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.\" Portions of this text are reprinted and reproduced in electronic form
.\" from IEEE Std 1003.1, 2004 Edition, Standard for Information Technology --
.\" Portable Operating System Interface (POSIX), The Open Group Base
.\" Specifications Issue 6, Copyright (C) 2001-2004 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.
.\"
.Dd September 28, 2019
.Dt MQ_OPEN 2
.Os
.Sh NAME
.Nm mq_open
.Nd "open a message queue (REALTIME)"
.Sh LIBRARY
.Lb librt
.Sh SYNOPSIS
.In mqueue.h
.Ft mqd_t
.Fn mq_open "const char *name" "int oflag" "..."
.Sh DESCRIPTION
The
.Fn mq_open
system call establishes the connection between a process and a message queue
with a message queue descriptor.
It creates 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
.Fa name
argument points to a string naming a message queue.
The
.Fa name
argument should conform to the construction rules for a pathname.
The
.Fa name
should begin with a slash character.
Processes calling
.Fn mq_open
with the same value of
.Fa name
refers to the same message queue object, as long as that name has not been
removed.
If the
.Fa name
argument is not the name of an existing message queue and creation is not
requested,
.Fn mq_open
will fail and return an error.
.Pp
The
.Fa oflag
argument requests the desired receive and/or send access to the message
queue.
The requested access permission to receive messages or send messages
would be granted if the calling process would be granted read or write access,
respectively, to an equivalently protected file.
.Pp
The value of
.Fa oflag
is the bitwise-inclusive OR of values from the following list.
Applications should specify exactly one of the first three values (access
modes) below in the value of
.Fa oflag :
.Bl -tag -width ".Dv O_NONBLOCK"
.It Dv O_RDONLY
Open the message queue for receiving messages.
The process can use the
returned message queue descriptor with
.Fn mq_receive ,
but not
.Fn mq_send .
A message queue may be open multiple times in the same or different processes
for receiving messages.
.It Dv O_WRONLY
Open the queue for sending messages.
The process can use the returned
message queue descriptor with
.Fn mq_send
but not
.Fn mq_receive .
A message queue may be open multiple times in the same or different processes
for sending messages.
.It Dv O_RDWR
Open the queue for both receiving and sending messages.
The process can use
any of the functions allowed for
.Dv O_RDONLY
and
.Dv O_WRONLY .
A message queue may be open multiple times in the same or different processes
for sending messages.
.El
.Pp
Any combination of the remaining flags may be specified in the value of
.Fa oflag :
.Bl -tag -width ".Dv O_NONBLOCK"
.It Dv O_CREAT
Create a message queue.
It requires two additional arguments:
.Fa mode ,
which is of type
.Vt mode_t ,
and
.Fa attr ,
which is a pointer to an
.Vt mq_attr
structure.
If the pathname
.Fa name
has already been used to create a message queue that still exists, then
this flag has no effect, except as noted under
.Dv O_EXCL .
Otherwise, a message queue will be created without any messages
in it.
The user ID of the message queue will be set to the effective user ID
of the process, and the group ID of the message queue will be set to the
effective group ID of the process.
The permission bits of the message queue
will be set to the value of the
.Fa mode
argument, except those set in the file mode creation mask of the process.
When bits in
.Fa mode
other than the file permission bits are specified, the effect is
unspecified.
If
.Fa attr
is
.Dv NULL ,
the message queue is created with implementation-defined default message
queue attributes.
If attr is
.Pf non- Dv NULL
and the calling process has the
appropriate privilege on name, the message queue
.Va mq_maxmsg
and
.Va mq_msgsize
attributes will be set to the values of the corresponding members in the
.Vt mq_attr
structure referred to by
.Fa attr .
If
.Fa attr
is
.Pf non- Dv NULL ,
but the calling process does not have the appropriate privilege
on name, the
.Fn mq_open
function will fail and return an error without creating the message queue.
.It Dv O_EXCL
If
.Dv O_EXCL
and
.Dv O_CREAT
are set,
.Fn mq_open
will fail if the message queue name exists.
.It Dv O_NONBLOCK
Determines whether an
.Fn mq_send
or
.Fn mq_receive
waits for resources or messages that are not currently available, or fails
with
.Va errno
set to
.Er EAGAIN ;
see
.Xr mq_send 2
and
.Xr mq_receive 2
for details.
.El
.Pp
The
.Fn mq_open
system call does not add or remove messages from the queue.
.Sh NOTES
.Fx
implements message queue based on file descriptor.
The descriptor
is inherited by child after
.Xr fork 2 .
The descriptor is closed in a new image after
.Xr exec 3 .
The
.Xr select 2
and
.Xr kevent 2
system calls are supported for message queue descriptor.
.Pp
Please see the
.Xr mqueuefs 5
man page for instructions on loading the module or compiling the service into
the kernel.
.Sh RETURN VALUES
Upon successful completion, the function returns a message queue
descriptor; otherwise, the function returns
.Po Vt mqd_t Pc Ns \-1
and sets the global variable
.Va errno
to indicate the error.
.Sh ERRORS
The
.Fn mq_open
system call
will fail if:
.Bl -tag -width Er
.It Bq Er EACCES
The message queue exists and the permissions specified by
.Fa oflag
are denied, or the message queue does not exist and permission to create the
message queue is denied.
.It Bq Er EEXIST
.Dv O_CREAT
and
.Dv O_EXCL
are set and the named message queue already exists.
.It Bq Er EINTR
The
.Fn mq_open
function was interrupted by a signal.
.It Bq Er EINVAL
The
.Fn mq_open
function is not supported for the given name.
.It Bq Er EINVAL
.Dv O_CREAT
was specified in
.Fa oflag ,
the value of
.Fa attr
is not
.Dv NULL ,
and either
.Va mq_maxmsg
or
.Va mq_msgsize
was less than or equal to zero.
.It Bq Er EMFILE
Too many message queue descriptors or file descriptors are currently in use
by this process.
.It Bq Er ENAMETOOLONG
The length of the
.Fa name
argument exceeds
.Brq Dv PATH_MAX
or a pathname component
is longer than
.Brq Dv NAME_MAX .
.It Bq Er ENFILE
Too many message queues are currently open in the system.
.It Bq Er ENOENT
.Dv O_CREAT
is not set and the named message queue does not exist.
.It Bq Er ENOSPC
There is insufficient space for the creation of the new message queue.
.El
.Sh SEE ALSO
.Xr mq_close 2 ,
.Xr mq_getattr 2 ,
.Xr mq_receive 2 ,
.Xr mq_send 2 ,
.Xr mq_setattr 2 ,
.Xr mq_timedreceive 3 ,
.Xr mq_timedsend 3 ,
.Xr mq_unlink 3 ,
.Xr mqueuefs 5
.Sh STANDARDS
The
.Fn mq_open
system call conforms to
.St -p1003.1-2004 .
.Sh HISTORY
Support for
.Tn POSIX
message queues first appeared in
.Fx 7.0 .
.Sh BUGS
This implementation places strict requirements on the value of
.Fa name :
it must begin with a slash
.Pq Ql /
and contain no other slash characters.
.Pp
The
.Fa mode
and
.Fa attr
arguments are variadic and may result in different calling conventions
than might otherwise be expected.
.Sh COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2004 Edition, Standard for Information Technology --
Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 6, Copyright (C) 2001-2004 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.