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Open a message queue
#include <mqueue.h>
mqd_t mq_open( const char * name,
int oflag,
... )
- 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
- The file permissions for the new queue.
For more information, see
“Access permissions”
in the documentation for 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.
- 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.
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.
|
Neutrino supports two implementations of message queues:
a traditional implementation, and an alternate one that uses
asynchronous messages.
For more information, see the entry for
mq and mqueue
in the Utilities Reference. |
The name is interpreted as follows:
name
|
Pathname space entry |
entry
|
cwd/entry |
/entry
|
/dev/mqueue/entry using the traditional
(mqueue) implementation, or
/dev/mq/entry using the alternate
(mq) implementation |
entry/newentry
|
cwd/entry/newentry |
/entry/newentry
|
/entry/newentry |
where cwd is the current working directory for the program at
the point that it calls mq_open().
|
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. |
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.
|
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(). |
A valid message queue descriptor if the queue is successfully created,
or -1 (errno is set).
- 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 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.
POSIX 1003.1 MSG
Safety: | |
Cancellation point |
No |
Interrupt handler |
No |
Signal handler |
No |
Thread |
Yes |
mq_close(),
mq_getattr(),
mq_notify(),
mq_receive(),
mq_send(),
mq_setattr(),
mq_timedreceive(),
mq_timedsend(),
mq_unlink()
mq, mqueue
in the Utilities Reference
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