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


See attributes(5) for descriptions of the following attributes:


+--------------------+-------------------+
| ATTRIBUTE TYPE | ATTRIBUTE VALUE |
+--------------------+-------------------+
|Interface Stability | Committed |
+--------------------+-------------------+
|MT-Level | MT-Safe |
+--------------------+-------------------+
|Standard | See standards(5). |
+--------------------+-------------------+

SEE ALSO


exec(2), exit(2), umask(2), sysconf(3C), mqueue.h(3HEAD), mq_close(3C),
mq_receive(3C), mq_send(3C), mq_setattr(3C), mq_unlink(3C),
attributes(5), standards(5)

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)