Open a message queue
Synopsis:
#include <mqueue.h>
#include <fcntl.h>
mqd_t mq_open( const char * name,
int oflag,
... )
Arguments:
- name
- The name of the message queue that you want to open; see below.
- oflag
- You must specify one of O_RDONLY (receive-only),
O_WRONLY (send-only) or O_RDWR (send-receive).
In addition, you can OR in the following constants to produce the following effects:
- O_CREAT — if name doesn't exist,
instruct the server to create a new message queue with the given name.
If you specify this flag, mq_open() uses its
mode and mq_attr arguments; see below.
- O_EXCL — if you set both O_EXCL and
O_CREAT, and a message queue name exists, the
call fails and errno is set to EEXIST.
Otherwise, the queue is created normally.
If you set O_EXCL without O_CREAT, it's ignored.
- O_NONBLOCK — under normal message queue operation,
a call to mq_send() or mq_receive() could block
if the message queue is full or empty.
If you set this flag, these calls never block.
If the queue isn't in a condition to perform the given call,
errno is set to EAGAIN and the call returns an error.
If you set O_CREAT in the oflag argument,
you must also pass these arguments to mq_open():
- mode_t mode
- The file permissions for the new queue.
For more information, see the entry for
struct stat.
If you set any bits other than file permission bits, they're ignored.
Read and write permissions are analogous to receive and send
permissions; execute permissions are ignored.
- struct mq_attr *mq_attr
- NULL, or a pointer to an mq_attr
structure that contains the attributes that you want to use for the new queue.
For more information, see
mq_getattr().
If mq_attr is NULL, the following default
attributes are used—depending on which implementation of
message queues you're using—provided that you didn't override
the defaults when you started the message-queue server:
| Attribute |
Traditional |
Alternate |
| mq_maxmsg |
1024 |
64 |
| mq_msgsize |
4096 |
256 |
| mq_flags |
0 |
0 |
If mq_attr isn't NULL, the new queue
adopts the mq_maxmsg and mq_msgsize of mq_attr.
The mq_flags flags field is ignored.
Description:
The mq_open() function opens a message queue referred to by name,
and returns a message queue descriptor by which the queue can be referenced in the future.
Note: The message queue manager needs to be running.
QNX Neutrino supports two
implementations of message queues: a traditional implementation, and an alternate one that
uses the
mq server and a queue in kernel space. For more information, see
the entries for
mq and
mqueue in the
Utilities Reference, as well as the
POSIX Message Queues: Two Implementations
technote.
The name is interpreted as follows:
- With the traditional (mqueue) implementation:
| name |
Pathname space entry |
| /entry |
/dev/mqueue/entry |
| /entry/newentry |
/entry/newentry |
| entry |
cwd/entry |
| entry/newentry |
cwd/entry/newentry |
where cwd is the current working directory for the program at
the point that it calls mq_open().
- With the alternate (mq) implementation, message queues
are always created under /dev/mq (or the path specified
with the -N option to
mq):
- If the name argument starts with a slash, the queue is given that name.
- If the name argument doesn't begin with a slash character, the queue is given that name,
prepended with the current working directory.
In either case, slash characters other than the leading slash character
aren't interpreted, and the specified name, including these slash
characters, is used to identify the message queue.
In other words, additional slashes don't create a directory structure under
/dev/mq.
For example, if your current directory is /tmp:
| name |
Pathname space entry |
| /entry |
/dev/mq/entry |
| entry |
/dev/mq/tmp/entry |
Note:
If you want to open a queue on another node, you have to use the traditional
(mqueue) implementation and specify the name as
/net/node/mqueue_location.
Here, node is the node name and mqeueue_location is
the full device path for the queue, which is stored under /dev/mqueue.
For instance, if you want to open the queue at /dev/mqueue/mymessagequeue
on a node named NTO1, you must use the name
/net/NTO1/dev/mqueue/mymessagequeue in calling mq_open().
If name doesn't exist, mq_open() examines the third and fourth
parameters: a mode_t and a pointer to an mq_attr structure.
The only time that a call to mq_open() with
O_CREAT set fails is if you open a message queue and
later unlink it, but never close it.
Like their file counterparts, an unlinked queue that hasn't yet been
closed must continue to exist; an attempt to recreate such a message
queue fails, and errno is set to ENOENT.
Note:
Message queues persist—like files—even after the processes
that created them end.
A message queue is destroyed when the last process connected to it unlinks
from the queue by calling
mq_unlink().
Returns:
A valid message queue descriptor if the queue is successfully created,
or -1 (errno is set).
Errors:
- EACCES
- The message queue exists, and you don't have
permission to open the queue under the given oflag,
or the message queue doesn't exist, and you don't have permission to create one.
- EEXIST
- You specified the O_CREAT and O_EXCL flags
in oflag, and the queue name exists.
- EINTR
- The operation was interrupted by a signal.
- EINVAL
- You specified the O_CREAT flag in oflag, and mq_attr
wasn't NULL, but some values in the mq_attr structure were invalid.
- ELOOP
- Too many levels of symbolic links or prefixes.
- EMFILE
- Too many message queue descriptors or file descriptors are in use by the calling process.
- ENAMETOOLONG
- The length of name exceeds PATH_MAX.
- ENFILE
- Too many message queues are open in the system.
- ENOENT
- You didn't set the O_CREAT flag, and the queue name doesn't exist.
- ENOSPC
- The message queue server has run out of memory.
- ENOSYS
- The mq_open() function isn't implemented for the filesystem specified
in name, or the message queue manager
(mq
or
mqueue)
isn't running.
Classification:
POSIX 1003.1 MSG
| Safety: |
|
|---|
| Cancellation point |
No |
| Interrupt handler |
No |
| Signal handler |
No |
| Thread |
Yes |