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.TH MQ_OPEN "3P" 2013 "IEEE/The Open Group" "POSIX Programmer's Manual"
.SH PROLOG
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.

.SH NAME
mq_open
\(em open a message queue
(\fBREALTIME\fP)
.SH SYNOPSIS
.LP
.nf
#include <mqueue.h>
.P
mqd_t mq_open(const char *\fIname\fP, int \fIoflag\fP, ...);
.fi
.SH DESCRIPTION
The
\fImq_open\fR()
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
.IR 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
.IR name
argument conforms to the construction rules for a pathname, except that
the interpretation of
<slash>
characters other than the leading
<slash>
character in
.IR name
is implementation-defined, and that the length limits for the
.IR name
argument are implementation-defined and need not be the same as the
pathname limits
{PATH_MAX}
and
{NAME_MAX}.
If
.IR name
begins with the
<slash>
character, then processes calling
\fImq_open\fR()
with the same value of
.IR name
shall refer to the same message queue object, as long as that name
has not been removed. If
.IR name
does not begin with the
<slash>
character, the effect is implementation-defined. If the
.IR name
argument is not the name of an existing message queue and creation is
not requested,
\fImq_open\fR()
shall fail and return an error.
.P
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.
.P
The
.IR 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.
.P
The value of
.IR 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
.IR oflag :
.IP O_RDONLY 12
Open the message queue for receiving messages. The process can use the
returned message queue descriptor with
\fImq_receive\fR(),
but not
\fImq_send\fR().
A message queue may be open multiple times in the same or different
processes for receiving messages.
.IP O_WRONLY 12
Open the queue for sending messages. The process can use the returned
message queue descriptor with
\fImq_send\fR()
but not
\fImq_receive\fR().
A message queue may be open multiple times in the same or different
processes for sending messages.
.IP O_RDWR 12
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.
.P
Any combination of the remaining flags may be specified in the value of
.IR oflag :
.IP O_CREAT 12
Create a message queue. It requires two additional arguments:
.IR mode ,
which shall be of type
.BR mode_t ,
and
.IR attr ,
which shall be a pointer to an
.BR mq_attr
structure. If the pathname
.IR 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
.IR 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
.IR mode
other than the file permission bits are specified, the effect is
unspecified. If
.IR attr
is NULL, the message queue shall be created with implementation-defined
default message queue attributes. If
.IR attr
is non-NULL and the calling process has appropriate privileges on
.IR name ,
the message queue
.IR mq_maxmsg
and
.IR mq_msgsize
attributes shall be set to the values of the corresponding members in the
.BR mq_attr
structure referred to by
.IR attr .
The values of the
.IR mq_flags
and
.IR mq_curmsgs
members of the
.BR mq_attr
structure shall be ignored. If
.IR attr
is non-NULL, but the calling process does not have appropriate
privileges on
.IR name ,
the
\fImq_open\fR()
function shall fail and return an error without creating the message
queue.
.IP O_EXCL 12
If O_EXCL and O_CREAT are set,
\fImq_open\fR()
shall fail if the message queue
.IR 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
\fImq_open\fR()
naming the same
.IR name
with O_EXCL and O_CREAT set. If O_EXCL is set and O_CREAT is not set,
the result is undefined.
.IP O_NONBLOCK 12
Determines whether an
\fImq_send\fR()
or
\fImq_receive\fR()
waits for resources or messages that are not currently available, or
fails with
.IR errno
set to
.BR [EAGAIN] ;
see
.IR "\fImq_send\fR\^(\|)"
and
.IR "\fImq_receive\fR\^(\|)"
for details.
.P
The
\fImq_open\fR()
function does not add or remove messages from the queue.
.SH "RETURN VALUE"
Upon successful completion, the function shall return a message queue
descriptor; otherwise, the function shall return (\fBmqd_t\fP)\(mi1
and set
.IR errno
to indicate the error.
.SH ERRORS
The
\fImq_open\fR()
function shall fail if:
.TP
.BR EACCES
The message queue exists and the permissions specified by
.IR oflag
are denied, or the message queue does not exist and permission to
create the message queue is denied.
.TP
.BR EEXIST
O_CREAT and O_EXCL are set and the named message queue already exists.
.TP
.BR EINTR
The
\fImq_open\fR()
function was interrupted by a signal.
.TP
.BR EINVAL
The
\fImq_open\fR()
function is not supported for the given name.
.TP
.BR EINVAL
O_CREAT was specified in
.IR oflag ,
the value of
.IR attr
is not NULL, and either
.IR mq_maxmsg
or
.IR mq_msgsize
was less than or equal to zero.
.TP
.BR EMFILE
Too many message queue descriptors or file descriptors are currently in
use by this process.
.TP
.BR ENFILE
Too many message queues are currently open in the system.
.TP
.BR ENOENT
O_CREAT is not set and the named message queue does not exist.
.TP
.BR ENOSPC
There is insufficient space for the creation of the new message queue.
.br
.P
If any of the following conditions occur, the
\fImq_open\fR()
function may return (\c
.BR mqd_t )\(mi1
and set
.IR errno
to the corresponding value.
.TP
.BR ENAMETOOLONG
.br
The length of the
.IR 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.
.LP
.IR "The following sections are informative."
.SH EXAMPLES
None.
.SH "APPLICATION USAGE"
None.
.SH RATIONALE
None.
.SH "FUTURE DIRECTIONS"
A future version might require the
\fImq_open\fR()
and
\fImq_unlink\fR()
functions to have semantics similar to normal file system operations.
.SH "SEE ALSO"
.IR "\fImq_close\fR\^(\|)",
.IR "\fImq_getattr\fR\^(\|)",
.IR "\fImq_receive\fR\^(\|)",
.IR "\fImq_send\fR\^(\|)",
.IR "\fImq_setattr\fR\^(\|)",
.IR "\fImq_unlink\fR\^(\|)",
.IR "\fImsgctl\fR\^(\|)",
.IR "\fImsgget\fR\^(\|)",
.IR "\fImsgrcv\fR\^(\|)",
.IR "\fImsgsnd\fR\^(\|)"
.P
The Base Definitions volume of POSIX.1\(hy2008,
.IR "\fB<mqueue.h>\fP"
.SH COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, Copyright (C) 2013 by the Institute of
Electrical and Electronics Engineers, Inc and The Open Group.
(This is POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) 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.unix.org/online.html .

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