.\" Copyright (c) 2001-2003 The Open Group, All Rights Reserved 
.TH "MQ_OPEN" 3P 2003 "IEEE/The Open Group" "POSIX Programmer's Manual"
.\" mq_open 
.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 \- open a message queue (\fBREALTIME\fP)
.SH SYNOPSIS
.LP
\fB#include <mqueue.h>
.br
.sp
mqd_t mq_open(const char *\fP\fIname\fP\fB, int\fP \fIoflag\fP\fB,
\&...); \fP
\fB
.br
\fP
.SH DESCRIPTION
.LP
The \fImq_open\fP() 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 \fIname\fP 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 \fIname\fP argument shall conform to the
construction rules for a pathname. If \fIname\fP begins with the slash
character, then processes calling \fImq_open\fP() with the
same value of \fIname\fP shall refer to the same message queue object,
as long as that name has not been removed. If \fIname\fP
does not begin with the slash character, the effect is implementation-defined.
The interpretation of slash characters other than
the leading slash character in \fIname\fP is implementation-defined.
If the \fIname\fP argument is not the name of an existing
message queue and creation is not requested, \fImq_open\fP() shall
fail and return an error.
.LP
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.
.LP
The \fIoflag\fP 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.
.LP
The value of \fIoflag\fP 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 \fIoflag\fP:
.TP 7
O_RDONLY
Open the message queue for receiving messages. The process can use
the returned message queue descriptor with \fImq_receive\fP(), but
not \fImq_send\fP(). A message
queue may be open multiple times in the same or different processes
for receiving messages.
.TP 7
O_WRONLY
Open the queue for sending messages. The process can use the returned
message queue descriptor with \fImq_send\fP() but not \fImq_receive\fP().
A message
queue may be open multiple times in the same or different processes
for sending messages.
.TP 7
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.
.sp
.LP
Any combination of the remaining flags may be specified in the value
of \fIoflag\fP:
.TP 7
O_CREAT
Create a message queue. It requires two additional arguments: \fImode\fP,
which shall be of type \fBmode_t\fP, and
\fIattr\fP, which shall be a pointer to an \fBmq_attr\fP structure.
If the pathname \fIname\fP 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,
and the group ID of the message queue shall be set to the effective
group ID of the process. The file permission bits shall be set
to the value of \fImode\fP. When bits in \fImode\fP other than file
permission bits are set, the effect is
implementation-defined. If \fIattr\fP is NULL, the message queue shall
be created with implementation-defined default message
queue attributes. If \fIattr\fP is non-NULL and the calling process
has the appropriate privilege on \fIname\fP, the message
queue \fImq_maxmsg\fP and \fImq_msgsize\fP attributes shall be set
to the values of the corresponding members in the
\fBmq_attr\fP structure referred to by \fIattr\fP. If \fIattr\fP is
non-NULL, but the calling process does not have the
appropriate privilege on \fIname\fP, the \fImq_open\fP() function
shall fail and return an error without creating the message
queue.
.TP 7
O_EXCL
If O_EXCL and O_CREAT are set, \fImq_open\fP() shall fail if the message
queue \fIname\fP 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\fP() naming the same \fIname\fP with O_EXCL and
O_CREAT set. If O_EXCL is set and O_CREAT is not set, the
result is undefined.
.TP 7
O_NONBLOCK
Determines whether an \fImq_send\fP() or \fImq_receive\fP() waits
for resources or messages that are not currently available, or fails
with \fIerrno\fP set to [EAGAIN]; see \fImq_send\fP() and \fImq_receive\fP()
for details.
.sp
.LP
The \fImq_open\fP() function does not add or remove messages from
the queue.
.SH RETURN VALUE
.LP
Upon successful completion, the function shall return a message queue
descriptor; otherwise, the function shall return
(\fBmqd_t\fP)-1 and set \fIerrno\fP to indicate the error.
.SH ERRORS
.LP
The \fImq_open\fP() function shall fail if:
.TP 7
.B EACCES
The message queue exists and the permissions specified by \fIoflag\fP
are denied, or the message queue does not exist and
permission to create the message queue is denied.
.TP 7
.B EEXIST
O_CREAT and O_EXCL are set and the named message queue already exists.
.TP 7
.B EINTR
The \fImq_open\fP() function was interrupted by a signal.
.TP 7
.B EINVAL
The \fImq_open\fP() function is not supported for the given name.
.TP 7
.B EINVAL
O_CREAT was specified in \fIoflag\fP, the value of \fIattr\fP is not
NULL, and either \fImq_maxmsg\fP or \fImq_msgsize\fP
was less than or equal to zero.
.TP 7
.B EMFILE
Too many message queue descriptors or file descriptors are currently
in use by this process.
.TP 7
.B ENAMETOOLONG
The length of the \fIname\fP argument exceeds {PATH_MAX} or a pathname
component is longer than {NAME_MAX}.
.TP 7
.B ENFILE
Too many message queues are currently open in the system.
.TP 7
.B ENOENT
O_CREAT is not set and the named message queue does not exist.
.TP 7
.B ENOSPC
There is insufficient space for the creation of the new message queue.
.sp
.LP
\fIThe following sections are informative.\fP
.SH EXAMPLES
.LP
None.
.SH APPLICATION USAGE
.LP
None.
.SH RATIONALE
.LP
None.
.SH FUTURE DIRECTIONS
.LP
None.
.SH SEE ALSO
.LP
\fImq_close\fP(), \fImq_getattr\fP(), \fImq_receive\fP(), \fImq_send\fP(),
\fImq_setattr\fP(), \fImq_timedreceive\fP(), \fImq_timedsend\fP(),
\fImq_unlink\fP(), \fImsgctl\fP(), \fImsgget\fP(), \fImsgrcv\fP(),
\fImsgsnd\fP(), the Base Definitions volume of IEEE\ Std\ 1003.1-2001,
\fI<mqueue.h>\fP
.SH COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 6, Copyright (C) 2001-2003 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 .