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Get a message queue's attributes
#include <mqueue.h> int mq_getattr( mqd_t mqdes, struct mq_attr* mqstat );
- The message-queue descriptor, returned by mq_open(), of the message queue that you want to get the attributes of.
- A pointer to a mq_attr structure where the function can store the attributes of the message queue. For more information, see below.
- For the traditional implementation, libc
Use the -l c option to qcc to link against this library. This library is usually included automatically.
- For the alternate implementation using asynchronous messages,
Use the -l mq option to qcc to link against this library.
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.
|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 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.
- 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.
|The alternate (mq) implementation of message queues doesn't see the non-POSIX mq_sendwait and mq_recvwait fields.|
-1 if an error occurred (errno is set). Any other value indicates success.
- Invalid message queue mqdes.
mq, mqueue in the Utilities Reference