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NAME

     msgctl - message control operations

LIBRARY

     Standard C Library (libc, -lc)

SYNOPSIS

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

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

DESCRIPTION

     The msgctl() system call performs some control operations on the message
     queue specified by msqid.

     Each message queue has a data structure associated with it, parts of
     which may be altered by msgctl() and parts of which determine the actions
     of msgctl().  The data structure is defined in #include <sys/msg.h>
     and contains (amongst others) the following members:

     struct msqid_ds {
             struct  ipc_perm msg_perm;      /* msg queue permission bits */
             struct  msg *msg_first; /* first message in the queue */
             struct  msg *msg_last;  /* last message in the queue */
             msglen_t msg_cbytes;    /* number of bytes in use on the queue */
             msgqnum_t msg_qnum;     /* number of msgs in the queue */
             msglen_t msg_qbytes;    /* max # of bytes on the queue */
             pid_t   msg_lspid;      /* pid of last msgsnd() */
             pid_t   msg_lrpid;      /* pid of last msgrcv() */
             time_t  msg_stime;      /* time of last msgsnd() */
             time_t  msg_rtime;      /* time of last msgrcv() */
             time_t  msg_ctime;      /* time of last msgctl() */
     };

     The ipc_perm structure used inside the msqid_ds structure is defined in
     #include <sys/ipc.h>
     and looks like this:

     struct ipc_perm {
             uid_t           cuid;   /* creator user id */
             gid_t           cgid;   /* creator group id */
             uid_t           uid;    /* user id */
             gid_t           gid;    /* group id */
             mode_t          mode;   /* r/w permission */
             unsigned short  seq;    /* sequence # (to generate unique ipcid) */
             key_t           key;    /* user specified msg/sem/shm key */
     };

     The operation to be performed by msgctl() is specified in cmd and is one
     of:

     IPC_STAT   Gather information about the message queue and place it in the
                structure pointed to by buf.

     IPC_SET    Set the value of the msg_perm.uid, msg_perm.gid, msg_perm.mode
                and msg_qbytes fields in the structure associated with msqid.
                The values are taken from the corresponding fields in the
                structure pointed to by buf.  This operation can only be
                executed by the super-user, or a process that has an effective
                user id equal to either msg_perm.cuid or msg_perm.uid in the
                data structure associated with the message queue.  The value
                of msg_qbytes can only be increased by the super-user.  Values
                for msg_qbytes that exceed the system limit (MSGMNB from are
                silently truncated to that limit.

     IPC_RMID   Remove the message queue specified by msqid and destroy the
                data associated with it.  Only the super-user or a process
                with an effective uid equal to the msg_perm.cuid or
                msg_perm.uid values in the data structure associated with the
                queue can do this.

     The permission to read from or write to a message queue (see msgsnd(2)
     and msgrcv(2)) is determined by the msg_perm.mode field in the same way
     as is done with files (see chmod(2)), but the effective uid can match
     either the msg_perm.cuid field or the msg_perm.uid field, and the
     effective gid can match either msg_perm.cgid or msg_perm.gid.

RETURN VALUES

     The msgctl() function returns the value 0 if successful; otherwise the
     value -1 is returned and the global variable errno is set to indicate the
     error.

ERRORS

     The msgctl() function will fail if:

     [EPERM]            The cmd argument is equal to IPC_SET or IPC_RMID and
                        the caller is not the super-user, nor does the
                        effective uid match either the msg_perm.uid or
                        msg_perm.cuid fields of the data structure associated
                        with the message queue.

                        An attempt is made to increase the value of msg_qbytes
                        through IPC_SET but the caller is not the super-user.

     [EACCES]           The command is IPC_STAT and the caller has no read
                        permission for this message queue.

     [EINVAL]           The msqid argument is not a valid message queue
                        identifier.

                        cmd is not a valid command.

     [EFAULT]           The buf argument specifies an invalid address.

SEE ALSO

     msgget(2), msgrcv(2), msgsnd(2)

HISTORY

     Message queues appeared in the first release of AT&T System V UNIX.