mq_getattr(3) — Linux manual pages

Linux manual pages	Section 3

Name

mq_getattr, mq_setattr — get/set message queue attributes

Synopsis

#include <mqueue.h>

`int mq_getattr(`	mqd_t `mqdes`,
	struct mq_attr *`attr)`;

`int mq_setattr(`	mqd_t `mqdes`,
	struct mq_attr *`newattr`,
	struct mq_attr *`oldattr)`;

	Note
	Link with `−lrt`.

DESCRIPTION

mq_getattr() and mq_setattr() respectively retrieve and modify attributes of the message queue referred to by the descriptor mqdes.

mq_getattr() returns an mq_attr structure in the buffer pointed by attr. This structure is defined as:

struct mq_attr {

long mq_flags;
/* Flags: 0 or O_NONBLOCK */

long mq_maxmsg;
/* Max. # of messages on queue */

long mq_msgsize;
/* Max. message size (bytes) */

long mq_curmsgs;
/* # of messages currently in queue */

};

The mq_flags field contains flags associated with the open message queue description. This field is initialized when the queue is created by mq_open(3). The only flag that can appear in this field is O_NONBLOCK.

The mq_maxmsg and mq_msgsize fields are set when the message queue is created by mq_open(3). The mq_maxmsg field is an upper limit on the number of messages that may be placed on the queue using mq_send(3). The mq_msgsize field is an upper limit on the size of messages that may be placed on the queue. Both of these fields must have a value greater than zero. Two /proc files that place ceilings on the values for these fields are described in mq_open(3).

The mq_curmsgs field returns the number of messages currently held in the queue.

mq_setattr() sets message queue attributes using information supplied in the mq_attr structure pointed to by newattr. The only attribute that can be modified is the setting of the O_NONBLOCK flag in mq_flags. The other fields in newattr are ignored. If the oldattr field is not NULL, then the buffer that it points to is used to return an mq_attr structure that contains the same information that is returned by mq_getattr().

RETURN VALUE

On success mq_getattr() and mq_setattr() return 0; on error, −1 is returned, with errno set to indicate the error.

ERRORS

EBADF: The descriptor specified in mqdes is invalid.
EINVAL: newattr−>mq_flags contained set bits other than O_NONBLOCK.

CONFORMING TO

POSIX.1-2001.

NOTES

On Linux, mq_getattr() and mq_setattr() are library functions layered on top of the mq_getsetattr(2) system call.

COLOPHON

This page is part of release 3.33 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be found at http://man7.org/linux/man-pages/.

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Copyright (C) 2006 Michael Kerrisk <mtk.manpagesgmail.com>

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struct	mq_attr {
	long	`mq_flags`;	/* Flags: 0 or O_NONBLOCK */
	long	`mq_maxmsg`;	/* Max. # of messages on queue */
	long	`mq_msgsize`;	/* Max. message size (bytes) */
	long	`mq_curmsgs`;	/* # of messages currently in queue */
};