The
mq_open()
system call establishes the connection between a process and a message queue
with a message queue descriptor.
It creates 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.
The
name
argument should conform to the construction rules for a pathname.
The
name
should begin with a slash character.
Processes calling
mq_open()
with the same value of
name
refers to the same message queue object, as long as that name has not been
removed.
If the
name
argument is not the name of an existing message queue and creation is not
requested,
mq_open()
will fail and return an error.
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
would 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 should 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 is of type
mode_t,
and
attr,
which is 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 has no effect, except as noted under
O_EXCL.
Otherwise, a message queue will be created without any messages
in it.
The user ID of the message queue will be set to the effective user ID
of the process, and the group ID of the message queue will be set to the
effective group ID of the process.
The permission bits of the message queue
will be set to the value of the
mode
argument, except those set in the file mode creation mask of the process.
When bits in
mode
other than the file permission bits are specified, the effect is
unspecified.
If
attr
is
NULL,
the message queue is 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 will 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 will fail and return an error without creating the message queue.
|
| O_EXCL
|
If
O_EXCL
and
O_CREAT
are set,
mq_open()
will fail if the message queue name exists.
|
| 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(2)
and
mq_receive(2)
for details.
|
|
The
mq_open()
system call does not add or remove messages from the queue.
