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RocketLink!--> Man page versions: OpenBSD FreeBSD NetBSD RedHat Others

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

     msgctl - message control operations

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

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

     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 shmid_ds  structure is defined in
     <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;   /* permission (9 bits, see chmod(2)) */
             u_short 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

     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

     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.

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

     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 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 permis-
                   sion 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.

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

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

OpenBSD 2.6                     August 17, 1995                              2

Source: OpenBSD 2.6 man pages. Copyright: Portions are copyrighted by BERKELEY
SOFTWARE DESIGN, INC., The Regents of the University of California, Massachusetts
Institute of Technology, Free Software Foundation, FreeBSD Inc., and others.

(Corrections, notes, and links courtesy of RocketAware.com)

[Detailed Topics]

[Overview Topics]

Up to: Local Process Communication - Communication between processes running on the same system. Synchronization. File locking. Signals. FIFOs, pipes, et al

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