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MSGCTL(2)                 NetBSD Programmer's Manual                 MSGCTL(2)


NAME

     msgctl - message control operations


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 <sys/msg.h> and contains
     (amongst others) the following members:

     struct msqid_ds {
         struct ipc_perm msg_perm;   /* msg queue permission bits */
         u_long msg_cbytes; /* # of bytes in use on the queue */
         u_long msg_qnum;   /* # of msgs in the queue */
         u_long 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 msgid_ds  structure is defined in
     <sys/ipc.h> and looks like this:

     struct ipc_perm {
         ushort cuid; /* creator user id */
         ushort cgid; /* creator group id */
         ushort uid;  /* user id */
         ushort gid;  /* group id */
         ushort mode; /* permission (9 bits, see chmod(2)) */
         ushort seq;  /* sequence # (to generate unique id) */
         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 exe-
                cuted 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
                <sys/msg.h>) 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 effec-
     tive gid can match either msg_perm.cgid or msg_perm.gid.


RETURN VALUES

     Upon successful completion, a value of 0 is returned. Otherwise, -1 is
     returned and the global variable errno is set to indicate the error.


ERRORS

     msgctl() will fail if:

     [EPERM]       cmd 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 was made to increase the value of msg_qbytes
                   through IPC_SET, but the caller is not the super-user.

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

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

                   cmd is not a valid command.

     [EFAULT]      buf 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.

NetBSD                          August 17, 1995                              2