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PROLOG | NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | EXAMPLES | APPLICATION USAGE | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT |
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MQ_OPEN(3P) POSIX Programmer's Manual MQ_OPEN(3P)
This manual page is part of the POSIX Programmer's Manual. The
Linux implementation of this interface may differ (consult the
corresponding Linux manual page for details of Linux behavior), or
the interface may not be implemented on Linux.
mq_open — open a message queue (REALTIME)
#include <mqueue.h>
mqd_t mq_open(const char *name, int oflag, ...);
The mq_open() function shall establish the connection between a
process and a message queue with a message queue descriptor. It
shall create an open message queue description that refers to the
message queue, and a message queue descriptor that refers to that
open message queue description. The message queue descriptor is
used by other functions to refer to that message queue. The name
argument points to a string naming a message queue. It is
unspecified whether the name appears in the file system and is
visible to other functions that take pathnames as arguments. The
name argument conforms to the construction rules for a pathname,
except that the interpretation of <slash> characters other than
the leading <slash> character in name is implementation-defined,
and that the length limits for the name argument are
implementation-defined and need not be the same as the pathname
limits {PATH_MAX} and {NAME_MAX}. If name begins with the <slash>
character, then processes calling mq_open() with the same value of
name shall refer to the same message queue object, as long as that
name has not been removed. If name does not begin with the <slash>
character, the effect is implementation-defined. If the name
argument is not the name of an existing message queue and creation
is not requested, mq_open() shall fail and return an error.
A message queue descriptor may be implemented using a file
descriptor, in which case applications can open up to at least
{OPEN_MAX} file and message queues.
The oflag argument requests the desired receive and/or send access
to the message queue. The requested access permission to receive
messages or send messages shall be granted if the calling process
would be granted read or write access, respectively, to an
equivalently protected file.
The value of oflag is the bitwise-inclusive OR of values from the
following list. Applications shall specify exactly one of the
first three values (access modes) below in the value of oflag:
O_RDONLY Open the message queue for receiving messages. The
process can use the returned message queue descriptor
with mq_receive(), but not mq_send(). A message queue
may be open multiple times in the same or different
processes for receiving messages.
O_WRONLY Open the queue for sending messages. The process can
use the returned message queue descriptor with
mq_send() but not mq_receive(). A message queue may
be open multiple times in the same or different
processes for sending messages.
O_RDWR Open the queue for both receiving and sending
messages. The process can use any of the functions
allowed for O_RDONLY and O_WRONLY. A message queue may
be open multiple times in the same or different
processes for sending messages.
Any combination of the remaining flags may be specified in the
value of oflag:
O_CREAT Create a message queue. It requires two additional
arguments: mode, which shall be of type mode_t, and
attr, which shall be a pointer to an mq_attr
structure. If the pathname name has already been used
to create a message queue that still exists, then this
flag shall have no effect, except as noted under
O_EXCL. Otherwise, a message queue shall be created
without any messages in it. The user ID of the message
queue shall be set to the effective user ID of the
process. The group ID of the message queue shall be
set to the effective group ID of the process; however,
if the name argument is visible in the file system,
the group ID may be set to the group ID of the
containing directory. When bits in mode other than the
file permission bits are specified, the effect is
unspecified. If attr is NULL, the message queue shall
be created with implementation-defined default message
queue attributes. If attr is non-NULL and the calling
process has appropriate privileges on name, the
message queue mq_maxmsg and mq_msgsize attributes
shall be set to the values of the corresponding
members in the mq_attr structure referred to by attr.
The values of the mq_flags and mq_curmsgs members of
the mq_attr structure shall be ignored. If attr is
non-NULL, but the calling process does not have
appropriate privileges on name, the mq_open() function
shall fail and return an error without creating the
message queue.
O_EXCL If O_EXCL and O_CREAT are set, mq_open() shall fail if
the message queue name exists. The check for the
existence of the message queue and the creation of the
message queue if it does not exist shall be atomic
with respect to other threads executing mq_open()
naming the same name with O_EXCL and O_CREAT set. If
O_EXCL is set and O_CREAT is not set, the result is
undefined.
O_NONBLOCK Determines whether an mq_send() or mq_receive() waits
for resources or messages that are not currently
available, or fails with errno set to [EAGAIN]; see
mq_send(3p) and mq_receive(3p) for details.
The mq_open() function does not add or remove messages from the
queue.
Upon successful completion, the function shall return a message
queue descriptor; otherwise, the function shall return (mqd_t)-1
and set errno to indicate the error.
The mq_open() function shall fail if:
EACCES The message queue exists and the permissions specified by
oflag are denied, or the message queue does not exist and
permission to create the message queue is denied.
EEXIST O_CREAT and O_EXCL are set and the named message queue
already exists.
EINTR The mq_open() function was interrupted by a signal.
EINVAL The mq_open() function is not supported for the given name.
EINVAL O_CREAT was specified in oflag, the value of attr is not
NULL, and either mq_maxmsg or mq_msgsize was less than or
equal to zero.
EMFILE Too many message queue descriptors or file descriptors are
currently in use by this process.
ENFILE Too many message queues are currently open in the system.
ENOENT O_CREAT is not set and the named message queue does not
exist.
ENOSPC There is insufficient space for the creation of the new
message queue.
If any of the following conditions occur, the mq_open() function
may return (mqd_t)-1 and set errno to the corresponding value.
ENAMETOOLONG
The length of the name argument exceeds {_POSIX_PATH_MAX}
on systems that do not support the XSI option or exceeds
{_XOPEN_PATH_MAX} on XSI systems, or has a pathname
component that is longer than {_POSIX_NAME_MAX} on systems
that do not support the XSI option or longer than
{_XOPEN_NAME_MAX} on XSI systems.
The following sections are informative.
None.
None.
None.
A future version might require the mq_open() and mq_unlink()
functions to have semantics similar to normal file system
operations.
mq_close(3p), mq_getattr(3p), mq_receive(3p), mq_send(3p),
mq_setattr(3p), mq_unlink(3p), msgctl(3p), msgget(3p), msgrcv(3p),
msgsnd(3p)
The Base Definitions volume of POSIX.1‐2017, mqueue.h(0p)
Portions of this text are reprinted and reproduced in electronic
form from IEEE Std 1003.1-2017, Standard for Information
Technology -- Portable Operating System Interface (POSIX), The
Open Group Base Specifications Issue 7, 2018 Edition, Copyright
(C) 2018 by the Institute of Electrical and Electronics Engineers,
Inc and The Open Group. In the event of any discrepancy between
this version and the original IEEE and The Open Group Standard,
the original IEEE and The Open Group Standard is the referee
document. The original Standard can be obtained online at
http://www.opengroup.org/unix/online.html .
Any typographical or formatting errors that appear in this page
are most likely to have been introduced during the conversion of
the source files to man page format. To report such errors, see
https://www.kernel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group 2017 MQ_OPEN(3P)
Pages that refer to this page: mqueue.h(0p), mq_close(3p), mq_getattr(3p), mq_notify(3p), mq_receive(3p), mq_send(3p), mq_setattr(3p), mq_unlink(3p), msgctl(3p), msgget(3p), msgrcv(3p), msgsnd(3p), umask(3p)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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