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39 .Nd message control operations
47 .Fn msgctl "int msqid" "int cmd" "struct msqid_ds *buf"
51 system call performs some control operations on the message queue specified
55 Each message queue has a data structure associated with it, parts of which
58 and parts of which determine the actions of
60 The data structure is defined in
62 and contains (amongst others) the following members:
65 struct ipc_perm msg_perm; /* msg queue permission bits */
66 msglen_t msg_cbytes; /* number of bytes in use on the queue */
67 msgqnum_t msg_qnum; /* number of msgs in the queue */
68 msglen_t msg_qbytes; /* max # of bytes on the queue */
69 pid_t msg_lspid; /* pid of last msgsnd() */
70 pid_t msg_lrpid; /* pid of last msgrcv() */
71 time_t msg_stime; /* time of last msgsnd() */
72 time_t msg_rtime; /* time of last msgrcv() */
73 time_t msg_ctime; /* time of last msgctl() */
79 structure used inside the
81 structure is defined in
86 uid_t cuid; /* creator user id */
87 gid_t cgid; /* creator group id */
88 uid_t uid; /* user id */
89 gid_t gid; /* group id */
90 mode_t mode; /* r/w permission */
91 unsigned short seq; /* sequence # (to generate unique ipcid) */
92 key_t key; /* user specified msg/sem/shm key */
96 The operation to be performed by
101 .Bl -tag -width IPC_RMIDX
103 Gather information about the message queue and place it in the
104 structure pointed to by
113 fields in the structure associated with
115 The values are taken from the corresponding fields in the structure
118 This operation can only be executed by the super-user, or a process that
119 has an effective user id equal to either
123 in the data structure associated with the message queue.
126 can only be increased by the super-user.
129 that exceed the system limit (MSGMNB from
131 are silently truncated to that limit.
133 Remove the message queue specified by
135 and destroy the data associated with it.
136 Only the super-user or a process
137 with an effective uid equal to the
141 values in the data structure associated with the queue can do this.
144 The permission to read from or write to a message queue (see
150 field in the same way as is
153 but the effective uid can match either the
158 effective gid can match either
174 is equal to IPC_SET or IPC_RMID and the caller is not the super-user, nor does
175 the effective uid match either the
179 fields of the data structure associated with the message queue.
181 An attempt is made to increase the value of
184 but the caller is not the super-user.
186 The command is IPC_STAT
187 and the caller has no read permission for this message queue.
192 is not a valid message queue identifier.
195 is not a valid command.
200 specifies an invalid address.
207 Message queues appeared in the first release of