Get a message queue's attributes
Synopsis:
#include <mqueue.h>
int mq_getattr( mqd_t mqdes,
struct mq_attr* mqstat );
Arguments:
- mqdes
- The message-queue descriptor, returned by
mq_open(),
of the message queue that you want to get the attributes of.
- mqstat
- A pointer to a mq_attr structure where the function
can store the attributes of the message queue.
For more information, see below.
Description:
The mq_getattr() function determines the current attributes of
the queue referenced by mqdes. These attributes are
stored in the location pointed to by mqstat.
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 fields of the mq_attr structure are as follows:
- long mq_flags
- The options set for this open message-queue description (i.e., these
options are for the given mqdes, not the queue as a whole).
This field may have been changed by call to
mq_setattr()
since you opened the queue.
- O_NONBLOCK — no call to
mq_receive()
or
mq_send()
will ever block on this queue.
If the queue is in such a condition that the given
operation can't be performed without blocking, then an error is
returned, and errno is set to EAGAIN.
- long mq_maxmsg
- The maximum number of messages that can be stored on the queue.
This value was set when the queue was created.
- long mq_msgsize
- The maximum size of each message on the given message queue.
This value was also set when the queue was created.
- long mq_curmsgs
- The number of messages currently on the given queue.
- long mq_sendwait
- The number of threads currently waiting to send a message.
This field was eliminated from the POSIX standard after draft 9, but has
been kept as a QNX Neutrino extension.
A nonzero value in this field implies that the queue is full.
- long mq_recvwait
- The number of threads currently waiting to receive a message.
Like mq_sendwait,
this field has been kept as a QNX Neutrino extension.
A nonzero value in this field implies that the queue is empty.
Note:
The alternate (mq) implementation of message queues doesn't
see the non-POSIX mq_sendwait and mq_recvwait fields.
Returns:
-1 if an error occurred
(errno is set).
Any other value indicates success.
Errors:
- EBADF
- Invalid message queue mqdes.
Classification:
POSIX 1003.1 MSG
Safety: |
|
Cancellation point |
No |
Interrupt handler |
No |
Signal handler |
No |
Thread |
Yes |