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NAME
mq_open - open a message queue (REALTIME)
SYNOPSIS
#include <mqueue.h> mqd_t mq_open(const char *name, int oflag, ...);
DESCRIPTION
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 shall conform to the
construction rules for a pathname. 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. The interpretation of slash characters other than the leading slash character in name 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, 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 mode. When bits in mode other than file permission bits are set,
the effect is implementation-defined. 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 the appropriate privilege 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. If attr is non-NULL, but the calling process does not have the appropriate
privilege 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() and mq_receive() for
details.
The mq_open() function does not add or remove messages from the queue.
RETURN VALUE
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.
ERRORS
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.
ENAMETOOLONG
The length of the name argument exceeds {PATH_MAX} or a pathname component is longer than
{NAME_MAX}.
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.
The following sections are informative.
EXAMPLES
None.
APPLICATION USAGE
None.
RATIONALE
None.
FUTURE DIRECTIONS
None.
SEE ALSO
mq_close() , mq_getattr() , mq_receive() , mq_send() , mq_setattr() , mq_timedreceive() , mq_timedsend()
, mq_unlink() , msgctl() , msgget() , msgrcv() , msgsnd() , the Base Definitions volume of
IEEE Std 1003.1-2001, <mqueue.h>
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 .