mq_getattr()
QNX SDP8.0C Library ReferenceAPIDeveloper
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.
Library:
libc
Use the -l c option to qcc to link against this library. This library is usually included automatically.
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. For more information, see the entry for mqueue in the Utilities Reference.
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 OS 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 OS extension. A nonzero value in this field implies that the queue is empty.
Returns:
-1 if an error occurred (errno is set). Any other value indicates success.
Errors:
- EBADF
- Invalid message queue mqdes.
Examples:
See the example for mq_open().
Classification:
| Safety: | |
|---|---|
| Cancellation point | No |
| Signal handler | No |
| Thread | Yes |
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