MSGCTL (2) | System calls | Unix Manual Pages

IPC_STAT
	Copy info from the message queue data structure associated with msqid into the structure pointed to by buf. The caller must have read permission on the message queue.
IPC_SET
	Write the values of some members of the msqid_ds structure pointed to by buf to the message queue data structure, updating also its msg_ctime member. The following members of the structure can be updated: msg_perm.uid msg_perm.gid msg_perm.mode /* only lowest 9-bits / msg_qbytes The calling process must have appropriate (probably, root) privileges or its effective user-ID must be that of the creator (msg_perm.cuid) or owner (msg_perm.uid) of the message queue. Appropriate (probably, root) privileges are required to raise the msg_qbytes* value beyond the system parameter MSGMNB.
IPC_RMID
	Immediately remove the message queue and its associated data structure, awakening all waiting reader and writer processes (with an error return and errno set to EIDRM). The calling process must have appropriate (probably, root) privileges or its effective user-ID must be either that of the creator or owner of the message queue.

EACCES	The argument cmd is equal to IPC_STAT but the calling process does not have read permission on the message queue msqid.
EFAULT	The argument cmd has the value IPC_SET or IPC_STAT, but the address pointed to by buf isn’t accessible.
EIDRM	The message queue was removed.
EINVAL	Invalid value for cmd or msqid.
EPERM	The argument cmd has the value IPC_SET or IPC_RMID, but the calling process has insufficient privileges to execute the command.

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