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.TH MQ_OPEN 3 2009-02-20 "Linux" "Linux Programmer's Manual"
.SH NAME
mq_open \- open a message queue
.SH SYNOPSIS
.nf
.BR "#include <fcntl.h>" "           /* For O_* constants */"
.BR "#include <sys/stat.h>" "        /* For mode constants */"
.B #include <mqueue.h>
.sp
.BI "mqd_t mq_open(const char *" name ", int " oflag );
.BI "mqd_t mq_open(const char *" name ", int " oflag ", mode_t " mode ,
.BI "              struct mq_attr *" attr );
.fi
.sp
Link with \fI\-lrt\fP.
.SH DESCRIPTION
.BR mq_open ()
creates a new POSIX message queue or opens an existing queue.
The queue is identified by
.IR name .
For details of the construction of
.IR name ,
see
.BR mq_overview (7).

The
.I oflag
argument specifies flags that control the operation of the call.
(Definitions of the flags values can be obtained by including
.IR <fcntl.h> .)
Exactly one of the following must be specified in
.IR oflag :
.TP
.B O_RDONLY
Open the queue to receive messages only.
.TP
.B O_WRONLY
Open the queue to send messages only.
.TP
.B O_RDWR
Open the queue to both send and receive messages.
.PP
Zero or more of the following flags can additionally be
.IR OR ed
in
.IR oflag :
.TP
.B O_NONBLOCK
Open the queue in nonblocking mode.
In circumstances where
.BR mq_receive (3)
and
.BR mq_send (3)
would normally block, these functions instead fail with the error
.BR EAGAIN .
.TP
.B 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.
.\" In reality the file system IDs are used on Linux.
.TP
.B O_EXCL
If
.B O_CREAT
was specified in
.IR oflag ,
and a queue with the given
.I name
already exists, then fail with the error
.BR EEXIST .
.PP
If
.B O_CREAT
is specified in
.IR oflag ,
then two additional arguments must be supplied.
The
.I mode
argument specifies the permissions to be placed on the new queue,
as for
.BR open (2).
(Symbolic definitions for the permissions bits can be obtained by including
.IR <sys/stat.h> .)
The permissions settings are masked against the process umask.
The
.I attr
argument specifies attributes for the queue.
See
.BR mq_getattr (3)
for details.
If
.I attr
is NULL, then the queue is created with implementation-defined
default attributes.
.SH RETURN VALUE
On success,
.BR mq_open ()
returns a message queue descriptor for use by other
message queue functions.
On error,
.BR mq_open ()
returns
.IR "(mqd_t)\ \-1",
with
.I errno
set to indicate the error.
.SH ERRORS
.TP
.B EACCES
The queue exists, but the caller does not have permission to
open it in the specified mode.
.TP
.B EACCES
.I name
contained more than one slash.
.\" Note that this isn't consistent with the same case for sem_open()
.TP
.B EEXIST
Both
.B O_CREAT
and
.B O_EXCL
were specified in
.IR oflag ,
but a queue with this
.I name
already exists.
.TP
.B EINVAL
.B O_CREAT
was specified in
.IR oflag ,
and
.I attr
was not NULL, but
.I attr\->mq_maxmsg
or
.I attr\->mq_msqsize
was invalid.
Both of these fields must be greater than zero.
In a process that is unprivileged (does not have the
.B CAP_SYS_RESOURCE
capability),
.I attr\->mq_maxmsg
must be less than or equal to the
.I msg_max
limit, and
.I attr\->mq_msgsize
must be less than or equal to the
.I msgsize_max
limit.
In addition, even in a privileged process,
.I attr\->mq_maxmsg
cannot exceed the
.B HARD_MAX
limit.
(See
.BR mq_overview (7)
for details of these limits.)
.TP
.B EMFILE
The process already has the maximum number of files and
message queues open.
.TP
.B ENAMETOOLONG
.I name
was too long.
.TP
.B ENFILE
The system limit on the total number of open files and message queues
has been reached.
.TP
.B ENOENT
The
.B O_CREAT
flag was not specified in
.IR oflag ,
and no queue with this
.I name
exists.
.TP
.B ENOENT
.I name
was just "/" followed by no other characters.
.\" Note that this isn't consistent with the same case for sem_open()
.TP
.B ENOMEM
Insufficient memory.
.TP
.B ENOSPC
Insufficient space for the creation of a new message queue.
This probably occurred because the
.I queues_max
limit was encountered; see
.BR mq_overview (7).
.SH CONFORMING TO
POSIX.1-2001.
.SH BUGS
In kernels before 2.6.14,
the process umask was not applied to the permissions specified in
.IR mode .
.SH "SEE ALSO"
.BR mq_close (3),
.BR mq_getattr (3),
.BR mq_notify (3),
.BR mq_receive (3),
.BR mq_send (3),
.BR mq_unlink (3),
.BR mq_overview (7)
.SH COLOPHON
This page is part of release 3.44 of the Linux
.I man-pages
project.
A description of the project,
and information about reporting bugs,
can be found at
http://www.kernel.org/doc/man-pages/.