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.
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 .
