Tribblix: manual page: mq

MQ_OPEN(3C) Standard C Library Functions MQ_OPEN(3C)

NAME

mq_open - open a message queue

SYNOPSIS

#include <mqueue.h>

mqd_t mq_open(const char *name, int oflag,
/* unsigned long mode, mq_attr attr */ ...);

DESCRIPTION

The mq_open() function establishes the connection between a process
and a message queue with a message queue descriptor. It creates a
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. The name
argument must conform to the construction rules for a path-name. If
name is not the name of an existing message queue and its creation is
not requested, mq_open() fails and returns an error. The first
character of name must be a slash (/) character and the remaining
characters of name cannot include any slash characters. For maximum
portability, name should include no more than 14 characters, but this
limit is not enforced.

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 is granted if the calling process would be
granted read or write access, respectively, to a file with the
equivalent permissions.

The value of oflag is the bitwise inclusive OR of values from the
following list. Applications must 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(3C), but not mq_send(3C). 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(3C)
but not mq_receive(3C). 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 additionally be specified
in the value of oflag:

O_CREAT
This option is used to create a message queue, and it
requires two additional arguments: mode, which is of
type mode_t, and attr, which is pointer to a mq_attr
structure. If the pathname, name, has already been
used to create a message queue that still exists, then
this flag has no effect, except as noted under O_EXCL
(see below). Otherwise, a message queue is created
without any messages in it.

The user ID of the message queue is set to the
effective user ID of process, and the group ID of the
message queue is set to the effective group ID of the
process. The file permission bits are set to the value
of mode, and modified by clearing all bits set in the
file mode creation mask of the process (see umask(2)).

If attr is non-NULL and the calling process has the
appropriate privilege on name, the message queue
mq_maxmsg and mq_msgsize attributes are 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 fails and
returns an error without creating the message queue.

O_EXCL
If both O_EXCL and O_CREAT are set, mq_open() will
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 are atomic with
respect to other processes executing mq_open() naming
the same name with both O_EXCL and O_CREAT set. If
O_EXCL and O_CREAT are not set, the result is
undefined.

O_NONBLOCK
The setting of this flag is associated with the open
message queue description and determines whether a
mq_send(3C) or mq_receive(3C) waits for resources or
messages that are not currently available, or fails
with errno set to EAGAIN. See mq_send(3C) and
mq_receive(3C) for details.

RETURN VALUES

Upon successful completion, mq_open() returns a message queue
descriptor; otherwise the function returns (mqd_t)-1 and sets errno
to indicate the error condition.

ERRORS

The mq_open() function will 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() operation was interrupted by a signal.

EINVAL
The mq_open() operation is not supported for the
given name, or 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
The number of open message queue descriptors in this
process exceeds MQ_OPEN_MAX, of the number of open
file descriptors in this process exceeds OPEN_MAX.

ENAMETOOLONG
The length of the name string exceeds PATH_MAX, or a
pathname component is longer than NAME_MAX while
_POSIX_NO_TRUNC is in effect.

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.

ENOSYS
The mq_open() function is not supported by the
system.

ATTRIBUTES

NOTES

Due to the manner in which message queues are implemented, they
should not be considered secure and should not be used in security-
sensitive applications.

Solaris 2.6 was the first release to support the Asynchronous Input
and Output option. Prior to this release, this function always
returned -1 and set errno to ENOSYS.

February 5, 2008 MQ_OPEN(3C)