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MSGCTL(2) NetBSD System Calls Manual MSGCTL(2)
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msgctl -- message control operations
Standard C Library (libc, -lc)
msgctl(int msqid, int cmd, struct msqid_ds *buf);
The msgctl() system call performs control operations on the message queue
specified by msqid.
Each message queue has a msqid_ds structure associated with it which con-
tains the following members:
struct ipc_perm msg_perm; /* msg queue permission bits */
msgqnum_t msg_qnum; /* # 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 msgid_ds structure is defined in
<sys/ipc.h> and contains the following members:
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 (lower 9 bits) */
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 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.
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 was made to increase the value of
msg_qbytes through IPC_SET, but the caller is not the
[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.
msgget(2), msgrcv(2), msgsnd(2)
The msgctl system call conforms to X/Open System Interfaces and Headers
Issue 5 (``XSH5'').
Message queues appeared in the first release of AT&T System V UNIX.
NetBSD 8.1 August 25, 1999 NetBSD 8.1