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 contains
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 executed
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 effective
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.
BSD August 25, 1999 BSD
[ Back ]