd(2).















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