mq_open(3) — Linux manual pages

Linux manual pages	Section 3

Name

mq_open — open a message queue

Synopsis

#include <fcntl.h>            /* For O_* constants */
#include <sys/stat.h>         /* For mode constants */
#include <mqueue.h>

`mqd_t mq_open(`	const char *`name`,
	int `oflag)`;

`mqd_t mq_open(`	const char *`name`,
	int `oflag`,
	mode_t `mode`,
	struct mq_attr *`attr)`;

	Note
	Link with `−lrt`.

DESCRIPTION

mq_open() creates a new POSIX message queue or opens an existing queue. The queue is identified by name. For details of the construction of name, see mq_overview(7).

The oflag argument specifies flags that control the operation of the call. (Definitions of the flags values can be obtained by including <fcntl.h> Exactly one of the following must be specified in oflag:

O_RDONLY: Open the queue to receive messages only.
O_WRONLY: Open the queue to send messages only.
O_RDWR: Open the queue to both send and receive messages.

Zero or more of the following flags can additionally be ORed in oflag:

O_CLOEXEC (since Linux 2.6.26): Set the close-on-exec flag for the message queue descriptor. See open(2) for a discussion of why this flag is useful.
O_CREAT: Create the message queue if it does not exist. The owner (user ID) of the message queue is set to the effective user ID of the calling process. The group ownership (group ID) is set to the effective group ID of the calling process.
O_EXCL: If O_CREAT was specified in oflag, and a queue with the given name already exists, then fail with the error EEXIST.
O_NONBLOCK: Open the queue in nonblocking mode. In circumstances where mq_receive(3) and mq_send(3) would normally block, these functions instead fail with the error EAGAIN.

If O_CREAT is specified in oflag, then two additional arguments must be supplied. The mode argument specifies the permissions to be placed on the new queue, as for open(2). (Symbolic definitions for the permissions bits can be obtained by including <sys/stat.h> The permissions settings are masked against the process umask.

The attr argument specifies attributes for the queue. See mq_getattr(3) for details. If attr is NULL, then the queue is created with implementation-defined default attributes. Since Linux 3.5, two /proc files can be used to control these defaults; see mq_overview(7) for details.

RETURN VALUE

On success, mq_open() returns a message queue descriptor for use by other message queue functions. On error, mq_open() returns (mqd_t) −1, with errno set to indicate the error.

ERRORS

EACCES: The queue exists, but the caller does not have permission to open it in the specified mode.
EACCES: name contained more than one slash.
EEXIST: Both O_CREAT and O_EXCL were specified in oflag, but a queue with this name already exists.
EINVAL: name doesn't follow the format in mq_overview(7).
EINVAL: O_CREAT was specified in oflag, and attr was not NULL, but attr−>mq_maxmsg or attr−>mq_msqsize was invalid. Both of these fields must be greater than zero. In a process that is unprivileged (does not have the CAP_SYS_RESOURCE capability), attr−>mq_maxmsg must be less than or equal to the msg_max limit, and attr−>mq_msgsize must be less than or equal to the msgsize_max limit. In addition, even in a privileged process, attr−>mq_maxmsg cannot exceed the HARD_MAX limit. (See mq_overview(7) for details of these limits.)
EMFILE: The per-process limit on the number of open file and message queue descriptors has been reached (see the description of RLIMIT_NOFILE in getrlimit(2)).
ENAMETOOLONG: name was too long.
ENFILE: The system-wide limit on the total number of open files and message queues has been reached.
ENOENT: The O_CREAT flag was not specified in oflag, and no queue with this name exists.
ENOENT: name was just "/" followed by no other characters.
ENOMEM: Insufficient memory.
ENOSPC: Insufficient space for the creation of a new message queue. This probably occurred because the queues_max limit was encountered; see mq_overview(7).

ATTRIBUTES

For an explanation of the terms used in this section, see attributes(7).

Interface	Attribute	Value
`mq_open`()	Thread safety	MT-Safe

CONFORMING TO

POSIX.1-2001, POSIX.1-2008.

NOTES

C library/kernel differences

The mq_open() library function is implemented on top of a system call of the same name. The library function performs the check that the name starts with a slash (/), giving the EINVAL error if it does not. The kernel system call expects name to contain no preceding slash, so the C library function passes name without the preceding slash (i.e., name+1) to the system call.

BUGS

In kernels before 2.6.14, the process umask was not applied to the permissions specified in mode.

COLOPHON

This page is part of release 4.07 of the Linux man-pages project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at https://www.kernel.org/doc/man−pages/.

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

%%%LICENSE_START(VERBATIM)
Permission is granted to make and distribute verbatim copies of this
manual provided the copyright notice and this permission notice are
preserved on all copies.

Permission is granted to copy and distribute modified versions of this
manual under the conditions for verbatim copying, provided that the
entire resulting derived work is distributed under the terms of a
permission notice identical to this one.

Since the Linux kernel and libraries are constantly changing, this
manual page may be incorrect or out-of-date.  The author(s) assume no
responsibility for errors or omissions, or for damages resulting from
the use of the information contained herein.  The author(s) may not
have taken the same level of care in the production of this manual,
which is licensed free of charge, as they might when working
professionally.

Formatted or processed versions of this manual, if unaccompanied by
the source, must acknowledge the copyright and authors of this work.
%%%LICENSE_END