The msgctl() function provides a variety of message control operations as specified by cmd. The following cmds are available:
IPC_STAT
Place the current value of each member of the data structure associated with msqid into the structure pointed to by buf. The contents of this structure are
defined in Intro(2).
IPC_SET
Set the value of the following members of the data structure associated with msqid to the corresponding value found in the structure pointed to by buf:
msg_perm.uid
msg_perm.gid
msg_perm.mode /* access permission bits only */
msg_qbytes
This command can be executed only by a process that has either the {PRIV_IPC_OWNER} privilege or an effective user ID equal to the value of msg_perm.cuid or msg_perm.uid in the data structure associated with msqid. Only a process with the {PRIV_SYS_IPC_CONFIG} privilege can raise the value of msg_qbytes.
IPC_RMID
Remove the message queue identifier specified by msqid from the system and destroy the message queue and data structure associated with it. This cmd can only be executed
by a process that has an effective user ID either with appropriate privileges asserted in the effective set or equal to the value of msg_perm.cuid or msg_perm.uid in the data structure associated with msqid.
The buf argument is ignored.
RETURN VALUES
Upon successful completion, msgctl() returns 0. Otherwise, it returns -1 and sets errno to indicate the error.
ERRORS
The msgctl() function will fail if:
EACCES
The cmd argument is IPC_STAT and operation permission is denied to the calling process (see Intro(2)).
EFAULT
The buf argument points to an illegal address.
EINVAL
The msqid argument is not a valid message queue identifier; or the cmd argument is not a valid command or is IPC_SET and msg_perm.uid or msg_perm.gid is not valid.
EOVERFLOW
The cmd argument is IPC_STAT and uid or gid is too large to be stored in the structure pointed to by buf.
EPERM
The cmd argument is IPC_RMID or IPC_SET, the {PRIV_SYS_IPC_OWNER} privilege is not asserted in the effective set of the calling
process, and is not equal to the value of msg_perm.cuid or msg_perm.uid in the data structure associated with msqid.
The cmd argument is IPC_SET, an attempt is being made to increase to the value of msg_qbytes, and the {PRIV_SYS_IPC_CONFIG} privilege is not asserted in the effective set of the calling process.
ATTRIBUTES
See attributes(5) for descriptions of the following attributes:
