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MSGCTL(2) System Calls Manual MSGCTL(2)

msgctl — message control operations

#include <sys/msg.h>

int
msgctl(int msqid, int cmd, struct msqid_ds *buf);

The () system call performs some control operations on the message queue specified by msqid.

Each message queue has a data structure associated with it, parts of which may be altered by () and parts of which determine the actions of msgctl(). The data structure is defined in <sys/msg.h> and contains (amongst others) the following members:

struct msqid_ds {
	struct ipc_perm msg_perm;	/* msg queue permission bits */
	msglen_t	msg_cbytes;	/* # of bytes in use on the queue */
	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 msqid_ds structure is defined in <sys/ipc.h> and looks like this:

struct ipc_perm {
	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 (9 bits, see chmod(2)) */
	u_short	seq;	/* sequence # (to generate unique id) */
	key_t	key;	/* user specified msg/sem/shm key */
};

The operation to be performed by () is specified in cmd and is one of:

Gather information about the message queue and place it in the structure pointed to by buf.
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 superuser, 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 superuser. Values for msg_qbytes that exceed the system limit (MSGMNB from <sys/msg.h>) are silently truncated to that limit.
Remove the message queue specified by msqid and destroy the data associated with it. Only the superuser 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 effective GID can match either msg_perm.cgid or msg_perm.gid.

Upon successful completion, the value 0 is returned; otherwise the value -1 is returned and the global variable errno is set to indicate the error.

msgctl() will fail if:

[]
cmd is equal to IPC_SET or IPC_RMID and the caller is not the superuser, 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 is made to increase the value of msg_qbytes through IPC_SET but the caller is not the superuser.

[]
The command is IPC_STAT and the caller has no read permission for this message queue.
[]
msqid is not a valid message queue identifier.

cmd is not a valid command.

[]
buf specifies an invalid address.

msgget(2), msgrcv(2), msgsnd(2)

The msgctl() function conforms to the X/Open System Interfaces option of IEEE Std 1003.1-2008 (“POSIX.1”).

Message queues first appeared in AT&T System V Release 1 UNIX and have been available since NetBSD 1.0.

April 30, 2024 OpenBSD-current