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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 struct msg *msg_first; /* first message in the queue */
67 struct msg *msg_last; /* last message in the queue */
68 msglen_t msg_cbytes; /* number of bytes in use on the queue */
69 msgqnum_t msg_qnum; /* number of msgs in the queue */
70 msglen_t msg_qbytes; /* max # of bytes on the queue */
71 pid_t msg_lspid; /* pid of last msgsnd() */
72 pid_t msg_lrpid; /* pid of last msgrcv() */
73 time_t msg_stime; /* time of last msgsnd() */
74 time_t msg_rtime; /* time of last msgrcv() */
75 time_t msg_ctime; /* time of last msgctl() */
81 structure used inside the
83 structure is defined in
88 uid_t cuid; /* creator user id */
89 gid_t cgid; /* creator group id */
90 uid_t uid; /* user id */
91 gid_t gid; /* group id */
92 mode_t mode; /* r/w permission */
93 unsigned short seq; /* sequence # (to generate unique ipcid) */
94 key_t key; /* user specified msg/sem/shm key */
98 The operation to be performed by
103 .Bl -tag -width IPC_RMIDX
105 Gather information about the message queue and place it in the
106 structure pointed to by
115 fields in the structure associated with
117 The values are taken from the corresponding fields in the structure
120 This operation can only be executed by the super-user, or a process that
121 has an effective user id equal to either
125 in the data structure associated with the message queue.
128 can only be increased by the super-user.
131 that exceed the system limit (MSGMNB from
133 are silently truncated to that limit.
135 Remove the message queue specified by
137 and destroy the data associated with it.
138 Only the super-user or a process
139 with an effective uid equal to the
143 values in the data structure associated with the queue can do this.
146 The permission to read from or write to a message queue (see
152 field in the same way as is
155 but the effective uid can match either the
160 effective gid can match either
176 is equal to IPC_SET or IPC_RMID and the caller is not the super-user, nor does
177 the effective uid match either the
181 fields of the data structure associated with the message queue.
183 An attempt is made to increase the value of
186 but the caller is not the super-user.
188 The command is IPC_STAT
189 and the caller has no read permission for this message queue.
194 is not a valid message queue identifier.
197 is not a valid command.
202 specifies an invalid address.
209 Message queues appeared in the first release of