msgctl(2) [netbsd man page]

MSGCTL(2)						      BSD System Calls Manual							 MSGCTL(2)

NAME

     msgctl -- message control operations

LIBRARY

     Standard C Library (libc, -lc)

SYNOPSIS

     #include <sys/msg.h>

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

DESCRIPTION

     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 of:

     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 associ-
		ated 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
     effective gid can match either msg_perm.cgid or msg_perm.gid.

RETURN VALUES

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

ERRORS

     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 super-user.

     [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.

SEE ALSO

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

STANDARDS

     The msgctl system call conforms to X/Open System Interfaces and Headers Issue 5 (``XSH5'').

HISTORY

     Message queues appeared in the first release of AT&T System V UNIX.

BSD
								  August 25, 1999							       BSD

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msgctl(2) System Calls Manual msgctl(2)

Name
msgctl - message control operations

Syntax
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/msg.h>

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

Description
The system call provides 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

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 /* only low 9 bits */
msg_qbytes

This cmd can be executed only by a process that has an effective user ID that is equal to superuser or the value of either msg_perm.uid or
msg_perm.cuid in the data structure associated with msqid. Only the superuser 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 command can only be executed by a process that has an effective user ID equal to either that of the
superuser or to the value of msg_perm.uid in the data structure associated with msquid.

Return Values
Upon successful completion, a value of 0 is returned. Otherwise, a value of -1 is returned and errno is set to indicate the error.

Diagnostics
The system call fails under the following conditions:

[EINVAL] The msqid is not a valid message queue identifier.

[EINVAL] The cmd is not a valid command.

[EACCES] The cmd is equal to IPC_STAT and read operation permission is denied to the calling process. For further information, see

[EPERM] The cmd is equal to IPC_RMID or IPC_SET and the effective user ID of the calling process is not equal to that of the supe-
ruser or to the value of msg_perm.uid in the data structure associated with msqid.

[EPERM] The cmd is equal to IPC_SET, an attempt is being made to increase to the value of msg_qbytes, and the effective user ID of
the calling process is not equal to that of superuser.

[EFAULT] The buf points to an illegal address.

Linux and UNIX Man Pages

msgctl(2) [netbsd man page]

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